Solar nebula origin for volatile gases in Halley's comet

Abstract

Modeling of physical and chemical processes in solar nebula environments is applied to the present data base on the composition of gases in Halley's comet to infer the conditions under which that comet formed. Key molecular ratios—CH 4 to CO, CO 2 to CO, and NH 3 to N 2—are compared to predictions of solar nebula models. Nebular thermochemistry is quantified for a range of solar elemental compositions corresponding to varying degrees of depletion of water in the inner part of the nebula. The edge of the chemically active region is fixed by rates of chemical reactions in the presence of grains. The mixing of molecules from chemically active regions to the outer nebula is parameterized using recent studies of this process. The incorporation of volatile molecules into water ice by clathration and absorption is then applied to yield predicted ratios of volatile species for comparison with the data. We find that, assuming chemistry in the inner nebula is catalyzed by reactions on grains, the abundances of the volatile carbon species CH 4, CO, and CO 2 in Halley could have been supplied by the solar nebula. The NH 3 abundance in Halley is, however, too high to have been derived from the nebula; an alternative source must be invoked. We suggest that the surrounding giant molecular cloud is a possible source for ammonia. This requires that nebular mixing not be 100% efficient. We also consider the heating and processing of grains entering the nebula by gas drag.