On the properties of comets, asteroids, and terrestrial planet impactors

Abstract

This review discusses problems related to understanding the compositional spectrum of the material interacting with Earth's atmosphere. The most important issue is therefore the properties of Earth-crossing objects, but these must be related to their place of origin and mode of transport and evolution. In terms of chemistry of minor bodies, the role of comets as an outward extension of the trends seen among the asteroids is briefly discussed. However, attention is focussed on bulk properties bearing on the question whether the Earth impactor population is dominated by asteroidal fragments originating in the main belt or if it has an important contribution from defunct comets. Cometary evolution is argued to be governed by the formation and blowoff of thin dust mantles, and it appears likely that a large fraction of the short-period comets reach a stage of complete coverage. If so, they form a rich source population for the supply of near-Earth asteroids but a fragile one in the sense that rejuvenation of cometary activity is easily induced by meteoroid impacts.