Comet reservoir gets more real.

Comets may make a spectacle of themselves when they near the sun, but it's tricky to determine exactly where any of them was born. Simulations tracing the orbits of comets back in time suggest that the solar system harbors two reservoirs of these icy bodies. A disk-shaped storehouse, known as the Kuiper belt, lies just beyond the orbits of Pluto and Neptune. A vast spherical region called the Oort cloud originated even closer to the sun, between the orbits of Uranus and Neptune, but was kicked out about 1,000 times farther than the belt.

Over the past 5 years, as astronomers have imaged more than 30 residents of the Kuiper belt, this proposed storehouse has become accepted as a source of cornets. Astronomers have little hope of detecting the present-day members of the more remote Oort cloud, however. Now, ultraviolet Studies of Comet Hale-Bopp provide support for the existence of the cloud.

Using the Extreme Ultraviolet Explorer satellite, Michael J. Mumma and Vladimir A. Krasnopolsky of NASA's Goddard Space Flight Center in Greenbelt, Md., and their colleagues recently searched for emissions from neon in the spectra of Hale-Bopp. They discovered that the abundance of neon relative to oxygen in the comet, thought to be an emigre from the Oort cloud, is at most one-twenty-fifth the abundance in the sun and could well be zero. The team had previously found that another Oort cloud comet, Hyakutake, also contains little or no neon.

Neon ice turns to vapor at 25 kelvins, readily escaping the nucleus of a cornet. The absence of neon suggests that the grains of interstellar ice that built Hale-Bopp and Hyakutake must have done so in a part of the solar system where temperatures are above 25 kelvins. The region between Uranus and Neptune meets this temperature criterion without being hot enough to boil off other ices found in the comet.

The match in temperature adds to the evidence that the Oort cloud did indeed assemble between these two planets and only later migrated to the fringes of the solar system. The team reported the neon measurement in an April 14 circular of the International Astronomical Union.