ORBITAL EVOLUTION OF IMPACT EJECTA FROM MARS

The orbital evolution of material ejected from Mars into heliocentric orbits is investigated, with particular emphasis on the origin of the shergottite, nakhlite, and chassignite achondrites. Two models are considered. In the first, meteorite-size bodies are ejected directly from Mars. In the second, the ejecta are ∼ 15 m diameter bodies, that are subsequently fragmented by collisions in space. In both cases a large fraction (∼ 35%) of the objects that will ever reach Earth do so within 10 m.y. For the “small body” model, it is found that about 0.03% of the Mars crater ejecta must be accelerated to the Mars escape velocity; the “large body” model requires an efficiency of 0.4%. Assuming that the acceleration of large bodies to be less probable, the results indicate that meteorites originating as small bodies should dominate the terrestrial flux of Mars ejecta. This result is in general agreement with data from SNC meteorites, but the reported short exposure age of EETA 79001 is hard to understand in a pure small body model. The yield of meteorites from Mercury is found to be at least a factor of 100 lower than from Mars.