CARBONACEOUS COMPONENTS IN THE COMET HALLEY DUST

Cometary grains containing large amounts of carbon and/or organic matt er (CHON) were discovered by in situ measurements of comet Halley dust composition during VEGA and GIOTTO flyby missions. In this paper, we report the classification of these cometary grains by means of cluster analysis, discuss the resulting compositional groups, and compare the m with substances observed or hypothesized in meteorites, interplaneta ry dust particles, and the interstellar medium. Grains dominated by ca rbon and/or organic matter (CHON grains) represent approximately 22% o f the total population of measured cometary dust particles. They usual ly contain a minor abundance of rock-forming elements as well. Grains having organic material are relatively more abundant in the vicinity o f the nucleus than in the outer regions of the coma, which suggests de composition of the organics in the coma environment. The majority of c omet Halley organic particles are multicomponent mixtures of carbon ph ases and organic compounds. Possibly, the cometary CHON grains may be related to kerogen material of an interstellar origin in carbonaceous meterorites. Pure carbon grains, hydrocarbons and polymers of cyanopol yynes, and multi-carbon monoxides are present in cometary dust as comp ositionally simple and distinctive components among a variety of other s. There is no clear evidence of significant presence of pure formalde hyde or HCN polymers in Halley dust particles. The diversity of types of cometary organic compounds is consistent with the interstellar dust model of comets and probably reflects differences in composition of p recursor dust. Preservation of this heterogeneity among submicron part icles suggests the gentle formation of cometary nucleus by aggregation of interstellar dust in the protosolar nebula without complete mixing or chemical homogenization at the submicron level.