On the Outgassing Profile of Comet Hale‐Bopp

Abstract

When Comet Hale-Bopp was Ðrst discovered at a heliocentric distance of D7.2 AU, its activity appeared to be controlled by the outgassing of the highly volatile CO molecule. Its gas production remained CO-driven until about 4 AU when the outgassing became controlled by the less volatile H 2 O molecule. While the outgassing around this distance is consistent with the sublimation from a large "" dirty snowball ÏÏ composed dominantly of the subsequent outgassing at smaller heliocentric disH 2 O, tance up to perihelion (d B 0.914 AU) falls consistently below the predictions of such a model. In this paper, we use the earliest chemical di †erentiation model of the cometary nucleus developed by Houpis et al., with a few modiÐcations, to show that the continuous chemical di †erentiation of a deepening layer of the cometary nucleus which has made multiple passages into the inner solar system can explain comet Hale-BoppÏs observed pre-perihelion production rate curve. It also predicts a hysteresis with the gas production rate proÐles falling systematically lower post-perihelion. The model suggests that comet Hale-Bopp is a complex combination of dust, clathrate, and volatile (mainly CO), the gas production of which changes from being CO-driven to driven around 4 AU. Furthermore, the thermal insulation pro-H 2 O vided by a growing dust mantle inside D3.5 AU causes the production rate curve to Ñatten-out. H 2