nice reference

ALH84001 is an Shergotty, Nakhla, and Chassigny (SNC) meteorite discovered in Antartica in the Allan Hills ice field in 1984. It is an old (4.5 by), 1.9 kg, softball sized meteorite. It was not recognized to be a Martian meteorite until 1994 when similarities to the other Martian meteorites (the SNC's) were noted. Why are the SNC's thought to be from Mars?

The SNC's have properties which suggest they come from a large planet--a planet-- rather than from an asteroid. All but one are young (<1.3 by), much younger than the typical ignoeous meteorites from asteroids (who are 4.5 by old), they have higher contents of volatiles (than older meteorites), and contain minerals with ferric iron. The strongest evidence for a Martian origin, however, comes from the study of gases trapped in their interiors (actually, the study of one SNC, EETA79001). The rather amazing result of

was found. The Martian atmospheric composition was determined by Viking in 1976.

How Did ALH84001 Get to Earth?

Between 3.6 and 4 billion years ago, water seeped below the surface of Mars through cracks in the crust. The water was saturated with carbon dioxide and carbonate minerals were deposited in the fractures. Biology could have contributed to the formation of the carbonates. The notion is then that 16 My ago, a large object struck Mars digging a crater 10 - 100 km in diameter which ejected ALH84001. It traveled through space for millions of years until it was captured 13,000 y ago.