The Volatile Composition of Comets

Abstract

Comets may be our best probes of the physical and chemical conditions in the outer regions of the solar nebula during that crucial period when the planets formed. The volatile composition of cometary nuclei, in particular, can be used to decide whether comets are the product of a condensation sequence similar to that invoked to explain the compositions of the planets and asteroids, or if comets are simply agglomerations of interstellar grains which ' have been insignificantly modified by the events that shaped the other bodies in the solar system. Although cometary nuclei are not generally accessible to observation, observations of comet_) comae can illuminate at least some of the mysteries of the nuclei provided one has a detailed knowledge of the excitation conditions in the coma and also has access to basic atomic and molecular data on the many species present in comets. This paper examines the status of our knowledge of the volatile composition of cometary nuclei and discusses how these data are obtained.

Chemical Composition of Dust Expected from Condensation Models

“…Volatile composition of comets is inferred from the gaseous composition observed in the coma. Table 2 summarizes candidate molecules composing cometary volatiles and their abundances observed in the comae of comets (Weaver, 1989). Although the abundance values are heavily weighted towards the results of comet Halley and have uncertainties, it is clear that both oxidized (CO, CO2) and reduced (CH4, NH3, HCN) species constitute cometary volatiles.…”