A suite of approximately 12 meteorites (called SNC meteorites) have been identified as having originated from the planet Mars. The assignment of a Martian origin to the 12-meteorite suite is well documented. Members of the suite are assigned on the basis of similar oxygen isotopic compositions. Some individuals within the suite, however, contain trapped gases that arc chemically and isotopically indistinguishable from the Viking lander analysis of the Martian atmosphere. This is an important observation as the Martian atmosphere is quite thin. Consequently, its chemical composition is continually being modified and is unique from Earth's and other planets.

The SNC acronym is short for shergottite-nakhlite-chassigny, three different lithologies within the suite, all of which are igneous. The shergottites are basalts or basaltic cumulates, the nakhlites are augitecumulates, and chassigny is an olivine cumulate. All members of the SNC suite have been subjected to isotopic analysis and radiometric dating. The nakhlites and chassigny meteorites yield consistent igneous ages of 1.2-1.3 billion years. This is old for an Earth rock, but is quite young for meteorites, which are typically 4.0-4.5 billion years old.

The shergottites, however, have been assigned a variety of igneous crystallization ages that range from 4.5 billion years to 350 million years. It will be argued that combining petrography with isotopic age dating allows a resolution of the shergottite age dilemma. The solution to this problem has interesting consequences for the geologic history of Mars.

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