Cover Page/Proposal Summary

ROSS-99    NRA 99-OSS-01

Date Due: 5/3/99

Atomic sodium (Na) has been detected in several comets, but Comet Hale-Bopp (C/1995 O1) offered the first opportunity to image long Na tails. Two types of sodium tails have been identified: A broad, diffuse Na tail superimposed on the dust tail and a narrow tail consisting of fast-moving Na atoms with a position angle about 15 degrees from the diffuse tail. The narrow tail appears to have its origin in the inner coma of the comet, while the diffuse tail appears to come from a steady release of sodium atoms in the dust tail. Preliminary analysis indicates that both sources are of about equal strength. It has been proposed that the sodium in the narrow tail comes in part directly from the nucleus and in part from a distributed source in the coma. The distributed source could be either Na-bearing molecules (or ions) or dust releasing Na by sputtering or vaporization. We will investigate all of these possible sources and add an additional possibility: Instead of a direct release from the nucleus, we propose a compact source of gas-entrained dust aggregates that rapidly fragment, thereby increasing fresh surface areas of dust to sputtering of Na in the coma near the nucleus. The probability of fragmentation decreases as particles become smaller with increasing distance from the nucleus and release of Na decreases as old surface areas become depleted. We will use long-slit and high-resolution spectroscopic observations to analyze and model the sources and mechanisms for sodium release leading to the production of the two distinct sodium tails. Potential benefits of the analysis are (1) increase in knowledge about the composition of comets, (2) a better measure of the total dust release rate, and (3) possibly a better understanding of fragmentation of dust clusters in the inner coma.