The Composition of Comets

Abstract

ABSTRACT. The chemical composition of gases in the coma of Comet Halley obtained from in situ and remote observations is reviewed. Water vapor with an abundance of approximately 80% by number is the dominant parent gas. Carbon monoxide is the second most abundant molecule, with about one-seventh of the water abundance. The other expected parent gasescarbon dioxide, methane, and ammonia-contributed with at most a few percent to the coma gas. Molecular nitrogen is found to be a minor species, with an abundance of less than one percent. Formaldehyde is relatively abundant, with a few percent relative to water. Gas-phase isotope ratios of nitrogen, oxygen, and sulfur are found to agree with solar system values within experimental errors. Halley's hydrogen isotope ratio is comparable t o t h e values found in other solar system objects poor in hydrogen, but distinctly different from t h e protosolar nebula and objects that accreted hydrogen in gaseous form. Carbon in the cyanide radical is enriched by 35% in 1 3 C compared to the bulk solar system value that indicates the presence of non-homogenized interstellar carbon in Halley. Volatiles released from grains contribute noticeably to the coma gas. Complex organic molecules, inferred from various observations as constituents of the dust grains, are believed to be the origin of the distributed C O source, CN, and other jet structures in the coma.

Molecular Abundances in Comets

“…H2CO was also observed in situ in P/Halley by the Ν MS and IMS instruments (Krankowsky 1991;Geiss et al 1991;Meier et al 1993). Both observations show that formaldehyde is released from a distributed source.…”

“…An inventory of known cometary volatiles will be presented, with a critical review of the evaluation of their abundances. Previous reviews on this topic were made recently by Krankowsky (1991) and Mumma et al (1993b) and as part of the comprehensive review of Festou et al (1993). The implications for the nucleus composition, structure, and evolution, as well as the possible link to the interstellar medium, are only briefly mentioned here, as they are treated elsewhere in this volume (Rickman 1994;Greenberg and Shalabiea, 1994).…”

“…Cometary isotopic ratios, which also contain a strong cosmogonie message, will not be discussed due to lack of space. A recent review is given by Krankowsky (1991) …”

On the role of solar EUV, photoelectrons, and auroral electrons in the chemistry of C(1D) and the production of C I 1931 Å in the cometary coma: A case for comet 1P/Halley

“…In the present study this model is employed, with few modifications, to investigate the chemistry of C(ID) atoms and the production of C I 1931 /• emission in the coma of Halley's comet. The modifications are (1) use of laboratory measured cross sections for the process CO + e ---) C + 0 [Cosby, 1993], (2) use of C(ID) + H20 quenching rate coefficient of 1.7X10 'll cm 3 s 'l [Schofield, 1979] During the comet 1P/Halley's exploration the data collected by various spacecraft and ground-satellite-based experiments have indicated that there are many C-bearing species and that carbon is present not only in gas phase but also in solid-dust form (see Grun and Jessberger [1990], Encrenaz and Knacke [1991], Krankowsky [ 1991 ] [Sahnow et al, 1993]. A full quantitative study taking both the solid-dust form and the gaseous form of the coma is very complex and is outside the scope of the present studies.…”

Comet Taxonomy and Evolution

“…In the last few years, observations of comets in the microwave region have become common, and have resulted in many new species being detected. The status of this new field and lists of the many species observed are given in several recent reviews (Krankowsky 1991;Mumma, Weissman, and Stern 1993;Crovisier 1992Crovisier , 1994.…”