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Martian meteorite. Artwork of a lump of rock (centre) ejected from Mars after a giant impact on Mars' surface (seen at upper left). Rocks such as this have fallen to Earth as meteorites, allowing study of Mars' history. A meteorite discovered in Antarctica in 1984, ALH84001, contained tube-like structures some thought to be the fossils of bacteria and similar microorganisms. The rock was around 4.5 billion years old. It was blasted from Mars by an impact around 16 million years ago, as seen here, before landing on Earth around 16,000 years ago. (KEYSTONE/SCIENCE PHOTO LIBRARY/DALE DARBY)