Thermal emission from the dust coma of comet bowell and a model for the grains

Abstract

We present 2.2-, 10-, and 20-μm photometry of Comet Bowell (1982 I) taken on 24 June 1982 when the comet was at 3.5 AU postperihelion. From these and earlier thermal emission measurements we conclude that the OH production in 1981 was probably supplied by large dirty-ice grains in the coma, as proposed by A'Hearn et al. (1984). The temperature of the grains must have been 140–155°K. Amorphous ice and the phase change from amorphous to cubic ice may have supplied much of the energy for sublimation. The much lower OH production in 1982 could have arisen from icy grains or from the nucleus. There is no evidence for an extremely low geometric albedo of the grains (<1%); in fact, much of the scattered light may have come froman additional component of cold icy grains.