How do we know a meteorite came from Mars?
Gas trapped in glass inside some Martian meteorites matches the unusual atmosphere measured on Mars by the Viking landers in 1976. Their young ages, chemistry and oxygen isotopes also set them apart from all other meteorites.
For years scientists suspected that a small group of meteorites was special. They were igneous rocks, formed from lava or magma, yet some were only a few hundred million years old, far younger than the roughly 4.5 billion years typical of asteroid meteorites. A small asteroid would have cooled long before then, so these rocks needed a large, geologically active parent body. Named after the meteorites Shergotty, Nakhla and Chassigny, the group was called the SNC meteorites, and Mars was the leading suspect.
The decisive evidence came from gas. In 1976 NASA's two Viking landers measured the Martian atmosphere directly, recording its particular mix of gases and isotopes, including nitrogen and noble gases such as argon and xenon. In the early 1980s researchers studied the shergottite EETA 79001, found in Antarctica, which contains pockets of glass melted by shock when it was blasted off its parent planet. Gas trapped in that glass matched the Viking measurements closely and did not match Earth's atmosphere. It was, in effect, a sealed sample of Martian air.
Other clues fit the same picture. All Martian meteorites share a distinctive oxygen isotope signature, a chemical fingerprint that differs from Earth, the Moon and most asteroid groups, showing that they come from one parent body. Their chemistry, including the proportions of certain elements, matches measurements made by rovers and orbiters, and they contain water-bearing minerals and oxidised iron consistent with Mars.
Today a few hundred Martian meteorites are known, including the famous ALH 84001, the 2011 fall Tissint, and the regolith breccia NWA 7034. Missions are now preparing to return samples collected by the Perseverance rover, which would allow direct comparison with known locations on Mars, although the timetable for that return is uncertain.