The formation of the atmospheres of the terrestrial planets by impact

It is generally supposed that the atmospheres of the terrestrial planets were formed by secondary degassing processes. We propose, instead, that they are of primary origin, forming as an immediate and necessary consequence of the final stages of planetary accretion. Once the planetary embryo reached a critical size, the impacting material began to vaporize. The atmosphere, so created, then decelerated other impacting material, thus limiting the rate of atmospheric growth. We show that, given reasonable assumptions concerning the chemical composition of the impacting material, an acceptable model for the early atmosphere of the Earth, and the present atmospheres of Venus and Mars results.A discussion of the noble gas data for the terrestrial atmosphere indicates that these can be readily reconciled with an impact origin.