Isotopic Ratios in Comets

Abstract

ABSTRACT. The isotopic abundances depend on the universal evolution of elements and on the individual history of particular objects. Since it is believed that unprocessed material of the solar nebula is preserved in comets, the data concerning the abundance of stable isotopes in these primitive bodies are of some importance in the cosmological context. The present status of this problem is reviewed. The reliability of results for nuclear species with cosmological and cosmogonical implications, such as D/H, C 12/13, N 14/15, O 16/18, and Mg 24/25/26, is discussed. Significant variation is found for the isotopic abundance of carbon, depending upon which carbon reservoir is sampled. Deuterium is probably enhanced relative to the interstellar ratio. For other isotopes, the ratios are close to those of the terrestrial data. The tendency of the D/H ratio to be at higher values indicates a low temperature in the environment of the comet's formation, and, together with similar effects in the outer planets, suggests that there were two different primordial reservoirs of deuterium in the solar system. The 12 C/ 13 C ratio inferred from in situ mass spectrometry of the dust, as well as from the ground-based optical spectra of the Swan band, tends to be approximately equal to the average terrestrial ratio (89) or larger. Recent results obtained from the CN band provide a significantly lower value (about 65), which corresponds to the carbon isotopic ratio in the diffuse interstellar clouds. The enhancement of deuterium and the possible differences of the carbon isotopic ratio in different species and refractory material are indicative of chemical fractionation processes in the protosolar nebula.

Laboratory simulation of the physical processes occurring on and near the surfaces of comet nuclei

“…The D/H ratios of Comets Halley, Hyakutake, and Hale-Bopp suggest an interstellar origin (see, e.g., Vanysek, 1991). The combined gas and dust released from the nucleus of Halley's comet has solar abundances of 0, C, and the nonvolatile elements, and is closer to solar abundance than CI chondrites for N, and may thus be 1976;Jessberger et al, 1988;Anders and Ebihara, 1982).…”

Commission 15: Physical Study of Comets. Minor Planets and meteorites (L’etude Physique des Cometes. des Petites Planetes Et des Meteorites)

“…Spectrophotometry and in situ measurements that led to the determination of the composition of comets were discussed in Krankowsky (1991), isotopic ratios are presented by Vanysek (1991). The various water production rate evaluations were compared by Festou (1990a) who found them to be sometimes slightly discrepant.…”

Comets and Constraints on Solar System Formation

“…The isotopic composition provides clues to the formation (and chemical history) of the cometary matter. Isotopic abundances derived from measurements in comet Halley gaseous coma have been reported and extensively discussed (Vanysek, 1991).…”