A 4-GYR SHOCK AGE FOR A MARTIAN METEORITE AND IMPLICATIONS FOR THE CRATERING HISTORY OF MARS

Analyses of meteorites that originated on Mars provide important insig hts into the geological and atmospheric evolution of the planet. Such analyses have hitherto been restricted to relatively young martian roc ks' (the oldest martian meteorites have an age of approximately 1.3 bi llion years). But the recently recognized(2) martian meteorite, Allan Hills 84001, which is distinct from the other martian meteorites(2-4), shows evidence for a much older age(5,6). Here we report an analysis of the shock-alteration history of this meteorite based on argon isoto pe dating, from which we derive a shock age of 4.0+/-0.1 billion years . The age and geological history of this meteorite suggest that it cam e from the heavily cratered Noachian-age terrains of Mars's southern h emisphere, and it may thus provide an absolute chronology for this reg ion of the planet, independent of that inferred from the cratering rec ord. The shock age of the meteorite also coincides with that of the so -called Lunar Cataclysm (a relatively short period during which many o f the craters on the Moon are believed to have formed), supporting the idea(7) that intense bombardment was widespread throughout the inner Solar System between 3.9 and 4.1 billion years ago.