A potato from another world
Inside a secure laboratory at Johnson Space Center lies a 1.93-kilogram rock that looks like a large potato. This stone, designated Allan Hills 84001, is an igneous rock that crystallized from magma on Mars about 4.091 billion years ago. It is one of the oldest known rocks from our solar system. For billions of years it sat on the Martian surface until a powerful asteroid impact blasted it into space around 17 million years ago. After a long journey through the solar system, it entered Earth's atmosphere and landed on an Antarctic ice field around 13,000 years ago.
On December 27, 1984, a team of American meteorite hunters with the ANSMET program discovered it in the Allan Hills region of Antarctica. Initially, its Martian origin was not recognized. It was classified as a diogenite meteorite and stored. A decade later, in 1993, chemical analysis revealed its true origin. Trapped gases within the meteorite perfectly matched the atmospheric composition of Mars as measured by the Viking lander in 1976, confirming its Martian origin. ALH84001 is unique among Martian meteorites because it formed during a period when liquid water may have been present on the planet's surface.