The Shergottite, Nakhlite, and Chassigny (SNC) meteorites are igneous rocks from Mars, inferred previously to have been propelled into space (and to Earth) by a single impact event. However, the sum of petrologic, chemical, and chronological data suggests two distinct sites of origin: S from one, NC from another. Differences between the S and NC include distributions of terrestrial residence ages, cosmic ray exposure ages, impact shock effects, preterrestrial aqueous alteration products, bulk magma compositions, trace element magma chemistries, and mantle source compositions. If the S and NC did not come from the same site on Mars, selection criteria for determining source craters are relaxed, and a numbr of craters become candidate sources. Formation of two meteorite source craters on young Martian surfaces (≤1300 Ma) within the last 20 m.y. (spanning the cosmic ray exposure ages) seems to require either smaller source craters or greater cratering rates than allowed in current theories. Both of these options suggest that meteorites from nonvolcanic and ancient Martian surfaces ought to be more common than the known types of Martian meteorites. The absence of such meteorites may reflect the physical properties of nonvolcanic and ancient surfaces, or a failure to recognize these rock types as meteorites. The ALH84001 meteorite, long classified as a diogenite, may be such a sample of the ancient Martian crust. ¿ American Geophysical Union 1995