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A Co-Crystal between Acetylene and Butane: A Potentially Ubiquitous Molecular Mineral on Titan

Morgan L. Cable*
Morgan L. Cable
NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, United States
*E-mail: [email protected].
More by Morgan L. Cable
Tuan H. Vu
Tuan H. Vu
NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, United States
More by Tuan H. Vu
Michael J. Malaska
Michael J. Malaska
NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, United States
More by Michael J. Malaska
Helen E. Maynard-Casely
Helen E. Maynard-Casely
Australian Nuclear Science and Technology Organisation, Kirrawee DC, New South Wales 2232, Australia
More by Helen E. Maynard-Casely
Mathieu Choukroun
Mathieu Choukroun
NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, United States
More by Mathieu Choukroun
http://orcid.org/0000-0001-7447-9139
Robert Hodyss
Robert Hodyss
NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, United States
More by Robert Hodyss

Cite this: ACS Earth Space Chem. 2019, 3, 12, 2808-2815

Publication Date (Web):November 15, 2019

https://doi.org/10.1021/acsearthspacechem.9b00275

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Abstract

Titan hosts a complex chemical engine producing a rich inventory of organic molecules in its thick atmosphere and on its surface. Some of these organics may be deposited in the liquid hydrocarbon lakes in the polar regions and form evaporite features when the lakes dry out as part of Titan’s methane/ethane cycle that is analogous to Earth’s hydrologic cycle. Modeling suggests that acetylene and butane would be the main components of such evaporite deposits. We have previously demonstrated that some organic molecules (such as benzene and ethane) readily form co-crystals in Titan-relevant conditions. We report here Raman spectroscopic evidence for a new co-crystal between acetylene and butane, which could be the most common organic co-crystal discovered so far of direct relevance to Titan’s surface. Intermolecular interactions such as those in the acetylene-butane co-crystal could modify the kinetics and equilibria of various processes (dissolution, reprecipitation, etc.) and therefore may play a key role in the formation mechanisms and timescales of landscape evolution on Titan.

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Introduction

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Titan, the largest satellite of Saturn, contains a wide distribution of organic molecules and is considered a prebiotic chemical laboratory on a planetary scale. Photochemistry in the atmosphere induced by solar radiation and energy from Saturn’s magnetosphere causes a chemical cascade as N₂ and CH₄ dissociate and generate organic molecules ranging from simple (ethane, acetylene, and HCN) to complex (>10,000 Da). These molecules continue to combine and react as they move through Titan’s atmosphere, forming aerosol haze layers and ultimately depositing on the surface.(1)

Some of these organic compounds are transported via fluvial (rivers and streams) or pluvial (rain) processes into the hydrocarbon lakes in Titan’s polar regions, comprised mostly of methane, ethane, dissolved molecular nitrogen, propane, and possibly 1-butene (C₄H₈).(2−4) Molecules that dissolve in these nonpolar liquids may precipitate via evaporation or other mechanisms (i.e., if the liquid recedes into a subsurface reservoir) and could be responsible for the 5 μm bright features observed by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS), and the high-backscatter regions observed by Cassini’s Synthetic Aperture Radar (SAR), around some of the lakes and in dry lakebeds in the north pole(5,6) and in other regions of Titan’s surface where liquid was present in the past.(7)

Acetylene (C₂H₂) is the primary solid photochemical product generated in Titan’s atmosphere.(8−12) It is believed to form via a multistep process starting with the photolysis of methane and leading to the photolysis of ethylene (Table 1).(8,10) Acetylene was first identified in the upper atmosphere of Titan by Voyager 1 and ground-based observations(13) and was later confirmed by the Composite Infrared Spectrometer(14) and Ion and Neutral Mass Spectrometer(15) instruments aboard Cassini. Acetylene was also detected on the surface by the gas chromatograph mass spectrometer of the Huygens probe upon landing(16) and has been tentatively identified in the equatorial region by Cassini’s VIMS.(17) Acetylene can be present in the solid form as one of two phases below the melting point (193 K) depending on the temperature: cubic (133–193 K) and orthorhombic (<133 K).(18−20) It is likely present in the low-temperature orthorhombic phase on Titan’s surface at 90 K.

Table 1. Primary Formation Mechanisms of Acetylene and n-Butane in the Atmosphere of Titan, Predicted Precipitation Rates onto the Surface, and Calculated Solubilities in Titan Lakes

species	formation reaction(s)a	precipitation rate (molecules·m^–2·s^–1)b	solubility (mole frac)c
acetylene	C₂H₄ + hν → C₂H₂ + 2H/H₂	5.1 × 10⁷	5.40 × 10^–2
butane	CH₃ + C₃H₇ + M → C₄H₁₀ + M	5.4 × 10⁷	1.22 × 10^–1
butane	2C₂H₅ + M → C₄H₁₀ + M	5.4 × 10⁷	1.22 × 10^–1

^{^a}

M is a third neutral molecule (most likely N₂) required for energy balance in the termolecular reactions.

^{^b}

Refs (10) and (27).

^{^c}

Ref (27). Calculated ideal mole fractions at saturation (assumes all activity coefficients are 1) for a solution of 89% ethane, 10% methane, and 1% nitrogen at 90 K.

Butane is expected to be abundant on Titan, though it is difficult to detect via remote sensing techniques. Propane is the largest saturated hydrocarbon detected in Titan’s atmosphere to date,(21) but n-butane (C₄H₁₀) and higher hydrocarbons are predicted by photochemical models(8−10) and laboratory experiments.(22,23)n-Butane is likely generated in the Titan atmosphere by the collisional addition of one methyl and one propyl radical or two ethyl radicals (Table 1); these radicals are produced via the photodissociation of hydrocarbons, such as methane and 1-butene, and/or abstraction reactions of propane with hydrogen and acetylene with ethane.(8,10,24) Butane detection on Titan via remote observation is difficult as the absorption bands overlap with other saturated hydrocarbons (propane, etc.), although data from the Infrared Space Observatory suggests that mole fractions of hydrocarbons in the atmosphere of Titan follow a power-law up to C₆ compounds.(25)n-Butane is known to have three solid forms below the melting point (133 K), all with a monoclinic symmetry, due to internal rotation about the central C–C single bond, though we note that these have been determined from deuterated butane.(26) Above 108 K, butane is a disordered structure, and below this (and therefore at Titan surface conditions), it can be present in two forms, both trans configurations, with one being metastable compared to the other.

The estimated precipitation rates for n-butane and acetylene on Titan are on the same order,(10,27) and their solubilities in liquid methane and ethane (Table 1) are relatively high compared to other solid organics on Titan.(28−31) Each of these compounds is estimated to comprise approximately 1% of the lake composition, with the other lake constituents being ethane (∼76–79%), propane (∼7–8%), methane (∼5–10%), hydrogen cyanide (∼2–3%), and butene (∼1%).(3) However, recent work suggests that the ethane content in the lakes and seas can vary significantly; the southern lakes and most equatorward of the north polar seas may be ethane-dominated(32) and could contain significant solutes and/or suspended materials, while the loss tangent of the northernmost seas suggests a composition of almost pure methane and dissolved molecular nitrogen(33) as supported by Cassini’s SAR measurements.(4)

Modeling suggests that precipitation from any Titan lake would result in an evaporite layer enriched in n-butane (C₄H₁₀) and acetylene (C₂H₂), regardless of whether the solvent is methane- or ethane-rich.(27,34) Previous studies(35−37) have shown that some organic molecules readily form co-crystals in Titan-relevant conditions, including acetylene.(38,39) These molecular minerals represent an exciting new class of compounds for Titan’s surface, which are in the initial stages of being catalogued.(40) We report here evidence for a new co-crystal between acetylene and n-butane that may be the most ubiquitous organic co-crystalline molecular mineral on Titan discovered thus far.

Experimental Methods

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Acetylene (Airgas, industrial grade, dissolved in acetone) and n-butane (Sigma-Aldrich, 99% purity) were either condensed sequentially or co-deposited from the gas phase into a liquid nitrogen-cooled cryostage (LTS 350, Linkam Scientific Instruments, Ltd.) maintained at 90 ± 0.1 K under an atmosphere of N₂ (1 bar). The acetylene was passed through a purifier (Micro Torr MC400-404F, SAES Pure Gas, Inc.) to remove organic impurities to <1 pptV and particles <0.003 μm prior to condensation in the cryostage (absence of features at 787, 1710, and 2922 cm^–1 in the Raman spectrum verified removal of acetone, a stabilizer added to acetylene to prevent self-polymerization). Another method implemented to improve the ratio of co-crystal to pure components was to condense n-butane and acetylene sequentially into a plastic pipette bulb (volume, ∼4 mL) immersed in an n-butane bath (130 K) in a custom-built cryostat under an atmosphere of N₂ (1 bar). After 30 min of equilibration, this mixture was transferred under N₂ atmosphere to the cryostage maintained at 90 K.

The optical cryostage was mounted onto an XYZ motorized translation stage underneath the Olympus BXFM objective turret of a high-resolution confocal dispersive micro-Raman spectrometer (Horiba Jobin-Yvon LabRam HR). The sample was observed continuously under various levels of magnification during the experiment. Raman spectra were obtained at 0.4 cm^–1/px resolution using an 1800 grooves/mm grating. All samples were excited by an external frequency-doubled Nd:YAG 532 nm laser with an output power of 50 mW. The silicon 520.7 cm^–1 peak was used for frequency calibration. Spectra were accumulated in duplicate or greater to improve the signal-to-noise ratio. Thermal stability studies were performed by warming in 5 K increments and obtaining Raman spectra following a 5 min equilibration time at each temperature point.

In an additional experiment designed to simulate an ethane pluvial (rain) event, acetylene and n-butane were condensed sequentially in the cryostage at 130 K. The cryostage was cooled to Titan surface temperature (90 K), and an aliquot of liquid ethane (Matheson Tri-Gas, ultrahigh purity grade (99.95%)) was then condensed onto the acetylene-butane deposit. The evolution of this ethane-wetted sample with time was monitored both with microscopic observations and collection of Raman spectra.

Results

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Formation of the Co-Crystal

Raman spectroscopy is an instructive tool in identification of organic co-crystals and their properties as it is sensitive to the chemical environments of the molecules probed. Co-crystal formation typically results in frequency shifts and sharpening of peaks as compared to the Raman spectra of pure compounds, enabling identification. Here, we compare the frequency shifts observed in the acetylene-butane co-crystal to the pure compounds and to acetylene clathrate hydrate. Acetylene clathrate hydrate is included for comparison due to its ease of formation under our experimental conditions(41) and the possibility of adventitious water in the sample.

Upon sequential deposition or co-deposition of acetylene and n-butane at 130 K, blue shifts of the ν₂ C≡C stretch (12 cm^–1) and ν₁ C–H symmetric stretch (20 cm^–1) of acetylene are observed, suggesting a distinctly different chemical environment for C₂H₂ compared to its solid phase (Figure 1). Shifts of similar magnitude have been observed in the vibrational spectra of other Titan-relevant co-crystals(35,38) and clathrate hydrates.(42) The blue shift of the acetylene ν₂ C≡C stretch, from 1955 cm^–1 in pure solid acetylene to 1967 cm^–1 in the presence of butane, is very similar to (but resolvable and distinct from) the acetylene clathrate hydrate(41) and a co-crystal of acetylene and CO₂ with a fixed 2:1 stoichiometry,(43) which exhibit features at 1966 and 1964 cm^–1, respectively, in the Raman and IR. The acetylene clathrate spectrum also features a broad absorption in the O–H region around 3100 cm^–1, which is absent in the co-crystal spectra (Figure 1 and Supporting Information, Figures S2 and S3). Therefore, we assign the observed spectral shifts to the formation of a co-crystal between acetylene and butane. This is additionally supported by a change in morphology upon the formation of the co-crystalline structure (see Supporting Information, Figure S1), similar to the recrystallization observed on the formation of the ethane-benzene co-crystal.(35)

Both solid phases of pure acetylene (cubic ≥133 K (Figure 1) and orthorhombic <133 K (Figure S4)) exhibit splitting in the ν₂ C≡C stretch and ν₁ C–H stretch due to crystal field effects.(44,45) These unique splittings indicate differences in the crystal structure and intermolecular interaction (primarily the C···H interaction) between the two acetylene solid phases. In the low-temperature orthorhombic phase, this splitting is also affected by the presence of a weak hydrogen bond between the C–H and the π-electrons.(44,46) Pressure studies of the orthorhombic acetylene phase show that splitting in the ν₂ C≡C stretch increases as the hydrogen-bonding interaction strengthens.(47) Crystal field splitting in the acetylene-butane co-crystal is expected to be suppressed because of the different crystal structures; this is consistent with the observed Raman spectra, where single peaks are observed at 1967.3 and 3355.2 cm^–1 for the ν₂ C≡C stretch and ν₁ C–H stretch, respectively (Figure 1 and Table 2).

Table 2. List of Selected Experimental Raman Shifts upon Co-Crystal Formation at 150 K, Compared to Reported Raman Values for Pure Components and the Acetylene Clathrate at a Similar Temperature

		Raman shift (cm^–1)
		pure component		co-crystal	clathrate	Δν between pure component and co-crystal
molecule	vibrational mode	this worka	reportedb	this work	reportedc	this workd
acetylene	¹³C≡C stretch	1928.8	1929.5	1935.8	1935.2	7.0
	ν₂ (C≡C stretch)	1955.1	1959	1967.3	1966.0	12.2
	ν₂ (C≡C stretch)	1960.7	1961.5
	ν₁ (C–H stretch)	3327.1	3329.5
	ν₁ (C–H stretch)	3333.2	3335	3355.2	3352.6	22.0
butane	ν₈ CH₃ rock, C–C stretch	1150.8	1150	1150.3		–0.5

^{^a}

Pure component spectra collected at 130 K.

^{^b}

Cubic C₂H₂ at 150 K;(45) liquid n-butane at 146 K.(48)

^{^c}

sI clathrate hydrate, large cage, collected at 200 K.(41)

^{^d}

A positive value of Δν indicates a blue shift; a negative value indicates a red shift.

Interestingly, the acetylene-ammonia co-crystal displays not a blue shift but a 7 cm^–1 red shift of the acetylene ν₂ C≡C stretch with respect to solid acetylene, to 1944 cm^–1.(38) This pattern is also present in the symmetric C–H stretching bands and may serve to highlight the differences in how the acetylene molecule is confined in the acetylene-butane co-crystalline structure compared to the acetylene-ammonia structure. As ammonia is capable of forming strong hydrogen bonds in the solid phase, its interaction with acetylene can be expected to be significantly different from that of the nonpolar n-butane molecules. In particular, the ammonia-acetylene co-crystal is governed by a network of specific C–H···N interactions,(39) giving rise to large red shifts in both the C≡C and C–H stretches of acetylene. The n-butane matrix, on the other hand, is bound together by weak hydrophobic interactions, implying that no specific bonding motif is likely involved in the formation of the co-crystal. Given the similarity between the shifts observed for acetylene confined in the co-crystal and those trapped in clathrate hydrates,(41) it is probable that the co-crystal exhibits a clathrate-like environment where acetylene molecules are loosely held in the butane structure. Such a configuration is also consistent with the fact that no significant shifts were observed in the low-frequency regions of lattice vibrations and C–H bending modes for the acetylene-butane co-crystal; in contrast, shifts on the order of 42–66 cm^–1 were observed in the acetylene-ammonia co-crystal.(38) Further crystallographic work is warranted in order to definitively correlate vibrational frequency shifts with co-crystal structures, although it does seem clear that not all co-crystals behave the same spectroscopically. All observed frequency shifts for the acetylene-butane co-crystal are reported in Table 2 (for shifts at 90 K, see Supporting Information, Table S1).

Thermal Stability

The co-crystal forms within minutes at 130 K and is stable when cooled to Titan surface temperatures (90 K) (see Supporting Information, Figure S2). This co-crystal remains intact up to 190 K (Figure 2), approximately 40° higher than the point at which acetylene sublimates in our experimental setup. This enhanced thermal stability is similar to that seen in the benzene-ethane co-crystal, which persisted up to 160 K, 15° higher than the temperature where we observed pure liquid ethane to boil off in the same experimental setup.(36) It is also distinct from the acetylene clathrate hydrate, which is stable up to 233 K.(41)

Influence of Solid-Phase Acetylene or Butane on Co-Crystal Stability

As described above, acetylene can be present in the solid form either in the cubic phase (≥133 K) or orthorhombic phase (<133 K).(18,19) The formation of the acetylene-butane co-crystal appears to be independent of the acetylene phase; the 1967 cm^–1 feature is present above and below the phase transition temperature of 133 K (see Supporting Information, Figure S4).(49) This phase transition from orthorhombic to cubic results in a blue shift in the C≡C region from 1951.9 to 1955.1 cm^–1 and the ν₁ C–H stretching modes from 3317.0 and 3325.4 cm^–1 to 3327.1 and 3333.2 cm^–1, respectively. While these shifts due to the phase transition of pure acetylene are still observed in the sample (containing both co-crystal and pure acetylene, see Table 2), the co-crystal features are unaffected.

n-Butane has multiple phases at a low temperature, including one liquid (>133 K), one disordered solid (108–133 K), and two ordered solid (≤108) phases. Stability of the acetylene-butane co-crystal appears to be independent of n-butane phase; the 1967 cm^–1 feature is present above and below the freezing point of n-butane and at temperatures where solid butane is in the ordered or disordered phase (see Supporting Information, Figures S5 and S6). It should be noted, however, that attempts to form the co-crystal from solid–solid contact of n-butane and acetylene at Titan surface temperature (90 K) were not successful over timescales achievable with our laboratory setup. The co-crystal forms rapidly (< 5 min) at 130 K when n-butane is liquid and remains stable when cooled to 90 K. This is unsurprising given the greater mobility of liquid butane. Further experiments on the kinetics of co-crystal formation may provide better constraints on the timescale required for its formation at Titan surface temperature.

Co-Crystal Stability after Exposure to Liquid Ethane

The stability of the co-crystal was investigated in the presence of liquid ethane to simulate exposure to hydrocarbon liquids. Ethane was selected over methane due to the greater solubilities of acetylene and butane in this solvent, and hence we expect any impact on co-crystal stability due to a fluvial or pluvial event to be more pronounced with ethane. After co-crystal formation was observed, the cryostage was cooled to Titan surface temperature, and ethane was condensed over the co-crystal. As shown in Figure 3, the co-crystal is still observed in the presence of liquid ethane after a brief (∼20 min) equilibration period. Subsequent warming in 5 K increments up to 135 K in the presence of liquid ethane showed a dramatic decrease in the ratio of co-crystal to pure acetylene when the temperature reached 120 K (Figure 4A); this was not coincident with the phase change of acetylene, which occurs at 130 K. It was, however, coincident with the phase change of n-butane to liquid (Figure 4B), which occurred at a lower temperature than the typical melting point of n-butane (133 K) perhaps due to freezing point depression. It is possible that, once the excess butane melted and dissolved in the liquid ethane, the butane in the co-crystal was able to more easily dissolve in the mixed solvent. The loss of the co-crystal with the change in phase of n-butane suggests that, above this temperature (120 K), the co-crystal is not stable in the presence of liquid hydrocarbons. Detailed kinetics studies of dissociation may further elucidate the stability of the acetylene-butane co-crystal in the presence of liquid hydrocarbons at Titan surface temperature (90–95 K). This may have important implications for geological resistance under Titan surface conditions.(50)

While the co-crystal is stable following exposure to liquid ethane below the depressed butane freezing point (120 K), attempts to form the co-crystal via dissolution of acetylene and butane in liquid ethane followed by evaporation to form a precipitate were not successful. This may be due to the limitations of the experimental setup as subsequent Raman analyses revealed that significant amounts of butane had dissolved in the ethane solvent over the ∼3 h equilibration period (in a custom cryostat maintained at 110 K with stirring), but very little if any acetylene was dissolved. This is consistent with the higher solubility of butane compared to acetylene in liquid hydrocarbons (Table 1).(27) It may not be possible to fully replicate the conditions of Titan in this respect, where acetylene would be expected to have sufficient time to fully equilibrate in a liquid methane/ethane lake over seasonal or geologic timescales.

Discussion

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The acetylene-butane co-crystal has been successfully formed via sequential deposition and co-deposition at 130 K. While many co-crystals have previously been reported with acetylene,(51) this is, to the best of our knowledge, the first report of a co-crystal between acetylene and butane. This is also the third organic co-crystal observed to be stable under simulated Titan surface conditions in addition to the benzene-ethane and acetylene-ammonia co-crystals.

The acetylene-butane co-crystal forms rapidly at 130 K and is stable at Titan surface temperatures (90 K). Given that butane and acetylene are predicted to be the most abundant evaporite materials around Titan lakes, the acetylene-butane co-crystal may be ubiquitous in these regions of Titan’s surface. Further, as this co-crystal may not be stable under constant or extensive fluvial/pluvial exposure to methane or ethane, the presence of this co-crystal on the surface of Titan may be an indicator of a dry lakebed with little to no liquid weathering (hydrocarbon rain or flooding events).

Co-crystals may influence Titan surface material characteristics such as particle size, dissolution rate, structural hardness, and resistance to erosion. Given that this is the second co-crystal discovered with an alkane (the first being the benzene-ethane co-crystal), it seems likely that lake edges and evaporite basins could serve as selective hydrocarbon reservoirs, possibly enriching these materials in ethane and butane over more unlikely co-crystal formers (to date) such as methane and propane.

Many of the smaller circular closed depressions in Titan’s polar regions are thought to have been formed from karstic dissolution or other phase change processes.(52) Most of these studies and modeling efforts have been via examination of one pure solid or mixture of solids, which behave as individual components.(30) If intimately associated molecules, as demonstrated in these laboratory experiments, exist on Titan’s surface, then intermolecular interactions could change dissolution and reprecipitation kinetics and equilibria, thus affecting the chemical erosion, transport, and deposition of molecules on Titan’s surface. Furthermore, if geologically significant quantities of these co-crystals exist, then they could play a key role in the formation mechanisms and timescales of landscape evolution, particularly in karstic or other phase change processes. For example, karstic dissolution and other phase change processes have been invoked in the formation of Titan’s uplifted plateau, referred to as labyrinth terrains.(53)

Recent investigations have identified enigmatic raised ramparts surrounding some of the northern Titan lakes.(54) These are positively expressed features up to 100 m and extending outward up to 10 km from the lakes. The near-infrared reflectance spectra of these features are similar to evaporitic materials found in some of the empty lake basins. One of the leading hypotheses for their formation is a “karstic hardening” due to dissolution/reprecipitation of dissolved materials and/or the surrounding geological substrate. Following landscape deflation from regional erosion, more resistant structures would be positively expressed. Thus, if co-crystal formation occurred during precipitation and percolation into the surrounding substrate, and if these materials were more resistant and durable than surrounding terrains comprised more “pure” molecular materials, they could explain the formation of this terrain. The exact formation mechanism of the ramparts, and other enigmatic features such as the high-walled lake rims, both could in part be affected by differential erosion rates of varied materials on Titan.(55) Understanding the complex chemical associations and chemical properties of Titan surface candidate materials could therefore have important implications for geological processes and features occurring on Titan.(40)

Differences in physical or mechanical properties may also lead to chemical gradients on Titan, which life could potentially exploit.(40) On Earth, in deepwater marine and freshwater environments, the obligate anaerobe Pelobacter acetylenicus is able to hydrolyze acetylene to acetaldehyde and use acetylene as both an energy source and its sole carbon source.(56−58) This bacterium uses a unique metalloenzyme, acetylene hydratase, that is able to perform this conversion.(59) The catalytic hydrogenation of acetylene to ethane has also been proposed as a possible energy-yielding reaction for metabolism.(60−62) It is possible that acetylene-based co-crystals might be a mechanism for storing acetylene, in a manner similar to how carbon dioxide is stored in carbonate deposits on Earth, where it might be more readily accessible to a putative microbial community either at the surface or in the deeper subsurface.

In conclusion, we have demonstrated the formation of a new co-crystal between acetylene and n-butane at Titan-relevant temperatures. This co-crystal may have implications in understanding some of the chemical and geological processes leading to surface features observed on this moon, as well as possibly sequestering nutrients for putative life. A future in situ Titan mission, such as the recently selected Dragonfly mission under the NASA New Frontiers program, may help to shed light on such tantalizing possibilities.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsearthspacechem.9b00275.

Raman microscopy images, a table of experimental Raman shifts upon co-crystal formation at 90 K, and high-resolution Raman spectra of the acetylene-butane co-crystal (PDF)

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Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The NASA Solar Systems Working (SSW) Program.

The authors declare no competing financial interest.

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Acknowledgments

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The authors gratefully acknowledge funding from the Solar System Workings Program. The authors also thank Jason Barnes and another anonymous reviewer for comments that helped improve this manuscript. This work was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.

ABBREVIATIONS
HCN	hydrogen cyanide
SAR	Synthetic Aperture Radar
VIMS	Visual and Infrared Mapping Spectrometer

References

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Brown, R. H.; Soderblom, L. A.; Soderblom, J. M.; Clark, R. N.; Jaumann, R.; Barnes, J. W.; Sotin, C.; Buratti, B.; Baines, K. H.; Nicholson, P. D.
Nature (London, United Kingdom) (2008), 454 (7204), 607-610CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
Titan was once thought to have global oceans of light hydrocarbons on its surface, but after 40 close flybys of Titan by the Cassini spacecraft, it has become clear that no such oceans exist. There are, however, features similar to terrestrial lakes and seas, and widespread evidence for fluvial erosion, presumably driven by pptn. of liq. methane from Titan's dense, nitrogen-dominated atm. The authors report IR spectroscopic data, obtained by the Visual and IR Mapping Spectrometer (VIMS) on board the Cassini spacecraft, that strongly indicate that ethane, probably in liq. soln. with methane, nitrogen and other low-mol.-mass hydrocarbons, is contained within Titan's Ontario Lacus, a lake-like feature.
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3
Cordier, D.; Mousis, O.; Lunine, J. I.; Lavvas, P.; Vuitton, V. An Estimate of the Chemical Composition of Titan’s Lakes. Astrophys. J. 2009, 707, L128– L131,
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An estimate of the chemical composition of Titan's lakes
Cordier, Daniel; Mousis, Olivier; Lunine, Jonathan I.; Lavvas, Panayotis; Vuitton, Veronique
Astrophysical Journal (2009), 707 (2, Pt. 2), L128-L131CODEN: ASJOAB; ISSN:0004-637X. (Institute of Physics Publishing)
Hundreds of radar-dark patches interpreted as lakes have been discovered in the north and south polar regions of Titan. We have estd. the compn. of these lakes by using the direct abundance measurements from the Gas Chromatograph Mass Spectrometer aboard the Huygens probe and recent photochem. models based on the vertical temp. profile derived by the Huygens Atm. Structure Instrument. Thermodn. equil. is assumed between the atm. and the lakes, which are also considered nonideal solns. We find that the main constituents of the lakes are ethane (C2H6) (∼76%-79%), propane (C3H8) (∼7%-8%), methane (CH4) (∼5%-10%), hydrogen cyanide (HCN) (∼2%-3%), butene (C4H8) (∼1%), butane (C4H10) (∼1%), and acetylene (C2H2) (∼1%). The calcd. compn. of lakes is then substantially different from what has been expected from models elaborated prior to the exploration of Titan by the Cassini-Huygens spacecraft.
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4
Mastrogiuseppe, M.; Poggiali, V.; Hayes, A. G.; Lunine, J. I.; Seu, R.; Mitri, G.; Lorenz, R. D. Deep and methane-rich lakes on Titan. Nat. Astron. 2019, 535,
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Barnes, J. W.; Bow, J.; Schwartz, J.; Brown, R. H.; Soderblom, J. M.; Hayes, A. G.; Vixie, G.; Le Mouélic, S.; Rodriguez, S.; Sotin, C.; Jaumann, R.; Stephan, K.; Soderblom, L. A.; Clark, R. N.; Buratti, B. J.; Baines, K. H.; Nicholson, P. D. Organic sedimentary deposits in Titan’s dry lakebeds: Probable evaporite. Icarus 2011, 216, 136– 140,
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Organic sedimentary deposits in Titan's dry lakebeds: Probable evaporite
Barnes, Jason W.; Bow, Jacob; Schwartz, Jacob; Brown, Robert H.; Soderblom, Jason M.; Hayes, Alexander G.; Vixie, Graham; Le Mouelic, Stephane; Rodriguez, Sebastien; Sotin, Christophe; Jaumann, Ralf; Stephan, Katrin; Soderblom, Laurence A.; Clark, Roger N.; Buratti, Bonnie J.; Baines, Kevin H.; Nicholson, Philip D.
Icarus (2011), 216 (1), 136-140CODEN: ICRSA5; ISSN:0019-1035. (Elsevier B.V.)
We report the discovery of org. sedimentary deposits at the bottom of dry lakebeds near Titan's north pole in observations from the Cassini Visual and IR Mapping Spectrometer (VIMS). We show evidence that the deposits are evaporitic, making Titan just the third known planetary body with evaporitic processes after Earth and Mars, and is the first that uses a solvent other than water.
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6
Barnes, J. W.; Brown, R. H.; Soderblom, J. M.; Soderblom, L. A.; Jaumann, R.; Jackson, B.; Le Mouélic, S.; Sotin, C.; Buratti, B. J.; Pitman, K. M.; Baines, K. H.; Clark, R. N.; Nicholson, P. D.; Turtle, E. P.; Perry, J. Shoreline features of Titan’s Ontario Lacus from Cassini/VIMS observations. Icarus 2009, 201, 217– 225,
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MacKenzie, S. M.; Barnes, J. W.; Sotin, C.; Soderblom, J. M.; Le Mouélic, S.; Rodriquez, S.; Baines, K. H.; Buratti, B. J.; Clark, R. N.; Nicholson, P. D.; McCord, T. B. Evidence of Titan’s climate history from evaporite distribution. Icarus 2014, 243, 191– 207,
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Lara, L. M.; Lellouch, E.; López-Moreno, J. J.; Rodrigo, R. Vertical distribution of Titan’s atmospheric neutral constituents. J. Geophys. Res. 1996, 101, 23261– 23283,
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Vertical distribution of Titan's atmospheric neutral constituents
Lara, L. M.; Lellouch, E.; Lopez-Moreno, J. J.; Rodrigo, R.
Journal of Geophysical Research, [Planets] (1996), 101 (E10), 23261-23283CODEN: JGPLEH; ISSN:1934-8592. (American Geophysical Union)
The vertical distribution of Titan's neutral atm. compds. is calcd. from a new photochem. model extending from 40 to 1432 km. This model makes use of many updated reaction rates, and of the new scheme for methane photolysis proposed by Mordaunt et al. [1993]. The model also includes a realistic treatment of the dissocn. of N2, of the deposition of water in the atm. from meteoritic ablation, and of condensation processes. The sensitivity of the results to the eddy diffusion coeff. profile is investigated. Fitting the methane thermospheric profile and the stratospheric abundance of the major hydrocarbons requires a methane stratospheric mixing ratio of 1.5-2% rather than 3%. Fitting the HCN stratospheric profile requires an eddy diffusion coeff. at 100-300 km that is 5-20 times larger than that necessary for the hydrocarbons. Most species are reasonably well reproduced, with the exception of CH3C2H and HC3N. The formation of CH3CN may involve the reaction of CN with either CH4 or (preferably) C2H6. The obsd. CO2 profile can be modeled by assuming an external source of water of ∼6 x 106 cm-2 s-1. For a nominal CO mixing ratio of 5 x 10-5, the chem. loss of CO exceeds its prodn. by ∼15%, and equil. is achieved for CO = 1 x 10-5.
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9
Lavvas, P. P.; Coustenis, A.; Vardavas, I. M. Coupling photochemistry with haze formation in Titan’s atmosphere, Part II: Results and validation with Cassini/Huygens data. Planet. Space Sci. 2008, 56, 67– 99,
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Coupling photochemistry with haze formation in Titan's atmosphere, Part II: Results and validation with Cassini/Huygens data
Lavvas, P. P.; Coustenis, A.; Vardavas, I. M.
Planetary and Space Science (2008), 56 (1), 67-99CODEN: PLSSAE; ISSN:0032-0633. (Elsevier B.V.)
The new one-dimensional radiative-convective/photochem./microphys. model described in Part I is applied to the study of Titan's atm. processes that lead to haze formation. Our model generates the haze structure from the gaseous species photochem. Model results are presented for the species vertical concn. profiles, haze formation and its radiative properties, vertical temp./d. profiles and geometric albedo. These are validated against Cassini/Huygens observations and other ground-based and space-borne measurements. The model reproduces well most of the latest measurements from the Cassini/Huygens instruments for the chem. compn. of Titan's atm. and the vertical profiles of the obsd. species. For the haze prodn. we have included pathways that are based on pure hydrocarbons, pure nitriles and hydrocarbon/nitrile copolymers. From these, the nitrile and copolymer pathways provide the stronger contribution, in agreement with the results from the ACP instrument, which support the incorporation of nitrogen in the pyrolized haze structures. Our haze model reveals a new second major peak in the vertical profile of haze prodn. rate between 500 and 900 km. This peak is produced by the copolymer family used and has important ramifications for the vertical atm. temp. profile and geometric albedo. In particular, the existence of this second peak dets. the vertical profile of haze extinction. Our model results have been compared with the DISR retrieved haze extinction profiles and are found to be in very good agreement. We have also incorporated in our model heterogeneous chem. on the haze particles that converts at. hydrogen to mol. hydrogen. The resultant H2 profile is closer to the INMS measurements, while the vertical profile of the diacetylene formed is found to be closer to that of the CIRS profile when this heterogeneous chem. is included.
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10
Lavvas, P. P.; Coustenis, A.; Vardavas, I. M. Coupling photochemistry with haze formation in Titan’s atmosphere, Part I: Model description. Planet. Space Sci. 2008, 56, 27– 66,
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Coupling photochemistry with haze formation in Titan's atmosphere, Part I: Model description
Lavvas, P. P.; Coustenis, A.; Vardavas, I. M.
Planetary and Space Science (2008), 56 (1), 27-66CODEN: PLSSAE; ISSN:0032-0633. (Elsevier B.V.)
We introduce a new 1D coupled Radiative/Convective-Photochem.-Microphys. model for a planetary atm. and apply it to Titan. The model incorporates detailed radiation transfer calcns. for the description of the shortwave and long wave fluxes which provide the vertical structure of the radiation field and temp. profile. These are used for the generation of the photochem. inside the atm. from the photolysis of Titan's main constituents, nitrogen (N2) and methane (CH4). The resulting hydrocarbons and nitriles are used for the prodn. of the haze precursors, whose evolution is described by the microphys. part of the model. The calcd. aerosol and gas opacities are iteratively included in the radiation transfer calcns. in order to investigate their effect on the resulting temp. profile and geometric albedo. The main purpose of this model is to help in the understanding of the missing link between the gas prodn. and particle transformation in Titan's atm. In this part, the basic phys. mechanisms included in the model are described. The final results regarding the eddy mixing profile, the chem. compn. and the role of the different haze precursors suggested in the literature are presented in Part II along with the sensitivity of the results to the mol. nitrogen photoinization scheme and the impact of galactic cosmic rays in the atm. chem.
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11
Krasnopolsky, V. A. A photochemical model of Titan’s atmosphere and ionosphere. Icarus 2009, 201, 226– 256,
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11
A photochemical model of Titan's atmosphere and ionosphere
Krasnopolsky, Vladimir A.
Icarus (2009), 201 (1), 226-256CODEN: ICRSA5; ISSN:0019-1035. (Elsevier B.V.)
A global-mean model of coupled neutral and ion chem. on Titan has been developed. Unlike the previous coupled models, the model involves ambipolar diffusion and escape of ions, hydrodynamic escape of light species, and calcs. the H2 and CO densities near the surface that were assigned in some previous models. We tried to reduce the nos. of species and reactions in the model and remove all species and reactions that weakly affect the obsd. species. Hydrocarbon chem. is extended to C12H10 for neutrals and C10H+11 for ions but does not include PAHs. The model involves 415 reactions of 83 neutrals and 33 ions, effects of magnetospheric electrons, protons, and cosmic rays. UV absorption by Titan's haze was calcd. using the Huygens observations and a code for the aggregate particles. Hydrocarbon, nitrile, and ion chemistries are strongly coupled on Titan, and attempt to calc. them sep. (e.g., in models of ionospheric compn.) may result in significant error. The model densities of various species are typically in good agreement with the observations except vertical profiles in the stratosphere that are steeper than the CIRS limb data. (A model with eddy diffusion that facilitates fitting to the CIRS limb data is considered as well.) The CO densities are supported by the O+ flux from Saturn's magnetosphere. The ionosphere includes a peak at 80 km formed by the cosmic rays, steplike layers at 500-700 and 700-900 km and a peak at 1060 km (SZA = 60°). Nighttime densities of major ions agree with the INMS data. Ion chem. dominates in the prodn. of bicyclic arom. hydrocarbons above 600 km. The model ests. of heavy pos. and neg. ions are in reasonable agreement with the Cassini results. The major haze prodn. is in the reactions C6H + C4H2, C3N + C4H2, and condensation of hydrocarbons below 100 km. Overall, pptn. rate of the photochem. products is equal to 4-7 kg cm-2 Byr-1 (50-90 m Byr-1 while the global-mean depth of the org. sediments is ∼3 m). Escape rates of methane and hydrogen are 2.9 and 1.4 kg cm-2 Byr-1, resp. The model does not support the low C/N ratio obsd. by the Huygens ACP in Titan's haze.
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12
Krasnopolsky, V. A. Chemical composition of Titan’s atmosphere and ionosphere: Observations and the photochemical model. Icarus 2014, 236, 83– 91,
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Chemical composition of Titan's atmosphere and ionosphere: Observations and the photochemical model
Krasnopolsky, Vladimir A.
Icarus (2014), 236 (), 83-91CODEN: ICRSA5; ISSN:0019-1035. (Elsevier Inc.)
Basic observational data on hydrocarbons, nitriles, and ions on Titan are compared with predictions of the photochem. model. Uncertainties of the obsd. abundances and differences between the data from different instruments and observing teams are comparable with the differences between the observations and the model results. Main reactions of prodn. and loss for each species are quant. assessed and briefly discussed. Formation of haze by polymn. of hydrocarbons and nitriles and recombination of heavy ions is calcd. along with condensation of various species near the tropopause. Overall deposition is a layer of 300 m thick for the age of the Solar System, and nitrogen constitutes 8% of the deposition. The model reproduces the basic observational data and adequately describes basic chem. processes in Titan's atm. and ionosphere. The presented model results and the observational data may be used as a ref. to chem. compn. of Titan's atm. and ionosphere.
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13
Gupta, S.; Ochiai, E.; Ponnamperuma, C. Organic synthesis in the atmosphere of Titan. Nature 1981, 293, 725– 727,
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Organic synthesis in the atmosphere of Titan
Gupta, S.; Ochiai, E.; Ponnamperuma, C.
Nature (London, United Kingdom) (1981), 293 (5835), 725-7CODEN: NATUAS; ISSN:0028-0836.
An explanation of the formation of org. compds. identified by Voyager in the atm. of Titan was attempted using results from particle and wave irradns. of an N2-CH4 gas mixt. The products formed by elec. discharge were also examd.
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14
Coustenis, A.; Achterberg, R. K.; Conrath, B. J.; Jennings, D. E.; Marten, A.; Gautier, D.; Nixon, C. A.; Flasar, F. M.; Teanby, N. A.; Bézard, B.; Samuelson, R. E.; Carlson, R. C.; Lellouch, E.; Bjoraker, G. L.; Romani, P. N.; Taylor, F. W.; Irwin, P. G. J.; Fouchet, T.; Hubert, A.; Orton, G. S.; Kunde, V. G.; Vinatier, S.; Mondellini, J.; Abbas, M. M.; Courtin, R. The composition of Titan’s stratosphere from Cassini/CIRS mid-infrared spectra. Icarus 2007, 189, 35– 62,
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The composition of Titan's stratosphere from Cassini/CIRS mid-infrared spectra
Coustenis, Athena; Achterberg, Richard K.; Conrath, Barney J.; Jennings, Donald E.; Marten, Andre; Gautier, Daniel; Nixon, Conor A.; Flasar, F. Michael; Teanby, Nick A.; Bezard, Bruno; Samuelson, Robert E.; Carlson, Ronald C.; Lellouch, Emmanuel; Bjoraker, Gordon L.; Romani, Paul N.; Taylor, Fred W.; Irwin, Patrick G. J.; Fouchet, Thierry; Hubert, Augustin; Orton, Glenn S.; Kunde, Virgil G.; Vinatier, Sandrine; Mondellini, Jacqueline; Abbas, Mian M.; Courtin, Regis
Icarus (2007), 189 (1), 35-62CODEN: ICRSA5; ISSN:0019-1035. (Elsevier)
An. anal. was done of data recorded by the Composite IR Spectrometer (CIRS) aboard the Cassini spacecraft during the Titan flybys T0-T10 (July 2004-Jan. 2006). The spectra characterize various regions on Titan from 70° S to 70° N with a variety of emission angles. The mol. signatures obsd. in the mid-IR CIRS detector arrays (FP3 and FP4, covering roughly the 600-1500 cm-1 spectral range with apodized resolns. of 2.54 or 0.53 cm-1) were studied. The composite spectrum shows several mol. signatures: hydrocarbons, nitriles and CO2. A firm detection of benzene is provided by CIRS at levels of about 3.5 × 10-9 around 70° N. We have used temp. profiles retrieved from the inversion of the emission obsd. in the methane ν4 band at 1304 cm-1 and a line-by-line radiative transfer code to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere. Temp. profiles retrieved from the inversion of the emission obsd. in the methane ν4 band at 1304 cm-1 and a line-by-line radiative transfer code were used to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere. Temp. profiles retrieved from the inversion of the emission obsd. in the methane ν4 band at 1304 cm-1 and a line-by-line radiative transfer code were used to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere. No longitudinal variations were found for these gases. Little or no change is obsd. generally in their abundances from the south to the equator. On the other hand, meridional variations retrieved for these trace constituents from the equator to the North ranged from almost zero (no or very little meridional variations) for C2H2, C2H6, C3H8, C2H4 and CO2 to a significant enhancement at high northern (early winter) latitudes for HCN, HC3N, C4H2, C3H4 and C6H6. For the more important increases in the northern latitudes, the transition occurs roughly between 30 and 50 degrees north latitude, depending on the mol. Note however that the very high-northern latitude results from tours TB-T10 bear large uncertainties due to few available data and problems with latitude smearing effects. The obsd. variations are consistent with some, but not all, of the predictions from dynamical-photochem. models. Constraints are set on the vertical distribution of C2H2, found to be compatible with 2-D equatorial predictions by global circulation models. The D/H ratio in the methane on Titan has been detd. from the CH3D band at 1156 cm-1 and found to be 1.17+-00..2238 × 10-4 . Implications of this deuterium enrichment, with respect to the protosolar abundance on the origin of Titan, are discussed. Results are compared with values retrieved by Voyager IRIS observations taken in 1980, as well as with more recent (1997) disk-averaged IR Space Observatory (ISO) results and with the latest Cassini-Huygens inferences from other instruments in an attempt to better comprehend the phys. phenomena on Titan.
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15
Waite, J. H., Jr.; Niemann, H.; Yelle, R. V.; Kasprzak, W. T.; Cravens, T. E.; Luhmann, J. G.; McNutt, R. L.; Ip, W.-H.; Gell, D.; De La Haye, V.; Müller-Wordag, I.; Magee, B.; Borggren, N.; Ledvina, S.; Fletcher, G.; Walter, E.; Miller, R.; Scherer, S.; Thorpe, R.; Xu, J.; Block, B.; Arnett, K. Ion neutral mass spectrometer results from the first flyby of Titan. Science 2005, 308, 982– 986,
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Ion neutral mass spectrometer results from the first flyby of Titan
Waite, J. Hunter, Jr.; Niemann, Hasso; Yelle, Roger V.; Kasprzak, Wayne T.; Cravens, Thomas E.; Luhmann, Janet G.; McNutt, Ralph L.; Ip, Wing-Huen; Gell, David; De La Haye, Virginie; Mueller-Wordag, Ingo; Magee, Brian; Borggren, Nathan; Ledvina, Steve; Fletcher, Greg; Walter, Erin; Miller, Ryan; Scherer, Stefan; Thorpe, Rob; Xu, Jing; Block, Bruce; Arnett, Ken
Science (Washington, DC, United States) (2005), 308 (5724), 982-986CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
The Cassini Ion Neutral Mass Spectrometer (INMS) has obtained the first in situ compn. measurements of the neutral densities of mol. nitrogen, methane, mol. hydrogen, argon, and a host of stable carbon-nitrile compds. in Titan's upper atm. INMS in situ mass spectrometry has also provided evidence for atm. waves in the upper atm. and the first direct measurements of isotopes of nitrogen, carbon, and argon, which reveal interesting clues about the evolution of the atm. The bulk compn. and thermal structure of the moon's upper atm. do not appear to have changed considerably since the Voyager 1 flyby.
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16
Niemann, H. B.; Atreya, S. K.; Demick, J. E.; Gautier, D.; Haberman, J. A.; Harpold, D. N.; Kasprzak, W. T.; Lunine, J. I.; Owen, T. C.; Raulin, F. Composition of Titan’s lower atmosphere and simple surface volatiles as measured by the Cassini-Huygens probe gas chromatograph mass spectrometer experiment. J. Geophys. Res. 2010, 115, E12006,
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Singh, S.; McCord, T. B.; Combe, J.-P.; Rodriguez, S.; Cornet, T.; Le Mouélic, S.; Clark, R. N.; Maltagliati, L.; Chevrier, V. F. Acetylene on Titan’s Surface. Astrophys. J. 2016, 828, 1– 8,
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Bottger, G. L.; Eggers, D. F., Jr. Infrared spectra of crystalline C₂H₂, C₂HD, and C₂D₂. J. Chem. Phys. 1964, 40, 2010– 2017,
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Infrared spectra of crystalline C2H2, C2HD, and C2D2
Bottger, G.L.; Eggers, D. F., Jr.
Journal of Chemical Physics (1964), 40 (7), 2010-17CODEN: JCPSA6; ISSN:0021-9606.
The infrared absorption spectra of polycryst. films of C2H2, C2HD, and C2D2 were examd. at 63 °K. and 4500 and 450 cm.-1 The spectra of the solid state and the gas phase of the various acetylenes are compared and the appropriate vibrational assignments are made. Several absorptions attributable to combinations of lattice modes and mol. fundamentals were found. The observed multiplet structure of the fundamental vibrations indicates that in the low-temp. modification of C2H2 and C2D2 the mols. are located at sites of C2h symmetry. Spectra of various solid solns. composed of mixts. of isotopic species were obtained and their significance discussed. The effect of isotopic substitution on the splittings in the ν5 region led to the conclusion that the dipole-dipole coupling model is sufficient to explain the observed spectra.
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19
van Nes, G. J. H.; van Bolhuis, F. Single-crystal structures and electron density distributions of ethane, ethylene and acetylene. II. Single-crystal X-ray structure determination of acetylene at 141 K. Acta Cryst. 1979, 35, 2580– 2593,
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Single-crystal structures and electron density distributions of ethane, ethylene and acetylene. II. Single-crystal x-ray structure determination of acetylene at 141 K
Van Nes, Gerard J. H.; Van Bolhuis, Fre
Acta Crystallographica, Section B: Structural Crystallography and Crystal Chemistry (1979), 35 (11), 2580-93CODEN: ACBCAR; ISSN:0567-7408.
Cryst. acetylene at 141 K is cubic, space group Pa3, with a 6.091(3) Å; Z = 4. The intensities of all 164 independent reflections up to sin θ/λ = 0.80 Å-1 were measured accurately on a Nonius four-circle diffractometer. For higher sin θ/λ, very few significant reflection intensities exist due to the strong thermal motion. A valence anal., with multipole deformation terms and restricted radial functions centered on the atoms, was performed for the 164 independent reflections by least-squares refinement on I. Both ζ and SCF scattering factors were considered. H was constrained to C. The radial functions corresponding to the SCF scattering factors were modified by introduction of an isotropic extinction parameter and an addnl. isotropic temp. factor for H in the refinement. Rw(I;ζ) = 0.0176 And Rw(I;SCF) = 0.0170, both with 11 independent parameters. For the (modified) SCF at. scattering factors the agreement with theory is better than for the ζ scattering factors. No reliable value could be found for the scale factor.
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20
Sugawara, T.; Kanda, E. The Crystal Structure of Acetylene. I. Ser. A, Phys. Chem. Metall. 1952, 4, 607– 614
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Nixon, C. A.; Jennings, D. E.; Flaud, J.-M.; Bézard, B.; Teanby, N. A.; Irwin, P. G. J.; Ansty, T. M.; Coustenis, A.; Vinatier, S.; Flasar, F. M. Titan’s prolific propane: The Cassini CIRS perspective. Planet. Space Sci. 2009, 57, 1573– 1585,
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Titan's prolific propane: The Cassini CIRS perspective
Nixon, C. A.; Jennings, D. E.; Flaud, J.-M.; Bezard, B.; Teanby, N. A.; Irwin, P. G. J.; Ansty, T. M.; Coustenis, A.; Vinatier, S.; Flasar, F. M.
Planetary and Space Science (2009), 57 (13), 1573-1585CODEN: PLSSAE; ISSN:0032-0633. (Elsevier B.V.)
Although propane gas (C3H8) was first detected in the stratosphere of Titan by the Voyager IRIS IR spectrometer in 1980, obtaining an accurate measurement of its abundance has proved difficult. All existing measurements have been made by modeling the ν26 band at 748 cm-1: however, different analyzes over time have yielded quite different results, and it also suffers from confusion with the strong nearby ν5 band of acetylene. In this paper we select large spectral avs. of data from the Cassini Composite IR Spectrometer (CIRS) obtained in limb-viewing mode at low latitudes (30° S - 30° N), greatly increasing the path length and hence signal-to-noise ratio for optically thin trace species such as propane. By modeling and subtracting the emissions of other gas species, we demonstrate that at least six IR bands of propane are detected by CIRS, including two not previously identified in Titan spectra. Using a new linelist for the range 1300-1400 cm - 1 , along with an existing GEISA list, we retrieve propane abundances from two bands at 748 and 1376 cm-1. At 748 cm-1 we retrieve 4.2 ± 0.5 × 10-7 (1 - σ error) at 2 mbar, in good agreement with previous studies, although lack of hotbands in the present spectral atlas remains a problem. We also det. 5.7 ± 0.8 × 10-7 at 2 mbar from the 1376 cm-1 band -a value that is probably affected by systematic errors including continuum gradients due to haze and also an imperfect model of the ν6 band of ethane. This study clearly shows for the first time the ubiquity of propane's emission bands across the thermal IR spectrum of Titan, and points to an urgent need for further lab. spectroscopy work, both to provide the line positions and intensities needed to model these bands, and also to further characterize haze spectral opacity. The present lack of accurate modeling capability for propane is an impediment not only for the measurement of propane itself, but also for the search for the emissions of new mols. in many spectral regions.
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22
Coll, P.; Coscia, D.; Smith, N.; Gazeau, M.-C.; Ramïrez, S. I.; Cernogora, G.; Israël, G.; Raulin, F. Experimental laboratory simulation of Titan’s atmosphere: Aerosols and gas phase. Planet. Space Sci. 1999, 47, 1331– 1340,
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Experimental laboratory simulation of Titan's atmosphere: Aerosols and gas phase
Coll, P.; Coscia, D.; Smith, N.; Gazeau, M.-C.; Ramirez, S. I.; Cernogora, G.; Israel, G.; Raulin, F.
Planetary and Space Science (1999), 47 (10/11), 1331-1340CODEN: PLSSAE; ISSN:0032-0633. (Elsevier Science Ltd.)
The discovery that Titan, the largest satellite of Saturn, has an atm. and that methane is a significant constituent of it, was the starting point for a systematic study of Titan's atm. org. chem. Since then, the results from numerous ground-based observations and two flybys of Titan, by Voyager I and II, have led to exptl. lab. simulation studies and photochem. and phys. modeling. All these works have provided a more detailed picture of Titan. The authors report a continuation of such a study performing an exptl. lab. simulation of Titan's atm. chem., and considering the two phys. phases involved: gases and aerosols. Concerning the gaseous phase, the first detection of C4N2 is reported, and possible atm. abundances are proposed for 70 org. compds. on Titan's upper atm. Concerning the solid phase, aerosol analogs were synthesized under conditions close to those of Titan's environment, using elemental anal., pyrolysis, soly. studies and IR spectroscopy.
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23
Tran, B. N.; Joseph, J. C.; Force, M.; Briggs, R. G.; Vuitton, V.; Ferris, J. P. Photochemical processes on Titan: Irradiation of mixtures of gases that simulate Titan’s atmosphere. Icarus 2005, 177, 106– 115,
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24
Laufer, A. H.; Gardner, E. P.; Kwok, T. L.; Yung, Y. L. Computations and estimates of rate coefficients for hydrocarbon reactions of interest to the atmospheres of the outer solar system. Icarus 1983, 56, 560– 567,
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Computations and estimates of rate coefficients for hydrocarbon reactions of interest to the atmospheres of the outer solar system
Laufer, A. H.; Gardner, E. P.; Kwok, T. L.; Yung, Y. L.
Icarus (1983), 56 (3), 560-7CODEN: ICRSA5; ISSN:0019-1035.
The rate coeffs., including Arrhenius parameters, were computed for a no. of chem. reactions involving hydrocarbon species for which exptl. data are not available and which are important in planetary atm. models. The techniques used to calc. the kinetic parameters include the Troe and semi-empirical bond energy-bond order or bond strength-bond length methods.
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Dobrijevic, M.; Dutour, I. The distribution of hydrocarbons in Titan’s atmosphere: An evolutionary algorithm-based model. Planet. Space Sci. 2007, 55, 2128– 2136,
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The distribution of hydrocarbons in Titan's atmosphere: An evolutionary algorithm-based model
Dobrijevic, M.; Dutour, I.
Planetary and Space Science (2007), 55 (14), 2128-2136CODEN: PLSSAE; ISSN:0032-0633. (Elsevier B.V.)
We propose a new approach to study the chem. complexity of Titan's atm. We have developed an evolutionary algorithm-based model that simulates the evolution of interacting elements with different valences. This abstr. model mimics a C-H-O-N system that might get an insight into the general properties of the chem. of Titan's atm. Comparison with detailed models like photochem. models is discussed to evaluate limitations and benefits of each approach. Comparison with observations suggests that Titan's atm. might self-organize to produce hydrocarbons with distributions that follow a power-law relation. If confirmed, this property makes possible some prediction about the abundance of heavy hydrocarbons in the atm. of Titan.
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26
Refson, K.; Pawley, G. S. The structure and orientational disorder in solid n-butane by neutron powder diffraction. Acta Cryst. B 1986, 42, 402– 410,
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Cordier, D.; Barnes, J. W.; Ferreira, A. G. On the chemical composition of Titan’s dry lakebed evaporites. Icarus 2013, 226, 1431– 1437,
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On the chemical composition of Titan's dry lakebed evaporites
Cordier, D.; Barnes, J. W.; Ferreira, A. G.
Icarus (2013), 226 (2), 1431-1437CODEN: ICRSA5; ISSN:0019-1035. (Elsevier Inc.)
Titan, the main satellite of Saturn, has an active cycle of methane in its troposphere. Among other evidence for a mechanism of evapn. at work on the ground, dry lakebeds have been discovered. Recent Cassini IR observations of these empty lakes have revealed a surface compn. poor in water ice compared to that of the surrounding terrains-suggesting the existence of org. evaporites deposits. The chem. compn. of these possible evaporites is unknown. In this paper, we study evaporite compn. using a model that treats both org. solids dissoln. and solvent evapn. Our results suggest the possibility of large abundances of butane and acetylene in the lake evaporites. However, due to uncertainties of the employed theory, these detns. have to be confirmed by lab. expts.
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Raulin, F. Organic Chemistry in the Oceans of Titan. Adv. Space Res. 1987, 7, 71– 81,
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Organic chemistry in the oceans of Titan
Raulin, F.
Advances in Space Research (1987), 7 (5), 71-81CODEN: ASRSDW; ISSN:0273-1177.
On Titan, most of the org. compds. present in the atm. must condense in the lower stratosphere and be solid near the surface, except CH4, C2H6, C3H8, C3H6, and 1-butene, which must be liq. and could form oceans contg. large fractions of dissolved N2. The solid org. compds., depending on their d. relatively to this liq., could accumulate at the surface or at the bottom of the oceans; they could also dissolve partly or totally in the oceans. . Chem. evolution on Titan involved phys. chem. processes in a cryogenic apolar solvent mainly composed of CH4-C2H6-N2. Systematic study of the vol. mass and soly. of the org. compds. in such a cryogenic mixt. of various compns. was made for 94 K by thermodn. modeling. The oceans of Titan could be free of icebergs of org. compds. These oceans could be very rich in dissolved org. compds., with relatively high concns. (∼1-10-6 M). In addn., the concn. of several of the org. solutes should be const., buffered by a bottom layer of the corresponding compd. in the solid phase.
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29
Glein, C. R.; Shock, E. L. A geochemical model of non-ideal solutions in the methane-ethane-propane-nitrogen-acetylene system on Titan. Geochim. Cosmochim. Acta 2013, 115, 217– 240,
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A geochemical model of non-ideal solutions in the methane-ethane-propane-nitrogen-acetylene system on Titan
Glein, Christopher R.; Shock, Everett L.
Geochimica et Cosmochimica Acta (2013), 115 (), 217-240CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
Saturn's largest moon, Titan, has an atm. and surface that are rich in org. compds. Liq. hydrocarbons exist on the surface, most famously as lakes. Photochem. reactions produce solid orgs. in Titan's atm., and these materials settle or snow onto the surface. At the surface, liqs. can interact with solids, and geochem. processes can occur. The consequences of these processes can be explored using a thermodn. model to calc. the solubilities of gases and solids in liq. hydrocarbons at cryogenic temps. The van Laar model developed in this study was parameterized using exptl. phase equil. data, and accurately represents the data for the CH4-C2H6-C3H8-N2-C2H2 chem. system from 90 to 110 K. The model generally gives more accurate results than existing models. The model also features a suitable balance between accuracy and simplicity, and can serve as a foundation for studies of fluvial geochem. on Titan because it can be extended to any no. of components while maintaining thermodn. consistency. Application of the model to Titan reveals that the equil. compn. of surface liqs. depends on the abundance of methane gas in the local atm., consistent with prior studies. The concn. of mol. nitrogen in Titan's lakes varies inversely with the ethane content of the lakes. The model indicates that solid acetylene should be quite sol. in surface liqs., which implies that acetylene-rich sedimentary rocks would be susceptible to chem. erosion, and acetylene evaporites may form on Titan. The geochem. character of acetylene in liq. hydrocarbons on Titan appears to be intermediate to those of calcite and gypsum in surface waters on Earth. Specific recommendations are given of observational, exptl., and theor. work that will lead to significant advancements in our knowledge of geochem. processes on Titan. This paper represents the beginning of a new kind of geochem., called cryogenic fluvial geochem., with Titan starring as the first example.
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30
Cornet, T.; Cordier, D.; Le Bahers, T.; Bourgeois, O.; Fleurant, C.; Le Mouélic, S.; Altobelli, N. Dissolution on Titan and on Earth: Toward the age of Titan’s karstic landscapes. J. Geophys. Res. Planets 2015, 120, 1044– 1074,
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Dissolution on Titan and on Earth: Toward the age of Titan's karstic landscapes
Cornet, Thomas; Cordier, Daniel; Le Bahers, Tangui; Bourgeois, Olivier; Fleurant, Cyril; Le Mouelic, Stephane; Altobelli, Nicolas
Journal of Geophysical Research: Planets (2015), 120 (6), 1044-1074CODEN: JGPLEH; ISSN:2169-9100. (Wiley-Blackwell)
Titan's polar surface is dotted with hundreds of lacustrine depressions. Based on the hypothesis that they are karstic in origin, we aim at detg. the efficiency of surface dissoln. as a landshaping process on Titan, in a comparative planetol. perspective with the Earth as ref. Our approach is based on the calcn. of solutional denudation rates and allow inference of formation timescales for topog. depressions developed by chem. erosion on both planetary bodies. The model depends on the soly. of solids in liqs., the d. of solids and liqs., and the av. annual net rainfall rates. We compute and compare the denudation rates of pure solid orgs. in liq. hydrocarbons and of minerals in liq. water over Titan and Earth timescales. We then investigate the denudation rates of a superficial org. layer in liq. methane over one Titan year. At this timescale, such a layer on Titan would behave like salts or carbonates on Earth depending on its compn., which means that dissoln. processes would likely occur but would be 30 times slower on Titan compared to the Earth due to the seasonality of pptn. Assuming an av. depth of 100 m for Titan's lacustrine depressions, these could have developed in a few tens of millions of years at polar latitudes higher than 70°N and S, and a few hundreds of million years at lower polar latitudes. The ages detd. are consistent with the youth of the surface (<1 Gyr) and the repartition of dissoln.-related landforms on Titan.
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31
Singh, S.; Combe, J.-P.; Cordier, D.; Wagner, A.; Chevrier, V. F.; McMahon, Z. Experimental determination of acetylene and ethylene solubility in liquid methane and ethane: Implications to Titan’s surface. Geochim. Cosmochim. Acta 2017, 208, 86– 101,
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31
Experimental determination of acetylene and ethylene solubility in liquid methane and ethane: Implications to Titan's surface
Singh, S.; Combe, J.-Ph.; Cordier, D.; Wagner, A.; Chevrier, V. F.; McMahon, Z.
Geochimica et Cosmochimica Acta (2017), 208 (), 86-101CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
In this study, the soly. of acetylene (or ethyne, C2H2) and ethylene (or ethene, C2H4) in liq. methane (CH4) and ethane (C2H6) has been exptl. detd. at Titan surface temp. (90 K) and pressure (1.5 bars). As predicted by theor. models, the solubilities of acetylene and ethylene are very large at Titan temp. and these species are most likely to be abundantly present in the lakes and as evaporites on the shores or dry lake beds. Our results indicate the soly. of 4.9 × 10-2 mole fraction for acetylene in methane and 48 × 10-2 mole fraction in ethane; for ethylene, 5.6 × 10-1 mole fraction in methane and 4.8 × 10-1 mole fraction in ethane. Assuming the mole fractions from atm. models in the lower stratosphere and equil. with the surface, we detd. that the lakes on Titan that cover ∼400,000 km2 are not satd. The liq. lakes on Titan act as an important reservoir for both acetylene and ethylene. Assuming difference of methane and ethane content in the lakes at different latitudes, the difference in soly. in liq. methane and ethane, solutes in lakes may change with the temporal evolution (such as; evapn. and condensation) over seasons and geol. time scales.
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32
Lorenz, R. D. The flushing of Ligeia: Composition variations across Titan’s seas in a simple hydrological model. Geophys. Res. Lett. 2014, 41, 5764– 5770,
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32
The flushing of Ligeia: Composition variations across Titan's seas in a simple hydrological model
Lorenz, Ralph D.
Geophysical Research Letters (2014), 41 (16), 5764-5770CODEN: GPRLAJ; ISSN:1944-8007. (Wiley-Blackwell)
We use a simple box model to explore possible differences in the liq. compn. of Titan's seas. Major variations in the abundance of involatile ethane, somewhat analogous to salinity in terrestrial waters, arise from the hydrol. cycle, which introduces more "fresh" methane rainfall at the highest latitudes in summer. The obsd. compn. of Ligeia Mare, flushed by methane rainfall and exporting its solutes to Kraken via a narrow labyrinth of channels, may have a methane-rich (>∼80%) compn., well out of thermodn. equil. with the atm., whereas the basins of Kraken are relatively well mixed and will have an ethane-dominated (∼60%) compn. These variations, analogous to Earth's salinity gradient between the Black Sea and the Mediterranean, may be detectable with Cassini measurements and are important for future exploration.
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Mitchell, K. L.; Barmatz, M. B.; Jamieson, C. S.; Lorenz, R. D.; Lunine, J. I. Laboratory measurements of cryogenic liquid alkane microwave absorptivity and implications for the composition of Ligeia Mare, Titan. Geophys. Res. Lett. 2015, 42, 1340– 1345,
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Laboratory measurements of cryogenic liquid alkane microwave absorptivity and implications for the composition of Ligeia Mare, Titan
Mitchell, Karl L.; Barmatz, Martin B.; Jamieson, Corey S.; Lorenz, Ralph D.; Lunine, Jonathan I.
Geophysical Research Letters (2015), 42 (5), 1340-1345CODEN: GPRLAJ; ISSN:1944-8007. (Wiley-Blackwell)
The complex dielec. consts. of liqs. methane and ethane were measured at 90 K and 14.1 GHz, close to the frequency of the Cassini RADAR. The liq. ethane loss tangent is far greater than that of liq. methane, facilitating discrimination by remote sensing. The results suggest a methane-dominated compn. for the northern sea, Ligeia Mare, on the basis of a recent loss tangent detn. using Cassini RADAR altimetry. This contrasts a previous far higher loss tangent for the southern lake, Ontario Lacus, which is inconsistent with simple mixts. of methane and ethane. The apparent nonequil. methane-to-ethane ratio of Ligeia Mare can be explained by poor admixt. of periodically cycled methane with a deeper ethane-rich alkanofer system, consistent with obliquity-driven volatile cycling, sequestration of ethane from the hydrocarbon cycle by incorporation into crustal clathrate hydrates, or periodic flushing of Ligeia Mare into adjacent Kraken Mare by fresh rainfall.
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34
Cordier, D.; Cornet, T.; Barnes, J. W.; MacKenzie, S. M.; Le Bahers, T.; Nna-Mvondo, D.; Rannou, P.; Ferreira, A. G. Structure of Titan’s evaporites. Icarus 2016, 270, 41– 56,
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34
Structure of Titan's evaporites
Cordier, D.; Cornet, T.; Barnes, J. W.; MacKenzie, S. M.; Le Bahers, T.; Nna-Mvondo, D.; Rannou, P.; Ferreira, A. G.
Icarus (2016), 270 (), 41-56CODEN: ICRSA5; ISSN:0019-1035. (Elsevier Inc.)
Numerous geol. features that could be evaporitic in origin have been identified on the surface of Titan. Although they seem to be water-ice poor, their main properties - chem. compn., thickness, stratification - are essentially unknown. In this paper, which follows on a previous one focusing on the surface compn. Cordier, D., Barnes, J.W., Ferreira, A.G. [2013b]. Icarus 226 2,1431-1437, we provide some answers to these questions derived from a new model. This model, based on the up-to-date thermodn. theory known as "PC-SAFT", has been validated with available lab. measurements and specifically developed for our purpose. 1-D models confirm the possibility of an acetylene and/or butane enriched central layer of evaporitic deposit. The estd. thickness of this acetylene-butane layer could explain the strong RADAR brightness of the evaporites. The 2-D computations indicate an accumulation of poorly sol. species at the deposit's margin. Among these species, HCN or aerosols similar to tholins could play a dominant role. Our model predicts the existence of chem. trimodal "bathtub rings" which is consistent with what it is obsd. at the south polar lake Ontario Lacus. This work also provides plausible explanations to the lack of evaporites in the south polar region and to the high radar reflectivity of dry lakebeds.
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35
Vu, T. H.; Cable, M. L.; Choukroun, M.; Hodyss, R.; Beauchamp, P. Formation of a new benzene-ethane co-crystalline structure under cryogenic conditions. J. Phys. Chem. A 2014, 118, 4087– 4094,
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35
Formation of a New Benzene-Ethane Co-Crystalline Structure Under Cryogenic Conditions
Vu, Tuan Hoang; Cable, Morgan L.; Choukroun, Mathieu; Hodyss, Robert; Beauchamp, Patricia
Journal of Physical Chemistry A (2014), 118 (23), 4087-4094CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
We report the first exptl. finding of a solid mol. complex between benzene and ethane, two small apolar hydrocarbons, at atm. pressure and cryogenic temps. Considerable amts. of ethane are found to be incorporated inside the benzene lattice upon the addn. of liq. ethane onto solid benzene at 90-150 K, resulting in formation of a distinctive co-cryst. structure that can be detected via micro-Raman spectroscopy. Two new features characteristic of these co-crystals are obsd. in the Raman spectra at 2873 and 1455 cm-1, which are red-shifted by 12 cm-1 from the υ1 (a1g) and υ11 (eg) stretching modes of liq. ethane, resp. Anal. of benzene and ethane vibrational bands combined with quantum mech. modeling of isolated mol. dimers reveal an interaction between the arom. ring of benzene and the hydrogen atoms of ethane in a C-H···π fashion. The most favored configuration for the benzene-ethane dimer is the monodentate-contact structure, with a calcd. interaction energy of 9.33 kJ/mol and an equil. bonding distance of 2.66 Å. These parameters are comparable to those for a T-shaped co-cryst. complex between benzene and acetylene that has been previously reported in the literature. These results are relevant for understanding the hydrocarbon cycle of Titan, where benzene and similar orgs. may act as potential hydrocarbon reservoirs due to this incorporation mechanism.
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36
Cable, M. L.; Vu, T. H.; Hodyss, R.; Choukroun, M.; Malaska, M. J.; Beauchamp, P. Experimental determination of the kinetics of formation of the benzene-ethane co-crystal and implications for Titan. Geophys. Res. Lett. 2014, 41, 5396– 5401,
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36
Experimental determination of the kinetics of formation of the benzene-ethane co-crystal and implications for Titan
Cable, Morgan L.; Vu, Tuan H.; Hodyss, Robert; Choukroun, Mathieu; Malaska, Michael J.; Beauchamp, Patricia
Geophysical Research Letters (2014), 41 (15), 5396-5401CODEN: GPRLAJ; ISSN:1944-8007. (Wiley-Blackwell)
Benzene is found on Titan and is a likely constituent of the putative evaporite deposits formed around the hydrocarbon lakes. We have recently demonstrated the formation of a benzene-ethane co-crystal under Titan-like surface conditions. Here we investigate the kinetics of formation of this new structure as a function of temp. We show that the formation process would reach completion under Titan surface conditions in ∼18 h and that benzene ppts. from liq. ethane as the co-crystal. This suggests that benzene-rich evaporite basins around ethane/methane lakes and seas may not contain pure cryst. benzene, but instead benzene-ethane co-crystals. This co-cryst. form of benzene with ethane represents a new class of materials for Titan's surface, analogous to hydrated minerals on Earth. This new structure may also influence evaporite characteristics such as particle size, dissoln. rate, and IR spectral properties.
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Maynard-Casely, H. E.; Hodyss, R.; Cable, M. L.; Vu, T. H.; Rahm, M. A co-crystal between benzene and ethane: A potential evaporite material for Saturn’s moon Titan. Int. Union Cryst. J. 2016, 3, 192– 199,
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38
Cable, M. L.; Vu, T. H.; Maynard-Casely, H. E.; Choukroun, M.; Hodyss, R. The Acetylene-Ammonia Co-crystal on Titan. ACS Earth Space Chem. 2018, 2, 366– 375,
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38
The Acetylene-Ammonia Co-crystal on Titan
Cable, Morgan L.; Vu, Tuan H.; Maynard-Casely, Helen E.; Choukroun, Mathieu; Hodyss, Robert
ACS Earth and Space Chemistry (2018), 2 (4), 366-375CODEN: AESCCQ; ISSN:2472-3452. (American Chemical Society)
Titan, the largest moon of Saturn, likely supports a rich org. mineralogy that we are only beginning to understand. Photochem. in the upper atm. generates a complex array of org. mols. from the simple precursors N2 and CH4. These orgs. continue to react and combine, forming aerosol layers and ultimately depositing on the surface. Orgs. are transported via pluvial (rain) and fluvial (rivers/flooding) processes into the methane-based hydrocarbon lakes, where evapn. of volatile liqs. can create evaporite deposits of remnant dissolved mols. Within such deposits, chem. and phys. processes may be occurring even at low temps. We have demonstrated in previous work that benzene and ethane rapidly form a unique and stable co-crystal at Titan surface temps. (90-95 K), akin to a salt on Earth, where the weak van der Waals interactions in the benzene-ethane co-crystal are analogous to the ionic bonds in a salt. Here, we report the formation of a second co-crystal between acetylene and ammonia, which forms even more quickly and is stable through anticipated conditions of Titan "rain" events of liq. methane, ethane, and propane. Such co-crystals represent an exciting new class of possible minerals for Titan's surface and may be responsible for processes such as selective sequestration and storage of species as well as having new properties for construction and erosive resistance of geol. materials.
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Boese, R.; Bläser, D.; Jansen, G. Synthesis and theoretical characterization of an acetylene-ammonia cocrystal. J. Am. Chem. Soc. 2009, 131, 2104– 2106,
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39
Synthesis and Theoretical Characterization of an Acetylene-Ammonia Cocrystal
Boese, Roland; Blaser, Dieter; Jansen, Georg
Journal of the American Chemical Society (2009), 131 (6), 2104-2106CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
One of the smallest mol. cocrystals consisting of acetylene and ammonia is grown in situ on the X-ray diffractometer. The resulting 1:1 mol. cocrystal is composed of antiparallel layers of zigzag chains. Subunits of the crystal structure are already stable as small mol. aggregates as shown by ab initio calcns. These aggregates may be considered as infant stages of cocrystal formation.
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40
Maynard-Casely, H. E.; Cable, M. L.; Malaska, M. J.; Vu, T. H.; Choukroun, M.; Hodyss, R. Prospects for Mineralogy on Titan. Am. Mineral. 2018, 103, 343– 349,
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Vu, T. H.; Hodyss, R.; Cable, M. L.; Choukroun, M. Raman signatures and thermal expansivity of acetylene clathrate hydrate. J. Phys. Chem. A 2019, 123, 7051– 7056,
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41
Raman Signatures and Thermal Expansivity of Acetylene Clathrate Hydrate
Vu, Tuan H.; Hodyss, Robert; Cable, Morgan L.; Choukroun, Mathieu
Journal of Physical Chemistry A (2019), 123 (32), 7051-7056CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
The vibrational signatures for the υ2 C≡C and υ1 sym. C-H stretches of acetylene in cubic structure I clathrate, synthesized under ambient pressure, are reported for the 1st time. The most diagnostic features are at 1966 for υ2 and 3353 cm-1 for υ1, resp., and are assigned to acetylene trapped in the large 51262 cages. The υ2 mode for acetylene occupying the small 512 cages is obsd. at 1972.5 cm-1, a red shift of 1.5 cm-1 from its gas phase frequency. Unit cell parameters and thermal expansion coeffs. are detd. via powder x-ray diffraction between 195 and 225 K and are in good correlation with previous single crystal data at 143 K. The calcd. d. for acetylene clathrate is also reported, with values ranging from 0.985 g/cm3 at 195 K to 0.976 g/cm3 at 225 K. These results are relevant for spectral detection of acetylene-contg. compds. on planetary bodies, as well as providing addnl. insights on the thermal behavior and phys. properties of acetylene clathrate.
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Subramanian, S.; Sloan, E. D. Trends in vibrational frequencies of guests trapped in clathrate hydrate cages. J. Phys. Chem. B 2002, 106, 4348– 4355,
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Trends in Vibrational Frequencies of Guests Trapped in Clathrate Hydrate Cages
Subramanian, Sivakumar; Sloan, E. Dendy, Jr.
Journal of Physical Chemistry B (2002), 106 (17), 4348-4355CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
Raman spectra of ethane trapped in the small 512 cage of sII hydrate (at ∼70 MPa), isobutane trapped in the large 51264 cage of sII hydrate, and the gauche form of n-butane trapped in the large 51264 cage of sII hydrate were obtained for the first time. These new Raman results are combined with existing Raman and IR results for various guests to infer general trends in vibrational frequencies of guest mols. trapped in clathrate hydrate cages as a function of cage size, guest size, guest vibrational mode, and pressure. The obsd. trend in stretching frequencies of guests with cage size, which can be stated as "the larger the cavity, the lower the frequency", is explained through the qual. "loose cage-tight cage" model of Pimentel and Charles (Pure Appl. Chem. 1963, 7, 111).
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Gough, T. E.; Wang, T. Vibrational spectroscopy of cocrystallized carbon dioxide and acetylene. J. Chem. Phys. 1995, 102, 3932,
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Vibrational spectroscopy of cocrystallized carbon dioxide and acetylene
Gough, T. E.; Wang, T.
Journal of Chemical Physics (1995), 102 (10), 3932-7CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
Fourier transform IR spectra of acetylene and carbon dioxide cocondensed in a diffusive trapping cell are presented, and interpreted in terms of the stoichiometric phase CO2.(C2H2)2. Simulations show that this novel phase is formed when the degrees of supersatn. of the constituent gases are of comparable values.
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Ito, M.; Yokoyama, T.; Suzuki, M. Raman spectra of acetylene crystals I and II. Spectrochim. Acta, Part A 1970, 26, 695– 705,
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Raman spectra of acetylene crystals I and II
Ito, Mitsuo; Yokoyama, Toru; Suzuki, Masako
Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy (1970), 26 (3), 695-705CODEN: SAMCAS; ISSN:1386-1425.
Laser Raman spectra have been obsd. for 2 modifications of the C2H2 crystal. The high temp. modification (crystal I) showed 3 bands owing to lattice vibrations of rotational origin. A normal coordinate calcn. indicates that their frequencies are reasonably explained with a potential based upon elec. quadrupole interaction. The spectrum of the low temp. modification (crystal II) exhibited large crystal-field splittings of intramol. bands and several sharp bands owing to lattice vibrations. The obsd. spectrum and some information from the x-ray study suggest that crystal II has a layer-type orthorhombic structure belonging to space group D182[.
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45
Anderson, A.; Andrews, B.; Torrie, B. H. Raman and Far Infrared Spectra of Crystalline Acetylene, C2H2 and C2D2. J. Raman Spectrosc. 1985, 16, 202– 207,
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Raman and far infrared spectra of crystalline acetylene, C2H2 and C2D2
Anderson, A.; Andrews, B.; Torrie, B. H.
Journal of Raman Spectroscopy (1985), 16 (3), 202-7CODEN: JRSPAF; ISSN:0377-0486.
Raman and far IR spectra of the orthorhombic and cubic phases of C2H2 and C2D2 were obtained. In the orthorhombic phase, 3 of the 4 expected librations were identified in the Raman spectrum and the 2 IR active translations were confirmed, for both samples. In the cubic phase, 2 of the 3 predicted librations in the Raman spectrum and the expected pair of translations in the far IR spectrum were obsd., again for both samples. Splittings in the internal mode Raman spectra are, with one exception, in agreement with the predictions of group theory.
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Marchi, M.; Righini, R. Intermolecular potential and lattice dynamics of orthorhombic acetylene. Chem. Phys. 1985, 94, 465,
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Intermolecular potential and lattice dynamics of orthorhombic acetylene
Marchi, M.; Righini, R.
Chemical Physics (1985), 94 (3), 465-73CODEN: CMPHC2; ISSN:0301-0104.
The crystal structure and lattice dynamics of orthorhombic acetylene were calcd. with an intermol. potential consisting of atom-atom and multipole-multipole interactions and including a H bond. A new assignment of the Raman lattice vibrations is discussed and utilized in the refinement of the potential parameters. The non-transferability of the potential to the cubic phase is attributed to the breaking of the H bond at the phase transition, and to the large anharmonicity expected for the high-temp. phase.
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47
Aoki, K.; Kakudate, Y.; Usuba, S.; Yoshida, M.; Tanaka, K.; Fujiwara, S. High-pressure Raman study of liquid and crystalline C₂H₂. J. Chem. Phys. 1988, 88, 4565– 4568,
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High-pressure Raman study of liquid and crystalline acetylene
Aoki, K.; Kakudate, Y.; Usuba, S.; Yoshida, M.; Tanaka, K.; Fujiwara, S.
Journal of Chemical Physics (1988), 88 (8), 4565-8CODEN: JCPSA6; ISSN:0021-9606.
Raman spectra were measured for liq. (0-0.7 GPa), cryst. cubic (0.7-0.9 GPa), and orthorhombic (0.9-3.5 GPa) phases in C2H2 at room temp. For the orthorhombic phase, the high-pressure behavior of the librational and internal vibrations was obtained in a wide pressure range. The frequencies of all the librational modes increased monotonically with pressure, while those of the internal modes showed a variety of frequency shifts depending on vibrational mode. These high-pressure data will be of great use for the construction of theor. models for the intermol. interactions in cryst. C2H2.
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48
Kint, S.; Scherer, J. R.; Snyder, R. G. Raman spectra of liquid n-alkanes. III. Energy difference between trans and gauche n-butane. J. Chem. Phys. 1980, 73, 2599– 2602,
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Raman spectra of liquid n-alkanes. III. Energy difference between trans and gauche n-butane
Kint, S.; Scherer, J. R.; Snyder, R. G.
Journal of Chemical Physics (1980), 73 (6), 2599-602CODEN: JCPSA6; ISSN:0021-9606.
The energy difference between the trans and gauche forms of liq. n-butane was detd. from the temp. dependence of Raman band intensities. A value of 557 ± 13 cal/mol was derived from the summed integrated intensities of six bands of each rotamer measured at nine temps. between 0 and -127°. This value is considerably lower than those detd. earlier from Raman spectra of the liq. and is in line with ΔE values for longer n-alkanes in the liq. state. However, there is a large difference between the value reported here for liq. n-butane and that reported for the gas (∼966 cal/mol).
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Schwartz, Y. A.; Ron, A.; Kimel, S. Far-infrared spectrum and phase transition of acetylene. J. Chem. Phys. 1969, 51, 1666– 1667,
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Far-infrared spectrum and phase transition of acetylene
Schwartz, Yvonne Aviva; Ron, Arza; Kimel, Sol
Journal of Chemical Physics (1969), 51 (4), 1666-7CODEN: JCPSA6; ISSN:0021-9606.
The far-ir spectrum (40-200 cm.-1) of solid C2H2 was studied at various temps. in the vicinity of the 1st-order phase transition to cubic at 133°K. in a closed cell. Below the transition point, the crystals are orthorhombic and exhibited 2 sharp bands with max. at about 95 and 115°K. (frequency shifted -0.12 cm.-1/°K. due to crystal imperfections). On warming, halfwidths increased. On heating samples through the temp. of the phase transition, these bands disappeared and were replaced by a broad and weak absorption at about 110 cm.-1 The spectral changes were completely reversible. The disappearance of the distinct spectral features is due to the higher degree of freedom of mol. motions in the cubic (higher temp.) phase.
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Malaska, M. J.; Hodyss, R. Dissolution of benzene, naphthalene, and biphenyl in a simulated Titan lake. Icarus 2014, 242, 74– 81,
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Dissolution of benzene, naphthalene, and biphenyl in a simulated Titan lake
Malaska, Michael J.; Hodyss, Robert
Icarus (2014), 242 (), 74-81CODEN: ICRSA5; ISSN:0019-1035. (Elsevier Inc.)
We constructed a lab. app. capable of measuring the satn. equil. concn. (csat) and dissoln. rate consts. (keff) of org. solutes in ethane at 94 K. We detd. a csat of 18.5 ± 1.9 mg L-1, 0.159 ± 0.003 mg L-1, and 0.039 ± 0.006 mg L-1 for benzene, naphthalene, and biphenyl, resp. The derived csat and keff can be used to predict the dissoln. behavior of the materials in ethane under Titan conditions. The arom. materials dissolved relatively quickly in liq. ethane at 94 K, reaching satn. in less than 2 h. The dissoln. characteristics of benzene in ethane at 94 K are compared to those of terrestrial karst-forming materials in water at 298 K, and are used to constrain Titan surface processes. We discuss the implications of our measurements on the formation of karst on Titan, the concn. of orgs. in Titan's lakes, and the formation of evaporite deposits during lake evapn.
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51
Kirchner, M. T.; Bläser, D.; Boese, R. Co-crystals with acetylene: Small is not simple!. Chem. – Eur. J. 2010, 16, 2131– 2146,
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Co-crystals with Acetylene: Small Is not Simple!
Kirchner, Michael T.; Blaeser, Dieter; Boese, Roland
Chemistry - A European Journal (2010), 16 (7), 2131-2146CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
Acetylene is an amazingly versatile component for the formation of co-crystals. It requires careful handling and special techniques for crystn., but the efforts seem to be rewarding when attaining co-crystals with small mols. as partners. Many basic questions such as the dominance of specific heterogeneous intermol. interactions, their driving force for the formation of multicomponent crystals instead of neat ones are expected to be easily analyzed. The underlying packing patterns and resulting stoichiometries based on the known supramol. synthons seem to be straightforward for such small mols. and crystal engineering, considered as the prototype of supramol. synthesis, should be a simple task. Nineteen co-crystals with acetylene are presented some of which were previously reported individually. An attempt was made to find features shared by the groups of co-crystals, including those that could not be co-crystd. But in spite of clear ideas and experiences from previous expts., surprisingly almost none of systems reached expectations. Intuitive approach was not fulfilled, which demonstrates that multicomponent crystals even of small mols. will remain a great challenge for theor. methods and the crystal structures shown herein represent good candidates for future testing. However, the authors wish to encourage other groups to present their views on the crystal structures with an unbiased approach that may offer a better explanation than the authors are able to outline.
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52
Hayes, A. G.; Birch, S. P. D.; Dietrich, W. E.; Howard, A. D.; Kirk, R. L.; Poggiali, V.; Mastroguiseppe, M.; Michaelides, R. J.; Corlies, P. M.; Moore, J. M.; Malaska, M. J.; Mitchell, K. L.; Lorenz, R. D.; Wood, C. A. Topographic constraints on the evolution and connectivity of Titan’s lacustrine basins. Geophys. Res. Lett. 2017, 44, 11– 745,
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Malaska, M. J.; Radebaugh, J.; Lopes, R. M. C.; Mitchell, K. L.; Verlander, T.; Schoenfeld, A. M.; Florence, M. M.; Le Gall, A.; Solomonidou, A.; Hayes, A. G.; Birch, S. P. D.; Janssen, M. A.; Shurmeier, L.; Cornet, T.; Ahrens, C.; Farr, T. G.; Cassini RADAR Team. Labyrinth Terrain on Titan. Submitted to Icarus.
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Solomonidou, A.; Le Gall, A.; Malaska, M.; Wall, S.; Lopes, R. M. C.; Coustenis, A.; Rodriguez, S.; Elachi, C.; Drossart, P.; Nasr, M.; Matsoukas, M.; Soderblom, J.; Janssen, M.; Lawrence, K.; Altobelli, N.; Witasse, O.; Radebaugh, J.; Schoenfeld, A.; Cassini RADAR Team Spectral and emissivity analysis of the raised ramparts around Titan’s northern lakes. Icarus 2019,
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Birch, S. P. D.; Hayes, A. G.; Poggiali, V.; Hofgartner, J. D.; Lunine, J. I.; Malaska, M. J.; Wall, S.; Lopes, R. M. C.; White, O. Raised rims around Titan’s sharp-edged depressions. Geophys. Res. Lett. 2018, 45, 1– 9,
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Schink, B. Fermentation of acetylene by an obligate anaerobe,Pelobacter acetylenicus sp. nov.. Arch. Microbiol. 1985, 142, 295– 301,
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Fermentation of acetylene by an obligate anaerobe, Pelobacter acetylenicus sp. nov
Schink, Bernhard
Archives of Microbiology (1985), 142 (3), 295-301CODEN: AMICCW; ISSN:0302-8933.
Four strains of strictly anaerobic gram-neg. rod-shaped nonsporeforming bacteria were enriched and isolated from marine and freshwater sediments with acetylene (ethyne) as sole source of C and energy. Acetylene, acetoin, ethanolamine, choline, 1,2-propanediol, and glycerol were the only substrates utilized for growth, the latter 2 only in the presence of small amts. of acetate. Substrates were fermented by disproportionation to acetate and EtOH or the resp. higher acids and alcs. No cytochromes were detectable; the guanine plus cytosine content of the DNA was 57.1 ± 0.2 mol%. Alc. dehydrogenase, aldehyde dehydrogenase, phosphate acetyltransferase, and acetate kinase were found in high activities in cell-free exts. of acetylene-grown cells, indicating that acetylene was metabolized via hydration to acetaldehyde. EtOH was oxidized to acetate in syntrophic coculture with H-scavenging anaerobes. The new isolates are described as a new species in the genus Pelobacter, P. acetylenicus.
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57
Oremland, R. S.; Voytek, M. A. Acetylene as fast food: Implications for development of life on anoxic primordial Earth and in the Outer Solar System. Astrobiology 2008, 8, 45– 58,
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Acetylene as fast food: Implications for development of life on anoxic primordial Earth and in the outer Solar System
Oremland, Ronald S.; Voytek, Mary A.
Astrobiology (2008), 8 (1), 45-58CODEN: ASTRC4; ISSN:1531-1074. (Mary Ann Liebert, Inc.)
Acetylene occurs, by photolysis of methane, in the atmospheres of jovian planets and Titan. In contrast, acetylene is only a trace component of Earth's current atm. Nonetheless, a methane-rich atm. has been hypothesized for early Earth; this atm. would also have been rich in acetylene. This poses a paradox, because acetylene is a potent inhibitor of many key anaerobic microbial processes, including methanogenesis, anaerobic methane oxidn., nitrogen fixation, and hydrogen oxidn. Fermn. of acetylene was discovered 25 years ago, and Pelobacter acetylenicus was shown to grow on acetylene by virtue of acetylene hydratase, which results in the formation of acetaldehyde. Acetaldehyde subsequently dismutates to ethanol and acetate (plus some hydrogen). However, acetylene hydratase is specific for acetylene and does not react with any analogous compds. We hypothesize that microbes with acetylene hydratase played a key role in the evolution of Earth's early biosphere by exploiting an available source of carbon from the atm. and in so doing formed protective niches that allowed for other microbial processes to flourish. Furthermore, the presence of acetylene in the atm. of a planet or planetoid could possibly represent evidence for an extraterrestrial anaerobic ecosystem.
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58
ten Brink, F. Living on Acetylene. A Primordial Energy Source. In The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment, Metal Ions in Life Sciences; Springer: Netherlands, 2014, Chapter 14. .
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Seiffert, G. B.; Ullmann, G. M.; Messerschmidt, A.; Schink, B.; Kroneck, P. M. H.; Einsle, O. Structure of the non-redox-active tungsten/[4Fe:4S] enzyme acetylene hydratase. Proc. Natl. Acad. Sci. 2007, 104, 3073– 3077,
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Structure of the non-redox-active tungsten/[4Fe:4S] enzyme acetylene hydratase
Seiffert, Grazyna B.; Ullmann, G. Matthias; Messerschmidt, Albrecht; Schink, Bernhard; Kroneck, Peter M. H.; Einsle, Oliver
Proceedings of the National Academy of Sciences of the United States of America (2007), 104 (9), 3073-3077CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
The W-Fe-S enzyme, acetylene hydratase (I) of Pelobacter acetylenicus, stands out from its class because it catalyzes a non-redox reaction, the hydration of acetylene to acetaldehyde. Sequence comparisons group I into the DMSO reductase family; it contains a bis-molybdopterin guanine dinucleotide-ligated W atom and a cubane-type [4Fe:4S] cluster. Here, the crystal structure of I was detd. at 1.26 Å and it showed that the W center bound a water mol. that was activated by an adjacent Asp residue, enabling it to attack acetylene bound in a distinct, hydrophobic pocket. This mechanism required a strong shift of pKa of the Asp residue, caused by a nearby low-potential [4Fe:4S] cluster. To access this previously unrecognized W-Asp active site, the protein evolved a new substrate channel distant from where it was found in other Mo- and W-contg. enzymes.
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60
Schulze-Makuch, D.; Grinspoon, D. H. Biologically enhanced energy and carbon cycling on Titan?. Astrobiology 2005, 5, 560– 567,
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Biologically Enhanced Energy and Carbon Cycling on Titan?
Schulze-Makuch, Dirk; Grinspoon, David H.
Astrobiology (2005), 5 (4), 560-567CODEN: ASTRC4; ISSN:1531-1074. (Mary Ann Liebert, Inc.)
With the Cassini-Huygens Mission in orbit around Saturn, the large moon Titan, with its reducing atm., rich org. chem., and heterogeneous surface, moves into the astrobiol. spotlight. Environmental conditions on Titan and Earth were similar in many respects 4 billion years ago, the approx. time when life originated on Earth. Life may have originated on Titan during its warmer early history and then developed adaptation strategies to cope with the increasingly cold conditions. If organisms originated and persisted, metabolic strategies could exist that would provide sufficient energy for life to persist, even today. Metabolic reactions might include the catalytic hydrogenation of photochem. produced acetylene, or involve the recombination of radicals created in the atm. by UV radiation. Metabolic activity may even contribute to the apparent youth, smoothness, and high activity of Titan's surface via biothermal energy.
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McKay, C. P.; Smith, H. D. Possibilities for methanogenic life in liquid methane on the surface of Titan. Icarus 2005, 178, 274– 276,
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Possibilities for methanogenic life in liquid methane on the surface of Titan
McKay, C. P.; Smith, H. D.
Icarus (2005), 178 (1), 274-276CODEN: ICRSA5; ISSN:0019-1035. (Elsevier)
Photochem. produced compds. on Titan, principally acetylene, ethane and org. solids, would release energy when consumed with atm. hydrogen, at levels of 334, 57, and 54 kJ mol-1, resp. On Earth methanogenic bacteria can survive on this energy level. Here we speculate on the possibility of widespread methanogenic life in liq. methane on Titan. Hydrogen may be the best mol. to show the affects of such life because it does not condense at the tropopause and has no sources or sinks in the troposphere. If life is consuming atm. hydrogen it will have a measurable effect on the hydrogen mixing ratio in the troposphere if the biol. consumption is greater than 108 cm-2s-1. Life could develop strategies to overcome the low soly. of orgs. in liq. methane and use catalysts to accelerate biochem. reactions despite the low temp. Expected results of the recent Huygens probe could indicate the presence of such life by anomalous depletions of acetylene and ethane as well as hydrogen at the surface.
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Tokano, T. Limnological structure of Titan’s hydrocarbon lakes and its astrobiological implication. Astrobiology 2009, 9, 147– 164,
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Limnological Structure of Titan's Hydrocarbon Lakes and Its Astrobiological Implication
Tokano, Tetsuya
Astrobiology (2009), 9 (2), 147-164CODEN: ASTRC4; ISSN:1531-1074. (Mary Ann Liebert, Inc.)
Cassini radar recently detected several putative liq. hydrocarbon lakes in the polar region of Saturn's moon Titan. Such lakes may contain org. sediments deposited from the atm. that would promote prebiotic-type chem. driven by cosmic rays, the result of which could be the prodn. of more complex mols. such as nitrogen-bearing org. polymer or azides. The phys. properties of the lake and their temporal evolution under Titan's present climatic setting were investigated by means of a one-dimensional lake thermal stratification model. Lakes can undergo various evolutions, depending on the initial compn. and depth of the lake and hydrocarbon abundance in the near-surface atm. Pure methane ponds, which may occasionally form when heavy methane hailstones reach the surface, would be transitory in that they would evap., freeze up, and eventually dry up. On the other hand, lakes filled with a mixt. of methane, ethane, and nitrogen would be more stable; and freezing or drying would not necessarily occur in most cases. Such lakes undergo a seasonal cycle of thermal stratification in spring and early summer and convective overturning in other seasons. The summer thermal stratification near the lake surface could be destabilized by bottom heating as a result of an enhanced geothermal heat flux, e.g., in the vicinity of cryovolcanoes. Most likely the compn. of the lake and atm. would come to equil. by way of a small amt. of evapn., but the lake-atm. system could be repeatedly brought out of equil. by irregular pptn. The viability of prebiotic-like chem. in the lake may depend on many lake parameters, such as temp., liq. or frozen state, and convective mixing. Moreover, convective mixing may drive suspension of solid acetylene and other sediments on the lake bottom and redistribution of dissolved gases, which might be relevant for putative life-forms that consume hydrogen and solid acetylene.
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Abstract
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Figure 1
Figure 1. (A–C) High-resolution Raman spectra of solid acetylene (blue), liquid butane (gray), the acetylene-butane co-crystal (orange), and the acetylene clathrate hydrate (yellow). Single-component spectra were collected at 130 K, and the co-crystal spectrum was collected at 150 K due to stability of the respective species within the experimental setup. The acetylene clathrate spectrum is from Vu et al.(35) and was collected at 200 K. Spectra are vertically offset for clarity. The clearest indications of co-crystal formation are the blue shifts in the C≡C stretch from 1955 to 1967 cm^–1 (inset B, orange spectrum multiplied by 20 for visibility) and the C–H stretch from 3333 to 3355 cm^–1 (inset C); these are distinct from the clathrate spectrum, which also exhibits a strong OH stretching vibration at ∼3120 cm^–1.
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Figure 2
Figure 2. Thermal stability study of the acetylene-butane co-crystal. The C≡C stretch at 1967 cm^–1 persists up to 190 K, above which temperature it is no longer 3σ above the noise. Spectra are scaled on the left and are vertically offset for clarity. The 1974 cm^–1 feature (dashed line) is acetylene in the gas phase, which increases with temperature relative to the co-crystal features as sublimated acetylene fills the headspace.
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Figure 3
Figure 3. (A) High-resolution Raman spectra of solid acetylene (black), liquid butane (gray), liquid ethane (green), and the acetylene-butane co-crystal exposed to liquid ethane (red). (B) Co-crystal feature at 1967 cm^–1 is still clearly visible in the red spectrum (arrow) when liquid ethane is present. The feature at 1974 cm^–1 is acetylene in the gas phase. All spectra collected at 90 K except for the ethane control (105 K). Spectra are vertically offset for clarity.
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Figure 4
Figure 4. High-resolution Raman spectra of the acetylene-butane co-crystal exposed to liquid ethane and warmed from 90 to 135 K. (A) At 120 K (green spectrum), the intensity of the co-crystal feature at 1967 cm^–1 (arrow) decreases substantially. (B) This coincides with butane features in the fingerprint region, which show a transition from solid to liquid butane at 120 K (also indicative of freezing point depression, as typically butane melts at 133 K). The phase transition of pure acetylene is also visible in (A) in the blue shift of the C≡C stretch from 1952.3 to 1955.1 cm^–1 at 130 K. The feature at 1974 cm^–1 is acetylene in the gas phase. Spectra are vertically offset for clarity.
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This article references 62 other publications.
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  A global-mean model of coupled neutral and ion chem. on Titan has been developed. Unlike the previous coupled models, the model involves ambipolar diffusion and escape of ions, hydrodynamic escape of light species, and calcs. the H2 and CO densities near the surface that were assigned in some previous models. We tried to reduce the nos. of species and reactions in the model and remove all species and reactions that weakly affect the obsd. species. Hydrocarbon chem. is extended to C12H10 for neutrals and C10H+11 for ions but does not include PAHs. The model involves 415 reactions of 83 neutrals and 33 ions, effects of magnetospheric electrons, protons, and cosmic rays. UV absorption by Titan's haze was calcd. using the Huygens observations and a code for the aggregate particles. Hydrocarbon, nitrile, and ion chemistries are strongly coupled on Titan, and attempt to calc. them sep. (e.g., in models of ionospheric compn.) may result in significant error. The model densities of various species are typically in good agreement with the observations except vertical profiles in the stratosphere that are steeper than the CIRS limb data. (A model with eddy diffusion that facilitates fitting to the CIRS limb data is considered as well.) The CO densities are supported by the O+ flux from Saturn's magnetosphere. The ionosphere includes a peak at 80 km formed by the cosmic rays, steplike layers at 500-700 and 700-900 km and a peak at 1060 km (SZA = 60°). Nighttime densities of major ions agree with the INMS data. Ion chem. dominates in the prodn. of bicyclic arom. hydrocarbons above 600 km. The model ests. of heavy pos. and neg. ions are in reasonable agreement with the Cassini results. The major haze prodn. is in the reactions C6H + C4H2, C3N + C4H2, and condensation of hydrocarbons below 100 km. Overall, pptn. rate of the photochem. products is equal to 4-7 kg cm-2 Byr-1 (50-90 m Byr-1 while the global-mean depth of the org. sediments is ∼3 m). Escape rates of methane and hydrogen are 2.9 and 1.4 kg cm-2 Byr-1, resp. The model does not support the low C/N ratio obsd. by the Huygens ACP in Titan's haze.
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  An. anal. was done of data recorded by the Composite IR Spectrometer (CIRS) aboard the Cassini spacecraft during the Titan flybys T0-T10 (July 2004-Jan. 2006). The spectra characterize various regions on Titan from 70° S to 70° N with a variety of emission angles. The mol. signatures obsd. in the mid-IR CIRS detector arrays (FP3 and FP4, covering roughly the 600-1500 cm-1 spectral range with apodized resolns. of 2.54 or 0.53 cm-1) were studied. The composite spectrum shows several mol. signatures: hydrocarbons, nitriles and CO2. A firm detection of benzene is provided by CIRS at levels of about 3.5 × 10-9 around 70° N. We have used temp. profiles retrieved from the inversion of the emission obsd. in the methane ν4 band at 1304 cm-1 and a line-by-line radiative transfer code to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere. Temp. profiles retrieved from the inversion of the emission obsd. in the methane ν4 band at 1304 cm-1 and a line-by-line radiative transfer code were used to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere. Temp. profiles retrieved from the inversion of the emission obsd. in the methane ν4 band at 1304 cm-1 and a line-by-line radiative transfer code were used to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere. No longitudinal variations were found for these gases. Little or no change is obsd. generally in their abundances from the south to the equator. On the other hand, meridional variations retrieved for these trace constituents from the equator to the North ranged from almost zero (no or very little meridional variations) for C2H2, C2H6, C3H8, C2H4 and CO2 to a significant enhancement at high northern (early winter) latitudes for HCN, HC3N, C4H2, C3H4 and C6H6. For the more important increases in the northern latitudes, the transition occurs roughly between 30 and 50 degrees north latitude, depending on the mol. Note however that the very high-northern latitude results from tours TB-T10 bear large uncertainties due to few available data and problems with latitude smearing effects. The obsd. variations are consistent with some, but not all, of the predictions from dynamical-photochem. models. Constraints are set on the vertical distribution of C2H2, found to be compatible with 2-D equatorial predictions by global circulation models. The D/H ratio in the methane on Titan has been detd. from the CH3D band at 1156 cm-1 and found to be 1.17+-00..2238 × 10-4 . Implications of this deuterium enrichment, with respect to the protosolar abundance on the origin of Titan, are discussed. Results are compared with values retrieved by Voyager IRIS observations taken in 1980, as well as with more recent (1997) disk-averaged IR Space Observatory (ISO) results and with the latest Cassini-Huygens inferences from other instruments in an attempt to better comprehend the phys. phenomena on Titan.
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15. 15
  Waite, J. H., Jr.; Niemann, H.; Yelle, R. V.; Kasprzak, W. T.; Cravens, T. E.; Luhmann, J. G.; McNutt, R. L.; Ip, W.-H.; Gell, D.; De La Haye, V.; Müller-Wordag, I.; Magee, B.; Borggren, N.; Ledvina, S.; Fletcher, G.; Walter, E.; Miller, R.; Scherer, S.; Thorpe, R.; Xu, J.; Block, B.; Arnett, K. Ion neutral mass spectrometer results from the first flyby of Titan. Science 2005, 308, 982– 986,
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  Ion neutral mass spectrometer results from the first flyby of Titan
  Waite, J. Hunter, Jr.; Niemann, Hasso; Yelle, Roger V.; Kasprzak, Wayne T.; Cravens, Thomas E.; Luhmann, Janet G.; McNutt, Ralph L.; Ip, Wing-Huen; Gell, David; De La Haye, Virginie; Mueller-Wordag, Ingo; Magee, Brian; Borggren, Nathan; Ledvina, Steve; Fletcher, Greg; Walter, Erin; Miller, Ryan; Scherer, Stefan; Thorpe, Rob; Xu, Jing; Block, Bruce; Arnett, Ken
  Science (Washington, DC, United States) (2005), 308 (5724), 982-986CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
  The Cassini Ion Neutral Mass Spectrometer (INMS) has obtained the first in situ compn. measurements of the neutral densities of mol. nitrogen, methane, mol. hydrogen, argon, and a host of stable carbon-nitrile compds. in Titan's upper atm. INMS in situ mass spectrometry has also provided evidence for atm. waves in the upper atm. and the first direct measurements of isotopes of nitrogen, carbon, and argon, which reveal interesting clues about the evolution of the atm. The bulk compn. and thermal structure of the moon's upper atm. do not appear to have changed considerably since the Voyager 1 flyby.
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16. 16
  Niemann, H. B.; Atreya, S. K.; Demick, J. E.; Gautier, D.; Haberman, J. A.; Harpold, D. N.; Kasprzak, W. T.; Lunine, J. I.; Owen, T. C.; Raulin, F. Composition of Titan’s lower atmosphere and simple surface volatiles as measured by the Cassini-Huygens probe gas chromatograph mass spectrometer experiment. J. Geophys. Res. 2010, 115, E12006,
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  Singh, S.; McCord, T. B.; Combe, J.-P.; Rodriguez, S.; Cornet, T.; Le Mouélic, S.; Clark, R. N.; Maltagliati, L.; Chevrier, V. F. Acetylene on Titan’s Surface. Astrophys. J. 2016, 828, 1– 8,
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  Bottger, G. L.; Eggers, D. F., Jr. Infrared spectra of crystalline C₂H₂, C₂HD, and C₂D₂. J. Chem. Phys. 1964, 40, 2010– 2017,
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  Infrared spectra of crystalline C2H2, C2HD, and C2D2
  Bottger, G.L.; Eggers, D. F., Jr.
  Journal of Chemical Physics (1964), 40 (7), 2010-17CODEN: JCPSA6; ISSN:0021-9606.
  The infrared absorption spectra of polycryst. films of C2H2, C2HD, and C2D2 were examd. at 63 °K. and 4500 and 450 cm.-1 The spectra of the solid state and the gas phase of the various acetylenes are compared and the appropriate vibrational assignments are made. Several absorptions attributable to combinations of lattice modes and mol. fundamentals were found. The observed multiplet structure of the fundamental vibrations indicates that in the low-temp. modification of C2H2 and C2D2 the mols. are located at sites of C2h symmetry. Spectra of various solid solns. composed of mixts. of isotopic species were obtained and their significance discussed. The effect of isotopic substitution on the splittings in the ν5 region led to the conclusion that the dipole-dipole coupling model is sufficient to explain the observed spectra.
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19. 19
  van Nes, G. J. H.; van Bolhuis, F. Single-crystal structures and electron density distributions of ethane, ethylene and acetylene. II. Single-crystal X-ray structure determination of acetylene at 141 K. Acta Cryst. 1979, 35, 2580– 2593,
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  Single-crystal structures and electron density distributions of ethane, ethylene and acetylene. II. Single-crystal x-ray structure determination of acetylene at 141 K
  Van Nes, Gerard J. H.; Van Bolhuis, Fre
  Acta Crystallographica, Section B: Structural Crystallography and Crystal Chemistry (1979), 35 (11), 2580-93CODEN: ACBCAR; ISSN:0567-7408.
  Cryst. acetylene at 141 K is cubic, space group Pa3, with a 6.091(3) Å; Z = 4. The intensities of all 164 independent reflections up to sin θ/λ = 0.80 Å-1 were measured accurately on a Nonius four-circle diffractometer. For higher sin θ/λ, very few significant reflection intensities exist due to the strong thermal motion. A valence anal., with multipole deformation terms and restricted radial functions centered on the atoms, was performed for the 164 independent reflections by least-squares refinement on I. Both ζ and SCF scattering factors were considered. H was constrained to C. The radial functions corresponding to the SCF scattering factors were modified by introduction of an isotropic extinction parameter and an addnl. isotropic temp. factor for H in the refinement. Rw(I;ζ) = 0.0176 And Rw(I;SCF) = 0.0170, both with 11 independent parameters. For the (modified) SCF at. scattering factors the agreement with theory is better than for the ζ scattering factors. No reliable value could be found for the scale factor.
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20. 20
  Sugawara, T.; Kanda, E. The Crystal Structure of Acetylene. I. Ser. A, Phys. Chem. Metall. 1952, 4, 607– 614
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21. 21
  Nixon, C. A.; Jennings, D. E.; Flaud, J.-M.; Bézard, B.; Teanby, N. A.; Irwin, P. G. J.; Ansty, T. M.; Coustenis, A.; Vinatier, S.; Flasar, F. M. Titan’s prolific propane: The Cassini CIRS perspective. Planet. Space Sci. 2009, 57, 1573– 1585,
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  Titan's prolific propane: The Cassini CIRS perspective
  Nixon, C. A.; Jennings, D. E.; Flaud, J.-M.; Bezard, B.; Teanby, N. A.; Irwin, P. G. J.; Ansty, T. M.; Coustenis, A.; Vinatier, S.; Flasar, F. M.
  Planetary and Space Science (2009), 57 (13), 1573-1585CODEN: PLSSAE; ISSN:0032-0633. (Elsevier B.V.)
  Although propane gas (C3H8) was first detected in the stratosphere of Titan by the Voyager IRIS IR spectrometer in 1980, obtaining an accurate measurement of its abundance has proved difficult. All existing measurements have been made by modeling the ν26 band at 748 cm-1: however, different analyzes over time have yielded quite different results, and it also suffers from confusion with the strong nearby ν5 band of acetylene. In this paper we select large spectral avs. of data from the Cassini Composite IR Spectrometer (CIRS) obtained in limb-viewing mode at low latitudes (30° S - 30° N), greatly increasing the path length and hence signal-to-noise ratio for optically thin trace species such as propane. By modeling and subtracting the emissions of other gas species, we demonstrate that at least six IR bands of propane are detected by CIRS, including two not previously identified in Titan spectra. Using a new linelist for the range 1300-1400 cm - 1 , along with an existing GEISA list, we retrieve propane abundances from two bands at 748 and 1376 cm-1. At 748 cm-1 we retrieve 4.2 ± 0.5 × 10-7 (1 - σ error) at 2 mbar, in good agreement with previous studies, although lack of hotbands in the present spectral atlas remains a problem. We also det. 5.7 ± 0.8 × 10-7 at 2 mbar from the 1376 cm-1 band -a value that is probably affected by systematic errors including continuum gradients due to haze and also an imperfect model of the ν6 band of ethane. This study clearly shows for the first time the ubiquity of propane's emission bands across the thermal IR spectrum of Titan, and points to an urgent need for further lab. spectroscopy work, both to provide the line positions and intensities needed to model these bands, and also to further characterize haze spectral opacity. The present lack of accurate modeling capability for propane is an impediment not only for the measurement of propane itself, but also for the search for the emissions of new mols. in many spectral regions.
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22. 22
  Coll, P.; Coscia, D.; Smith, N.; Gazeau, M.-C.; Ramïrez, S. I.; Cernogora, G.; Israël, G.; Raulin, F. Experimental laboratory simulation of Titan’s atmosphere: Aerosols and gas phase. Planet. Space Sci. 1999, 47, 1331– 1340,
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  Experimental laboratory simulation of Titan's atmosphere: Aerosols and gas phase
  Coll, P.; Coscia, D.; Smith, N.; Gazeau, M.-C.; Ramirez, S. I.; Cernogora, G.; Israel, G.; Raulin, F.
  Planetary and Space Science (1999), 47 (10/11), 1331-1340CODEN: PLSSAE; ISSN:0032-0633. (Elsevier Science Ltd.)
  The discovery that Titan, the largest satellite of Saturn, has an atm. and that methane is a significant constituent of it, was the starting point for a systematic study of Titan's atm. org. chem. Since then, the results from numerous ground-based observations and two flybys of Titan, by Voyager I and II, have led to exptl. lab. simulation studies and photochem. and phys. modeling. All these works have provided a more detailed picture of Titan. The authors report a continuation of such a study performing an exptl. lab. simulation of Titan's atm. chem., and considering the two phys. phases involved: gases and aerosols. Concerning the gaseous phase, the first detection of C4N2 is reported, and possible atm. abundances are proposed for 70 org. compds. on Titan's upper atm. Concerning the solid phase, aerosol analogs were synthesized under conditions close to those of Titan's environment, using elemental anal., pyrolysis, soly. studies and IR spectroscopy.
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23. 23
  Tran, B. N.; Joseph, J. C.; Force, M.; Briggs, R. G.; Vuitton, V.; Ferris, J. P. Photochemical processes on Titan: Irradiation of mixtures of gases that simulate Titan’s atmosphere. Icarus 2005, 177, 106– 115,
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  Laufer, A. H.; Gardner, E. P.; Kwok, T. L.; Yung, Y. L. Computations and estimates of rate coefficients for hydrocarbon reactions of interest to the atmospheres of the outer solar system. Icarus 1983, 56, 560– 567,
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  Computations and estimates of rate coefficients for hydrocarbon reactions of interest to the atmospheres of the outer solar system
  Laufer, A. H.; Gardner, E. P.; Kwok, T. L.; Yung, Y. L.
  Icarus (1983), 56 (3), 560-7CODEN: ICRSA5; ISSN:0019-1035.
  The rate coeffs., including Arrhenius parameters, were computed for a no. of chem. reactions involving hydrocarbon species for which exptl. data are not available and which are important in planetary atm. models. The techniques used to calc. the kinetic parameters include the Troe and semi-empirical bond energy-bond order or bond strength-bond length methods.
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25. 25
  Dobrijevic, M.; Dutour, I. The distribution of hydrocarbons in Titan’s atmosphere: An evolutionary algorithm-based model. Planet. Space Sci. 2007, 55, 2128– 2136,
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  The distribution of hydrocarbons in Titan's atmosphere: An evolutionary algorithm-based model
  Dobrijevic, M.; Dutour, I.
  Planetary and Space Science (2007), 55 (14), 2128-2136CODEN: PLSSAE; ISSN:0032-0633. (Elsevier B.V.)
  We propose a new approach to study the chem. complexity of Titan's atm. We have developed an evolutionary algorithm-based model that simulates the evolution of interacting elements with different valences. This abstr. model mimics a C-H-O-N system that might get an insight into the general properties of the chem. of Titan's atm. Comparison with detailed models like photochem. models is discussed to evaluate limitations and benefits of each approach. Comparison with observations suggests that Titan's atm. might self-organize to produce hydrocarbons with distributions that follow a power-law relation. If confirmed, this property makes possible some prediction about the abundance of heavy hydrocarbons in the atm. of Titan.
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26. 26
  Refson, K.; Pawley, G. S. The structure and orientational disorder in solid n-butane by neutron powder diffraction. Acta Cryst. B 1986, 42, 402– 410,
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  Cordier, D.; Barnes, J. W.; Ferreira, A. G. On the chemical composition of Titan’s dry lakebed evaporites. Icarus 2013, 226, 1431– 1437,
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  On the chemical composition of Titan's dry lakebed evaporites
  Cordier, D.; Barnes, J. W.; Ferreira, A. G.
  Icarus (2013), 226 (2), 1431-1437CODEN: ICRSA5; ISSN:0019-1035. (Elsevier Inc.)
  Titan, the main satellite of Saturn, has an active cycle of methane in its troposphere. Among other evidence for a mechanism of evapn. at work on the ground, dry lakebeds have been discovered. Recent Cassini IR observations of these empty lakes have revealed a surface compn. poor in water ice compared to that of the surrounding terrains-suggesting the existence of org. evaporites deposits. The chem. compn. of these possible evaporites is unknown. In this paper, we study evaporite compn. using a model that treats both org. solids dissoln. and solvent evapn. Our results suggest the possibility of large abundances of butane and acetylene in the lake evaporites. However, due to uncertainties of the employed theory, these detns. have to be confirmed by lab. expts.
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  Raulin, F. Organic Chemistry in the Oceans of Titan. Adv. Space Res. 1987, 7, 71– 81,
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  Organic chemistry in the oceans of Titan
  Raulin, F.
  Advances in Space Research (1987), 7 (5), 71-81CODEN: ASRSDW; ISSN:0273-1177.
  On Titan, most of the org. compds. present in the atm. must condense in the lower stratosphere and be solid near the surface, except CH4, C2H6, C3H8, C3H6, and 1-butene, which must be liq. and could form oceans contg. large fractions of dissolved N2. The solid org. compds., depending on their d. relatively to this liq., could accumulate at the surface or at the bottom of the oceans; they could also dissolve partly or totally in the oceans. . Chem. evolution on Titan involved phys. chem. processes in a cryogenic apolar solvent mainly composed of CH4-C2H6-N2. Systematic study of the vol. mass and soly. of the org. compds. in such a cryogenic mixt. of various compns. was made for 94 K by thermodn. modeling. The oceans of Titan could be free of icebergs of org. compds. These oceans could be very rich in dissolved org. compds., with relatively high concns. (∼1-10-6 M). In addn., the concn. of several of the org. solutes should be const., buffered by a bottom layer of the corresponding compd. in the solid phase.
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29. 29
  Glein, C. R.; Shock, E. L. A geochemical model of non-ideal solutions in the methane-ethane-propane-nitrogen-acetylene system on Titan. Geochim. Cosmochim. Acta 2013, 115, 217– 240,
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  A geochemical model of non-ideal solutions in the methane-ethane-propane-nitrogen-acetylene system on Titan
  Glein, Christopher R.; Shock, Everett L.
  Geochimica et Cosmochimica Acta (2013), 115 (), 217-240CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
  Saturn's largest moon, Titan, has an atm. and surface that are rich in org. compds. Liq. hydrocarbons exist on the surface, most famously as lakes. Photochem. reactions produce solid orgs. in Titan's atm., and these materials settle or snow onto the surface. At the surface, liqs. can interact with solids, and geochem. processes can occur. The consequences of these processes can be explored using a thermodn. model to calc. the solubilities of gases and solids in liq. hydrocarbons at cryogenic temps. The van Laar model developed in this study was parameterized using exptl. phase equil. data, and accurately represents the data for the CH4-C2H6-C3H8-N2-C2H2 chem. system from 90 to 110 K. The model generally gives more accurate results than existing models. The model also features a suitable balance between accuracy and simplicity, and can serve as a foundation for studies of fluvial geochem. on Titan because it can be extended to any no. of components while maintaining thermodn. consistency. Application of the model to Titan reveals that the equil. compn. of surface liqs. depends on the abundance of methane gas in the local atm., consistent with prior studies. The concn. of mol. nitrogen in Titan's lakes varies inversely with the ethane content of the lakes. The model indicates that solid acetylene should be quite sol. in surface liqs., which implies that acetylene-rich sedimentary rocks would be susceptible to chem. erosion, and acetylene evaporites may form on Titan. The geochem. character of acetylene in liq. hydrocarbons on Titan appears to be intermediate to those of calcite and gypsum in surface waters on Earth. Specific recommendations are given of observational, exptl., and theor. work that will lead to significant advancements in our knowledge of geochem. processes on Titan. This paper represents the beginning of a new kind of geochem., called cryogenic fluvial geochem., with Titan starring as the first example.
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30. 30
  Cornet, T.; Cordier, D.; Le Bahers, T.; Bourgeois, O.; Fleurant, C.; Le Mouélic, S.; Altobelli, N. Dissolution on Titan and on Earth: Toward the age of Titan’s karstic landscapes. J. Geophys. Res. Planets 2015, 120, 1044– 1074,
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  Dissolution on Titan and on Earth: Toward the age of Titan's karstic landscapes
  Cornet, Thomas; Cordier, Daniel; Le Bahers, Tangui; Bourgeois, Olivier; Fleurant, Cyril; Le Mouelic, Stephane; Altobelli, Nicolas
  Journal of Geophysical Research: Planets (2015), 120 (6), 1044-1074CODEN: JGPLEH; ISSN:2169-9100. (Wiley-Blackwell)
  Titan's polar surface is dotted with hundreds of lacustrine depressions. Based on the hypothesis that they are karstic in origin, we aim at detg. the efficiency of surface dissoln. as a landshaping process on Titan, in a comparative planetol. perspective with the Earth as ref. Our approach is based on the calcn. of solutional denudation rates and allow inference of formation timescales for topog. depressions developed by chem. erosion on both planetary bodies. The model depends on the soly. of solids in liqs., the d. of solids and liqs., and the av. annual net rainfall rates. We compute and compare the denudation rates of pure solid orgs. in liq. hydrocarbons and of minerals in liq. water over Titan and Earth timescales. We then investigate the denudation rates of a superficial org. layer in liq. methane over one Titan year. At this timescale, such a layer on Titan would behave like salts or carbonates on Earth depending on its compn., which means that dissoln. processes would likely occur but would be 30 times slower on Titan compared to the Earth due to the seasonality of pptn. Assuming an av. depth of 100 m for Titan's lacustrine depressions, these could have developed in a few tens of millions of years at polar latitudes higher than 70°N and S, and a few hundreds of million years at lower polar latitudes. The ages detd. are consistent with the youth of the surface (<1 Gyr) and the repartition of dissoln.-related landforms on Titan.
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31. 31
  Singh, S.; Combe, J.-P.; Cordier, D.; Wagner, A.; Chevrier, V. F.; McMahon, Z. Experimental determination of acetylene and ethylene solubility in liquid methane and ethane: Implications to Titan’s surface. Geochim. Cosmochim. Acta 2017, 208, 86– 101,
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  Experimental determination of acetylene and ethylene solubility in liquid methane and ethane: Implications to Titan's surface
  Singh, S.; Combe, J.-Ph.; Cordier, D.; Wagner, A.; Chevrier, V. F.; McMahon, Z.
  Geochimica et Cosmochimica Acta (2017), 208 (), 86-101CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
  In this study, the soly. of acetylene (or ethyne, C2H2) and ethylene (or ethene, C2H4) in liq. methane (CH4) and ethane (C2H6) has been exptl. detd. at Titan surface temp. (90 K) and pressure (1.5 bars). As predicted by theor. models, the solubilities of acetylene and ethylene are very large at Titan temp. and these species are most likely to be abundantly present in the lakes and as evaporites on the shores or dry lake beds. Our results indicate the soly. of 4.9 × 10-2 mole fraction for acetylene in methane and 48 × 10-2 mole fraction in ethane; for ethylene, 5.6 × 10-1 mole fraction in methane and 4.8 × 10-1 mole fraction in ethane. Assuming the mole fractions from atm. models in the lower stratosphere and equil. with the surface, we detd. that the lakes on Titan that cover ∼400,000 km2 are not satd. The liq. lakes on Titan act as an important reservoir for both acetylene and ethylene. Assuming difference of methane and ethane content in the lakes at different latitudes, the difference in soly. in liq. methane and ethane, solutes in lakes may change with the temporal evolution (such as; evapn. and condensation) over seasons and geol. time scales.
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32. 32
  Lorenz, R. D. The flushing of Ligeia: Composition variations across Titan’s seas in a simple hydrological model. Geophys. Res. Lett. 2014, 41, 5764– 5770,
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  The flushing of Ligeia: Composition variations across Titan's seas in a simple hydrological model
  Lorenz, Ralph D.
  Geophysical Research Letters (2014), 41 (16), 5764-5770CODEN: GPRLAJ; ISSN:1944-8007. (Wiley-Blackwell)
  We use a simple box model to explore possible differences in the liq. compn. of Titan's seas. Major variations in the abundance of involatile ethane, somewhat analogous to salinity in terrestrial waters, arise from the hydrol. cycle, which introduces more "fresh" methane rainfall at the highest latitudes in summer. The obsd. compn. of Ligeia Mare, flushed by methane rainfall and exporting its solutes to Kraken via a narrow labyrinth of channels, may have a methane-rich (>∼80%) compn., well out of thermodn. equil. with the atm., whereas the basins of Kraken are relatively well mixed and will have an ethane-dominated (∼60%) compn. These variations, analogous to Earth's salinity gradient between the Black Sea and the Mediterranean, may be detectable with Cassini measurements and are important for future exploration.
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33. 33
  Mitchell, K. L.; Barmatz, M. B.; Jamieson, C. S.; Lorenz, R. D.; Lunine, J. I. Laboratory measurements of cryogenic liquid alkane microwave absorptivity and implications for the composition of Ligeia Mare, Titan. Geophys. Res. Lett. 2015, 42, 1340– 1345,
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  Laboratory measurements of cryogenic liquid alkane microwave absorptivity and implications for the composition of Ligeia Mare, Titan
  Mitchell, Karl L.; Barmatz, Martin B.; Jamieson, Corey S.; Lorenz, Ralph D.; Lunine, Jonathan I.
  Geophysical Research Letters (2015), 42 (5), 1340-1345CODEN: GPRLAJ; ISSN:1944-8007. (Wiley-Blackwell)
  The complex dielec. consts. of liqs. methane and ethane were measured at 90 K and 14.1 GHz, close to the frequency of the Cassini RADAR. The liq. ethane loss tangent is far greater than that of liq. methane, facilitating discrimination by remote sensing. The results suggest a methane-dominated compn. for the northern sea, Ligeia Mare, on the basis of a recent loss tangent detn. using Cassini RADAR altimetry. This contrasts a previous far higher loss tangent for the southern lake, Ontario Lacus, which is inconsistent with simple mixts. of methane and ethane. The apparent nonequil. methane-to-ethane ratio of Ligeia Mare can be explained by poor admixt. of periodically cycled methane with a deeper ethane-rich alkanofer system, consistent with obliquity-driven volatile cycling, sequestration of ethane from the hydrocarbon cycle by incorporation into crustal clathrate hydrates, or periodic flushing of Ligeia Mare into adjacent Kraken Mare by fresh rainfall.
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34. 34
  Cordier, D.; Cornet, T.; Barnes, J. W.; MacKenzie, S. M.; Le Bahers, T.; Nna-Mvondo, D.; Rannou, P.; Ferreira, A. G. Structure of Titan’s evaporites. Icarus 2016, 270, 41– 56,
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  Structure of Titan's evaporites
  Cordier, D.; Cornet, T.; Barnes, J. W.; MacKenzie, S. M.; Le Bahers, T.; Nna-Mvondo, D.; Rannou, P.; Ferreira, A. G.
  Icarus (2016), 270 (), 41-56CODEN: ICRSA5; ISSN:0019-1035. (Elsevier Inc.)
  Numerous geol. features that could be evaporitic in origin have been identified on the surface of Titan. Although they seem to be water-ice poor, their main properties - chem. compn., thickness, stratification - are essentially unknown. In this paper, which follows on a previous one focusing on the surface compn. Cordier, D., Barnes, J.W., Ferreira, A.G. [2013b]. Icarus 226 2,1431-1437, we provide some answers to these questions derived from a new model. This model, based on the up-to-date thermodn. theory known as "PC-SAFT", has been validated with available lab. measurements and specifically developed for our purpose. 1-D models confirm the possibility of an acetylene and/or butane enriched central layer of evaporitic deposit. The estd. thickness of this acetylene-butane layer could explain the strong RADAR brightness of the evaporites. The 2-D computations indicate an accumulation of poorly sol. species at the deposit's margin. Among these species, HCN or aerosols similar to tholins could play a dominant role. Our model predicts the existence of chem. trimodal "bathtub rings" which is consistent with what it is obsd. at the south polar lake Ontario Lacus. This work also provides plausible explanations to the lack of evaporites in the south polar region and to the high radar reflectivity of dry lakebeds.
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35. 35
  Vu, T. H.; Cable, M. L.; Choukroun, M.; Hodyss, R.; Beauchamp, P. Formation of a new benzene-ethane co-crystalline structure under cryogenic conditions. J. Phys. Chem. A 2014, 118, 4087– 4094,
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  Formation of a New Benzene-Ethane Co-Crystalline Structure Under Cryogenic Conditions
  Vu, Tuan Hoang; Cable, Morgan L.; Choukroun, Mathieu; Hodyss, Robert; Beauchamp, Patricia
  Journal of Physical Chemistry A (2014), 118 (23), 4087-4094CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  We report the first exptl. finding of a solid mol. complex between benzene and ethane, two small apolar hydrocarbons, at atm. pressure and cryogenic temps. Considerable amts. of ethane are found to be incorporated inside the benzene lattice upon the addn. of liq. ethane onto solid benzene at 90-150 K, resulting in formation of a distinctive co-cryst. structure that can be detected via micro-Raman spectroscopy. Two new features characteristic of these co-crystals are obsd. in the Raman spectra at 2873 and 1455 cm-1, which are red-shifted by 12 cm-1 from the υ1 (a1g) and υ11 (eg) stretching modes of liq. ethane, resp. Anal. of benzene and ethane vibrational bands combined with quantum mech. modeling of isolated mol. dimers reveal an interaction between the arom. ring of benzene and the hydrogen atoms of ethane in a C-H···π fashion. The most favored configuration for the benzene-ethane dimer is the monodentate-contact structure, with a calcd. interaction energy of 9.33 kJ/mol and an equil. bonding distance of 2.66 Å. These parameters are comparable to those for a T-shaped co-cryst. complex between benzene and acetylene that has been previously reported in the literature. These results are relevant for understanding the hydrocarbon cycle of Titan, where benzene and similar orgs. may act as potential hydrocarbon reservoirs due to this incorporation mechanism.
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36. 36
  Cable, M. L.; Vu, T. H.; Hodyss, R.; Choukroun, M.; Malaska, M. J.; Beauchamp, P. Experimental determination of the kinetics of formation of the benzene-ethane co-crystal and implications for Titan. Geophys. Res. Lett. 2014, 41, 5396– 5401,
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  Experimental determination of the kinetics of formation of the benzene-ethane co-crystal and implications for Titan
  Cable, Morgan L.; Vu, Tuan H.; Hodyss, Robert; Choukroun, Mathieu; Malaska, Michael J.; Beauchamp, Patricia
  Geophysical Research Letters (2014), 41 (15), 5396-5401CODEN: GPRLAJ; ISSN:1944-8007. (Wiley-Blackwell)
  Benzene is found on Titan and is a likely constituent of the putative evaporite deposits formed around the hydrocarbon lakes. We have recently demonstrated the formation of a benzene-ethane co-crystal under Titan-like surface conditions. Here we investigate the kinetics of formation of this new structure as a function of temp. We show that the formation process would reach completion under Titan surface conditions in ∼18 h and that benzene ppts. from liq. ethane as the co-crystal. This suggests that benzene-rich evaporite basins around ethane/methane lakes and seas may not contain pure cryst. benzene, but instead benzene-ethane co-crystals. This co-cryst. form of benzene with ethane represents a new class of materials for Titan's surface, analogous to hydrated minerals on Earth. This new structure may also influence evaporite characteristics such as particle size, dissoln. rate, and IR spectral properties.
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37. 37
  Maynard-Casely, H. E.; Hodyss, R.; Cable, M. L.; Vu, T. H.; Rahm, M. A co-crystal between benzene and ethane: A potential evaporite material for Saturn’s moon Titan. Int. Union Cryst. J. 2016, 3, 192– 199,
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  Cable, M. L.; Vu, T. H.; Maynard-Casely, H. E.; Choukroun, M.; Hodyss, R. The Acetylene-Ammonia Co-crystal on Titan. ACS Earth Space Chem. 2018, 2, 366– 375,
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  The Acetylene-Ammonia Co-crystal on Titan
  Cable, Morgan L.; Vu, Tuan H.; Maynard-Casely, Helen E.; Choukroun, Mathieu; Hodyss, Robert
  ACS Earth and Space Chemistry (2018), 2 (4), 366-375CODEN: AESCCQ; ISSN:2472-3452. (American Chemical Society)
  Titan, the largest moon of Saturn, likely supports a rich org. mineralogy that we are only beginning to understand. Photochem. in the upper atm. generates a complex array of org. mols. from the simple precursors N2 and CH4. These orgs. continue to react and combine, forming aerosol layers and ultimately depositing on the surface. Orgs. are transported via pluvial (rain) and fluvial (rivers/flooding) processes into the methane-based hydrocarbon lakes, where evapn. of volatile liqs. can create evaporite deposits of remnant dissolved mols. Within such deposits, chem. and phys. processes may be occurring even at low temps. We have demonstrated in previous work that benzene and ethane rapidly form a unique and stable co-crystal at Titan surface temps. (90-95 K), akin to a salt on Earth, where the weak van der Waals interactions in the benzene-ethane co-crystal are analogous to the ionic bonds in a salt. Here, we report the formation of a second co-crystal between acetylene and ammonia, which forms even more quickly and is stable through anticipated conditions of Titan "rain" events of liq. methane, ethane, and propane. Such co-crystals represent an exciting new class of possible minerals for Titan's surface and may be responsible for processes such as selective sequestration and storage of species as well as having new properties for construction and erosive resistance of geol. materials.
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39. 39
  Boese, R.; Bläser, D.; Jansen, G. Synthesis and theoretical characterization of an acetylene-ammonia cocrystal. J. Am. Chem. Soc. 2009, 131, 2104– 2106,
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  Synthesis and Theoretical Characterization of an Acetylene-Ammonia Cocrystal
  Boese, Roland; Blaser, Dieter; Jansen, Georg
  Journal of the American Chemical Society (2009), 131 (6), 2104-2106CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  One of the smallest mol. cocrystals consisting of acetylene and ammonia is grown in situ on the X-ray diffractometer. The resulting 1:1 mol. cocrystal is composed of antiparallel layers of zigzag chains. Subunits of the crystal structure are already stable as small mol. aggregates as shown by ab initio calcns. These aggregates may be considered as infant stages of cocrystal formation.
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  Maynard-Casely, H. E.; Cable, M. L.; Malaska, M. J.; Vu, T. H.; Choukroun, M.; Hodyss, R. Prospects for Mineralogy on Titan. Am. Mineral. 2018, 103, 343– 349,
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  Vu, T. H.; Hodyss, R.; Cable, M. L.; Choukroun, M. Raman signatures and thermal expansivity of acetylene clathrate hydrate. J. Phys. Chem. A 2019, 123, 7051– 7056,
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  Raman Signatures and Thermal Expansivity of Acetylene Clathrate Hydrate
  Vu, Tuan H.; Hodyss, Robert; Cable, Morgan L.; Choukroun, Mathieu
  Journal of Physical Chemistry A (2019), 123 (32), 7051-7056CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  The vibrational signatures for the υ2 C≡C and υ1 sym. C-H stretches of acetylene in cubic structure I clathrate, synthesized under ambient pressure, are reported for the 1st time. The most diagnostic features are at 1966 for υ2 and 3353 cm-1 for υ1, resp., and are assigned to acetylene trapped in the large 51262 cages. The υ2 mode for acetylene occupying the small 512 cages is obsd. at 1972.5 cm-1, a red shift of 1.5 cm-1 from its gas phase frequency. Unit cell parameters and thermal expansion coeffs. are detd. via powder x-ray diffraction between 195 and 225 K and are in good correlation with previous single crystal data at 143 K. The calcd. d. for acetylene clathrate is also reported, with values ranging from 0.985 g/cm3 at 195 K to 0.976 g/cm3 at 225 K. These results are relevant for spectral detection of acetylene-contg. compds. on planetary bodies, as well as providing addnl. insights on the thermal behavior and phys. properties of acetylene clathrate.
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42. 42
  Subramanian, S.; Sloan, E. D. Trends in vibrational frequencies of guests trapped in clathrate hydrate cages. J. Phys. Chem. B 2002, 106, 4348– 4355,
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  Trends in Vibrational Frequencies of Guests Trapped in Clathrate Hydrate Cages
  Subramanian, Sivakumar; Sloan, E. Dendy, Jr.
  Journal of Physical Chemistry B (2002), 106 (17), 4348-4355CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
  Raman spectra of ethane trapped in the small 512 cage of sII hydrate (at ∼70 MPa), isobutane trapped in the large 51264 cage of sII hydrate, and the gauche form of n-butane trapped in the large 51264 cage of sII hydrate were obtained for the first time. These new Raman results are combined with existing Raman and IR results for various guests to infer general trends in vibrational frequencies of guest mols. trapped in clathrate hydrate cages as a function of cage size, guest size, guest vibrational mode, and pressure. The obsd. trend in stretching frequencies of guests with cage size, which can be stated as "the larger the cavity, the lower the frequency", is explained through the qual. "loose cage-tight cage" model of Pimentel and Charles (Pure Appl. Chem. 1963, 7, 111).
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  Vibrational spectroscopy of cocrystallized carbon dioxide and acetylene
  Gough, T. E.; Wang, T.
  Journal of Chemical Physics (1995), 102 (10), 3932-7CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
  Fourier transform IR spectra of acetylene and carbon dioxide cocondensed in a diffusive trapping cell are presented, and interpreted in terms of the stoichiometric phase CO2.(C2H2)2. Simulations show that this novel phase is formed when the degrees of supersatn. of the constituent gases are of comparable values.
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  Ito, M.; Yokoyama, T.; Suzuki, M. Raman spectra of acetylene crystals I and II. Spectrochim. Acta, Part A 1970, 26, 695– 705,
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  Raman spectra of acetylene crystals I and II
  Ito, Mitsuo; Yokoyama, Toru; Suzuki, Masako
  Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy (1970), 26 (3), 695-705CODEN: SAMCAS; ISSN:1386-1425.
  Laser Raman spectra have been obsd. for 2 modifications of the C2H2 crystal. The high temp. modification (crystal I) showed 3 bands owing to lattice vibrations of rotational origin. A normal coordinate calcn. indicates that their frequencies are reasonably explained with a potential based upon elec. quadrupole interaction. The spectrum of the low temp. modification (crystal II) exhibited large crystal-field splittings of intramol. bands and several sharp bands owing to lattice vibrations. The obsd. spectrum and some information from the x-ray study suggest that crystal II has a layer-type orthorhombic structure belonging to space group D182[.
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45. 45
  Anderson, A.; Andrews, B.; Torrie, B. H. Raman and Far Infrared Spectra of Crystalline Acetylene, C2H2 and C2D2. J. Raman Spectrosc. 1985, 16, 202– 207,
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  Raman and far infrared spectra of crystalline acetylene, C2H2 and C2D2
  Anderson, A.; Andrews, B.; Torrie, B. H.
  Journal of Raman Spectroscopy (1985), 16 (3), 202-7CODEN: JRSPAF; ISSN:0377-0486.
  Raman and far IR spectra of the orthorhombic and cubic phases of C2H2 and C2D2 were obtained. In the orthorhombic phase, 3 of the 4 expected librations were identified in the Raman spectrum and the 2 IR active translations were confirmed, for both samples. In the cubic phase, 2 of the 3 predicted librations in the Raman spectrum and the expected pair of translations in the far IR spectrum were obsd., again for both samples. Splittings in the internal mode Raman spectra are, with one exception, in agreement with the predictions of group theory.
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  Marchi, M.; Righini, R. Intermolecular potential and lattice dynamics of orthorhombic acetylene. Chem. Phys. 1985, 94, 465,
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  Intermolecular potential and lattice dynamics of orthorhombic acetylene
  Marchi, M.; Righini, R.
  Chemical Physics (1985), 94 (3), 465-73CODEN: CMPHC2; ISSN:0301-0104.
  The crystal structure and lattice dynamics of orthorhombic acetylene were calcd. with an intermol. potential consisting of atom-atom and multipole-multipole interactions and including a H bond. A new assignment of the Raman lattice vibrations is discussed and utilized in the refinement of the potential parameters. The non-transferability of the potential to the cubic phase is attributed to the breaking of the H bond at the phase transition, and to the large anharmonicity expected for the high-temp. phase.
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  Aoki, K.; Kakudate, Y.; Usuba, S.; Yoshida, M.; Tanaka, K.; Fujiwara, S. High-pressure Raman study of liquid and crystalline C₂H₂. J. Chem. Phys. 1988, 88, 4565– 4568,
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  High-pressure Raman study of liquid and crystalline acetylene
  Aoki, K.; Kakudate, Y.; Usuba, S.; Yoshida, M.; Tanaka, K.; Fujiwara, S.
  Journal of Chemical Physics (1988), 88 (8), 4565-8CODEN: JCPSA6; ISSN:0021-9606.
  Raman spectra were measured for liq. (0-0.7 GPa), cryst. cubic (0.7-0.9 GPa), and orthorhombic (0.9-3.5 GPa) phases in C2H2 at room temp. For the orthorhombic phase, the high-pressure behavior of the librational and internal vibrations was obtained in a wide pressure range. The frequencies of all the librational modes increased monotonically with pressure, while those of the internal modes showed a variety of frequency shifts depending on vibrational mode. These high-pressure data will be of great use for the construction of theor. models for the intermol. interactions in cryst. C2H2.
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48. 48
  Kint, S.; Scherer, J. R.; Snyder, R. G. Raman spectra of liquid n-alkanes. III. Energy difference between trans and gauche n-butane. J. Chem. Phys. 1980, 73, 2599– 2602,
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  Raman spectra of liquid n-alkanes. III. Energy difference between trans and gauche n-butane
  Kint, S.; Scherer, J. R.; Snyder, R. G.
  Journal of Chemical Physics (1980), 73 (6), 2599-602CODEN: JCPSA6; ISSN:0021-9606.
  The energy difference between the trans and gauche forms of liq. n-butane was detd. from the temp. dependence of Raman band intensities. A value of 557 ± 13 cal/mol was derived from the summed integrated intensities of six bands of each rotamer measured at nine temps. between 0 and -127°. This value is considerably lower than those detd. earlier from Raman spectra of the liq. and is in line with ΔE values for longer n-alkanes in the liq. state. However, there is a large difference between the value reported here for liq. n-butane and that reported for the gas (∼966 cal/mol).
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  Far-infrared spectrum and phase transition of acetylene
  Schwartz, Yvonne Aviva; Ron, Arza; Kimel, Sol
  Journal of Chemical Physics (1969), 51 (4), 1666-7CODEN: JCPSA6; ISSN:0021-9606.
  The far-ir spectrum (40-200 cm.-1) of solid C2H2 was studied at various temps. in the vicinity of the 1st-order phase transition to cubic at 133°K. in a closed cell. Below the transition point, the crystals are orthorhombic and exhibited 2 sharp bands with max. at about 95 and 115°K. (frequency shifted -0.12 cm.-1/°K. due to crystal imperfections). On warming, halfwidths increased. On heating samples through the temp. of the phase transition, these bands disappeared and were replaced by a broad and weak absorption at about 110 cm.-1 The spectral changes were completely reversible. The disappearance of the distinct spectral features is due to the higher degree of freedom of mol. motions in the cubic (higher temp.) phase.
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  Malaska, M. J.; Hodyss, R. Dissolution of benzene, naphthalene, and biphenyl in a simulated Titan lake. Icarus 2014, 242, 74– 81,
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  Dissolution of benzene, naphthalene, and biphenyl in a simulated Titan lake
  Malaska, Michael J.; Hodyss, Robert
  Icarus (2014), 242 (), 74-81CODEN: ICRSA5; ISSN:0019-1035. (Elsevier Inc.)
  We constructed a lab. app. capable of measuring the satn. equil. concn. (csat) and dissoln. rate consts. (keff) of org. solutes in ethane at 94 K. We detd. a csat of 18.5 ± 1.9 mg L-1, 0.159 ± 0.003 mg L-1, and 0.039 ± 0.006 mg L-1 for benzene, naphthalene, and biphenyl, resp. The derived csat and keff can be used to predict the dissoln. behavior of the materials in ethane under Titan conditions. The arom. materials dissolved relatively quickly in liq. ethane at 94 K, reaching satn. in less than 2 h. The dissoln. characteristics of benzene in ethane at 94 K are compared to those of terrestrial karst-forming materials in water at 298 K, and are used to constrain Titan surface processes. We discuss the implications of our measurements on the formation of karst on Titan, the concn. of orgs. in Titan's lakes, and the formation of evaporite deposits during lake evapn.
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  Kirchner, M. T.; Bläser, D.; Boese, R. Co-crystals with acetylene: Small is not simple!. Chem. – Eur. J. 2010, 16, 2131– 2146,
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  Co-crystals with Acetylene: Small Is not Simple!
  Kirchner, Michael T.; Blaeser, Dieter; Boese, Roland
  Chemistry - A European Journal (2010), 16 (7), 2131-2146CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Acetylene is an amazingly versatile component for the formation of co-crystals. It requires careful handling and special techniques for crystn., but the efforts seem to be rewarding when attaining co-crystals with small mols. as partners. Many basic questions such as the dominance of specific heterogeneous intermol. interactions, their driving force for the formation of multicomponent crystals instead of neat ones are expected to be easily analyzed. The underlying packing patterns and resulting stoichiometries based on the known supramol. synthons seem to be straightforward for such small mols. and crystal engineering, considered as the prototype of supramol. synthesis, should be a simple task. Nineteen co-crystals with acetylene are presented some of which were previously reported individually. An attempt was made to find features shared by the groups of co-crystals, including those that could not be co-crystd. But in spite of clear ideas and experiences from previous expts., surprisingly almost none of systems reached expectations. Intuitive approach was not fulfilled, which demonstrates that multicomponent crystals even of small mols. will remain a great challenge for theor. methods and the crystal structures shown herein represent good candidates for future testing. However, the authors wish to encourage other groups to present their views on the crystal structures with an unbiased approach that may offer a better explanation than the authors are able to outline.
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  Hayes, A. G.; Birch, S. P. D.; Dietrich, W. E.; Howard, A. D.; Kirk, R. L.; Poggiali, V.; Mastroguiseppe, M.; Michaelides, R. J.; Corlies, P. M.; Moore, J. M.; Malaska, M. J.; Mitchell, K. L.; Lorenz, R. D.; Wood, C. A. Topographic constraints on the evolution and connectivity of Titan’s lacustrine basins. Geophys. Res. Lett. 2017, 44, 11– 745,
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  Malaska, M. J.; Radebaugh, J.; Lopes, R. M. C.; Mitchell, K. L.; Verlander, T.; Schoenfeld, A. M.; Florence, M. M.; Le Gall, A.; Solomonidou, A.; Hayes, A. G.; Birch, S. P. D.; Janssen, M. A.; Shurmeier, L.; Cornet, T.; Ahrens, C.; Farr, T. G.; Cassini RADAR Team. Labyrinth Terrain on Titan. Submitted to Icarus.
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  Fermentation of acetylene by an obligate anaerobe, Pelobacter acetylenicus sp. nov
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  Acetylene occurs, by photolysis of methane, in the atmospheres of jovian planets and Titan. In contrast, acetylene is only a trace component of Earth's current atm. Nonetheless, a methane-rich atm. has been hypothesized for early Earth; this atm. would also have been rich in acetylene. This poses a paradox, because acetylene is a potent inhibitor of many key anaerobic microbial processes, including methanogenesis, anaerobic methane oxidn., nitrogen fixation, and hydrogen oxidn. Fermn. of acetylene was discovered 25 years ago, and Pelobacter acetylenicus was shown to grow on acetylene by virtue of acetylene hydratase, which results in the formation of acetaldehyde. Acetaldehyde subsequently dismutates to ethanol and acetate (plus some hydrogen). However, acetylene hydratase is specific for acetylene and does not react with any analogous compds. We hypothesize that microbes with acetylene hydratase played a key role in the evolution of Earth's early biosphere by exploiting an available source of carbon from the atm. and in so doing formed protective niches that allowed for other microbial processes to flourish. Furthermore, the presence of acetylene in the atm. of a planet or planetoid could possibly represent evidence for an extraterrestrial anaerobic ecosystem.
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  The W-Fe-S enzyme, acetylene hydratase (I) of Pelobacter acetylenicus, stands out from its class because it catalyzes a non-redox reaction, the hydration of acetylene to acetaldehyde. Sequence comparisons group I into the DMSO reductase family; it contains a bis-molybdopterin guanine dinucleotide-ligated W atom and a cubane-type [4Fe:4S] cluster. Here, the crystal structure of I was detd. at 1.26 Å and it showed that the W center bound a water mol. that was activated by an adjacent Asp residue, enabling it to attack acetylene bound in a distinct, hydrophobic pocket. This mechanism required a strong shift of pKa of the Asp residue, caused by a nearby low-potential [4Fe:4S] cluster. To access this previously unrecognized W-Asp active site, the protein evolved a new substrate channel distant from where it was found in other Mo- and W-contg. enzymes.
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  With the Cassini-Huygens Mission in orbit around Saturn, the large moon Titan, with its reducing atm., rich org. chem., and heterogeneous surface, moves into the astrobiol. spotlight. Environmental conditions on Titan and Earth were similar in many respects 4 billion years ago, the approx. time when life originated on Earth. Life may have originated on Titan during its warmer early history and then developed adaptation strategies to cope with the increasingly cold conditions. If organisms originated and persisted, metabolic strategies could exist that would provide sufficient energy for life to persist, even today. Metabolic reactions might include the catalytic hydrogenation of photochem. produced acetylene, or involve the recombination of radicals created in the atm. by UV radiation. Metabolic activity may even contribute to the apparent youth, smoothness, and high activity of Titan's surface via biothermal energy.
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  Photochem. produced compds. on Titan, principally acetylene, ethane and org. solids, would release energy when consumed with atm. hydrogen, at levels of 334, 57, and 54 kJ mol-1, resp. On Earth methanogenic bacteria can survive on this energy level. Here we speculate on the possibility of widespread methanogenic life in liq. methane on Titan. Hydrogen may be the best mol. to show the affects of such life because it does not condense at the tropopause and has no sources or sinks in the troposphere. If life is consuming atm. hydrogen it will have a measurable effect on the hydrogen mixing ratio in the troposphere if the biol. consumption is greater than 108 cm-2s-1. Life could develop strategies to overcome the low soly. of orgs. in liq. methane and use catalysts to accelerate biochem. reactions despite the low temp. Expected results of the recent Huygens probe could indicate the presence of such life by anomalous depletions of acetylene and ethane as well as hydrogen at the surface.
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  Cassini radar recently detected several putative liq. hydrocarbon lakes in the polar region of Saturn's moon Titan. Such lakes may contain org. sediments deposited from the atm. that would promote prebiotic-type chem. driven by cosmic rays, the result of which could be the prodn. of more complex mols. such as nitrogen-bearing org. polymer or azides. The phys. properties of the lake and their temporal evolution under Titan's present climatic setting were investigated by means of a one-dimensional lake thermal stratification model. Lakes can undergo various evolutions, depending on the initial compn. and depth of the lake and hydrocarbon abundance in the near-surface atm. Pure methane ponds, which may occasionally form when heavy methane hailstones reach the surface, would be transitory in that they would evap., freeze up, and eventually dry up. On the other hand, lakes filled with a mixt. of methane, ethane, and nitrogen would be more stable; and freezing or drying would not necessarily occur in most cases. Such lakes undergo a seasonal cycle of thermal stratification in spring and early summer and convective overturning in other seasons. The summer thermal stratification near the lake surface could be destabilized by bottom heating as a result of an enhanced geothermal heat flux, e.g., in the vicinity of cryovolcanoes. Most likely the compn. of the lake and atm. would come to equil. by way of a small amt. of evapn., but the lake-atm. system could be repeatedly brought out of equil. by irregular pptn. The viability of prebiotic-like chem. in the lake may depend on many lake parameters, such as temp., liq. or frozen state, and convective mixing. Moreover, convective mixing may drive suspension of solid acetylene and other sediments on the lake bottom and redistribution of dissolved gases, which might be relevant for putative life-forms that consume hydrogen and solid acetylene.
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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsearthspacechem.9b00275.
- Raman microscopy images, a table of experimental Raman shifts upon co-crystal formation at 90 K, and high-resolution Raman spectra of the acetylene-butane co-crystal (PDF)
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A Co-Crystal between Acetylene and Butane: A Potentially Ubiquitous Molecular Mineral on Titan

Publication History

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Citations

Abstract

Introduction

Experimental Methods

Results

Formation of the Co-Crystal

Figure 1

Thermal Stability

Figure 2

Influence of Solid-Phase Acetylene or Butane on Co-Crystal Stability

Co-Crystal Stability after Exposure to Liquid Ethane

Figure 3

Figure 4

Discussion

Supporting Information

pdf

Terms & Conditions

Acknowledgments

References

Cited By

Abstract

Figure 1

Figure 2

Figure 3

Figure 4

References

Supporting Information

Supporting Information

pdf

Terms & Conditions

STEP 1:

STEP 2:

OOPS

A Co-Crystal between Acetylene and Butane: A Potentially Ubiquitous Molecular Mineral on Titan

Article Views

Altmetric

Citations

Abstract

Introduction

Experimental Methods

Results

Formation of the Co-Crystal

Figure 1

Thermal Stability

Figure 2

Influence of Solid-Phase Acetylene or Butane on Co-Crystal Stability

Co-Crystal Stability after Exposure to Liquid Ethane

Figure 3

Figure 4

Discussion

Supporting Information

pdf

Terms & Conditions

Acknowledgments

References

Cited By

Abstract

Figure 1

Figure 2

Figure 3

Figure 4

References

Supporting Information

Supporting Information

pdf

Terms & Conditions

STEP 1:

STEP 2:

OOPS

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