Introduction to special issue

Inside Out; Outside In

Science  14 Oct 2005:
Vol. 310, Issue 5746, pp. 257
DOI: 10.1126/science.310.5746.257

Comets harbor ice and organic material that formed early in our solar system and was partly incorporated into the planets, including ours. They often appear to be huge in the sky, but this is deceptive—their visible coma is produced from a relatively small nucleus, typically a few tens of kilometers across or less. Their small size may have helped to preserve a pristine record of early solar system chemistry. On 3 July 2005, Deep Impact penetrated the nucleus of comet 9P/Tempel 1, ejecting dust and debris from inside the nucleus that was studied by a nearby spacecraft and dozens of other telescopes and spacecraft worldwide. Results from the observations of this truly international experiment (originally published online on 8 and 15 September 2005) are described in six papers in this special section.

Our solar system has two populations of comets: One now resides in a region near and just beyond the orbit of Pluto (the Kuiper Belt), and one group of these (including comet 9P/Tempel 1) gets scattered into orbits that bring them closer to the Sun and Earth; another population now resides much farther out in the Oort Cloud and includes many of our famous comets (Halley, for example). Although now far distant, it is thought that the Oort comets originated near Neptune and Uranus—closer in than the Kuiper Belt comets—but were flung outward (many may have left our solar system entirely) as these and the other giant planets formed.

CREDIT: NASA/JPL/UNIVERSITY OF MARYLAND

As discussed in these papers, the Deep Impact experiment revealed several surprises about the nature of the nucleus of Tempel 1 and the origin of comets (see also the News story by Kerr in the 9 September 2005 issue of Science). On approach, Deep Impact found that the nucleus contained regions with different layering, perhaps suggesting the amalgamation of two or more separate pieces. Observations from the supporting telescopes, and later from the spacecraft, revealed several episodic outbursts of material from the nucleus. Its surface also contained impact craters, the first seen on a cometary nucleus. Data from the impact itself imply that the nucleus was loosely consolidated and had a low density, consistent with porous ice. Clear evidence of some complex internal layering in the nucleus was not evident. Together these observations reveal a complex history and help tie in features of the nucleus to the formation of its coma. Analysis of the debris cone identified organic molecules, water ice, and silicate-rich dust. Interestingly, the composition and relative abundance of these gases and particles in this Kuiper Belt comet are similar to those of several Oort comets previously studied. If so, then at least some Kuiper Belt comets may have originated close to or within the giant-planet region, implying a more common origin for now-distant comets and suggesting interesting dynamics within this region of the early solar system.

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