Kuiper belt comets not so pristine?

Preserved in the deep freeze of space, comets don't look a day older than they did 4.5 billion years ago, when they formed during the birth of the solar system. At least that's what most planetary scientists have thought. Two researchers now challenge that assumption.

Although comets came in from the cold, they may not all represent pristine relics. A significant number may be mere fragments of objects that originally assembled during the earliest days of the solar system, assert Paolo Farinella of the University of Pisa in Italy and Donald R. Davis of the Planetary Science Institute in Tucson. The researchers base their view on a numerical model that charts the evolution of the Kuiper belt, a flattened reservoir of comets lying just beyond the orbits of Neptune and Pluto (SN: 7/27/96, p. 60).

Astronomers have so far identified 32 objects in the Kuiper belt, each of them at least 100 kilometers in diameter, or about 10 times larger than a typical comet. In addition, the Hubble Space Telescope has found evidence of about 30 smaller, comet-size bodies in the same region (SN: 6/22/96, p. 395). Kuiper belt comets that visit the inner solar system have orbital periods of 60 years or less and move in the same plane as the planets. Using established estimates of the density and size distribution of objects in the Kuiper belt, the researchers deduce that comets originate when larger parent bodies in the storehouse smash into each other at speeds of up to several hundred meters per second. Such collisions would have cut some of these icy bodies down to comet size. A few of these fragments may also have been compressed by the collisions, which could explain why some comets are less fragile than others, notes Davis. He and Farinella present their work in the Aug. 16 Science.

The team calculates that Kuiper belt residents larger than 100 km resisted breaking apart during collisions, so they may truly represent pristine bodies from the solar system's formation. However, about 75 percent of the residents that are as small as 10 km in diameter, and virtually all objects 1 km or smaller, arose from collisions of larger parent bodies, the researchers conclude. Their results generally agree with a different, less detailed model proposed by S. Alan Stern of the Boulder, Colo., branch of the Southwest Research Institute.

Scientists have proposed that other comets, those that visit the inner solar system less frequently, come from an even more distant reservoir called the Oort cloud. Davis notes that this reservoir is thought to be so vast that its members would rarely have a chance to collide. Thus, these comets would more likely bear witness to conditions during the early solar system. Astronomers are just beginning to record spectra of Kuiper belt objects.

Tetsuo Yamamoto of Hokkaido University in Sapporo, Japan, notes that if the surface spectra of the smaller objects show a wide range of composition and colors, it would indicate that collisions played a key role in sculpting these comets.