ALH84001 is the oldest-known SNC meteorite. It originated on the surface of Mars, is gray-green in color, measures 15 × 10 × 8 centimeters (6 × 4 × 3 inches), and weighs 1.94 kilograms (4.2 pounds). It was recovered by a team from the ANSMET program on 27 December 1984, from the Allan Hills region of Antarctica, where it had lain undisturbed since its arrival on Earth about 13,000 years ago.


Geological analysis of ALH84001 has revealed something of its history. It formed originally from molten lava which possibly issued from an ancient martian volcano, about 4.5 billion years ago. Some 4 billion years ago, it was heated again and deformed by a strong shock, probably resulting from the nearby impact of an asteroid or large meteorite. Then, about 3.6 billion years ago, some kind of liquid flowed through the rock and deposited rounded globules of carbonate minerals. About 15 million years ago, ALH84001 is believed to have been hurled from the surface of Mars when an asteroid or comet collided obliquely with the planet. Finally, it became the center of a major controversy in 1996 when scientists at the NASA Johnson Space Center, led by David S. McKay, claimed that they had found inside the meteorite very small fossils and other evidence of martian biological activity centered around the carbonate deposits.