CAUGHT IN THE ACT Comet LINEAR was just a bright spot in the sky, as seen by the Hubble Telescope on July 5, 2000 (top). But then on July 6, it blew off a piece of its crust (center), which can be seen the following day floating away along the comet's tail. After passing around the sun, C/LINEAR had melted away into a brilliant smear by July 26. NASA/HUBBLE TELESCOPE IMAGES

With the well-timed demise of C/LINEAR, as it's known, scientists peered for the first time into a comet's exposed nucleus. The wealth of information from their numerous observations has yielded a collection of six papers in last week's Science, which includes results that may fuel the debate over whether comets seeded Earth's oceans and delivered the building blocks of life.

Several research groups studied the comet's breakup, which is still a relative mystery. For example, one study shows that the comet rotated slowly--eliminating the theory that fast rotation could have pulled it apart [Science, 292, 1348 (2001)].

Scientists also confirmed the mechanism by which comets produce X-rays: Ions in the solar wind exchange charges with neutral gases in the coma [Science, 292, 1343 (2001)].

C/LINEAR also sports a chemical makeup unlike any of its recent, well-studied cometary brethren: Hale-Bopp, Hyakutake, Halley, and Lee. According to Michael J. Mumma, chief scientist for planetary research at NASA Goddard Space Flight Center in Greenbelt, Md., and colleagues, the latter comets are loaded with volatile organics such as methanol and methane, but the levels of these compounds were very depleted in C/LINEAR [Science, 292, 1334 (2001)].

This indicates that the comet formed in a relatively warm region where volatile compounds would have escaped and later condensed after being chemically processed into more complex organics in the solar nebula.

During the accretion of the solar system, comets formed from icy particles in a region extending from Jupiter to Neptune and later were ejected to great distances, forming what's known as the Oort cloud. Oort cloud comets like Hale-Bopp and Hyakutake are thought to have originated from the outer edges of the giant planets' region, between Uranus and Neptune. C/LINEAR, however, likely formed closer to Jupiter, where temperatures were somewhat warmer, the authors say.

Comets like Hale-Bopp have high ratios of deuterium to hydrogen, consistent with their formation in colder regions. But comets formed near Jupiter should have much lower D/H ratios--a result of their formation at warmer temperatures.

This is important, Mumma says, because although the idea that raining comets seeded Earth's primordial oceans is tantalizing, the oceans have relatively low D/H ratios. The number of comets with high D/H ratios, therefore, is at odds with that theory.

However, if C/LINEAR can be shown to have a low D/H ratio, and if there are many such Jupiter area comets, the issue will be reopened, Mumma says. And because the bulk of the nebular mass was near Jupiter, the groups believe that that scenario is likely.

Exploring that question will require more data points, notes David C. Jewitt, an astronomer at the Institute for Astronomy in Honolulu. "We need to get measurements of many more comets with the level of detail" shown in Mumma's paper, he says. And right now, measuring the D/H ratio in comets is difficult because the isotopic spectral lines are so faint, he adds.

Mumma and his colleagues are planning to study two more newly discovered comets in the next year, which should help clarify things. "It's quite an exciting time to be involved in this field," Mumma says.

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