When were meteorites from Mars first recognised?
In the early 1980s. Gas trapped in the Antarctic meteorite EETA79001 matched the Martian atmosphere measured by the Viking landers, confirming a Martian origin.
Some meteorites had long puzzled scientists. The shergottites, nakhlites and chassignites, named after falls at Shergotty in India in 1865, Nakhla in Egypt in 1911 and Chassigny in France in 1815, were volcanic rocks much younger than most meteorites, crystallising about 150 million to 1.3 billion years ago. They needed a parent body that had been volcanically active recently. Mars was a candidate, but how could rocks escape from a planet?
Clues from the 1970s. As dating improved, the young ages of these meteorites, together called SNC meteorites after their initials, became clearer. Some scientists suggested a Martian origin.
The key evidence. NASA's Viking landers, which arrived on Mars in 1976, measured the composition of the Martian atmosphere, including gases such as nitrogen, argon, krypton and xenon, with distinctive isotope ratios. In 1983, researchers analysed gas trapped in pockets of shock-melted glass in EETA79001, a shergottite found in the Elephant Moraine area of Antarctica in 1979. The trapped gas matched the Viking measurements closely, and differed from Earth's atmosphere. This was strong evidence that the rock came from Mars.
How rocks escaped. Studies of impact physics showed that large impacts could launch surface rocks at speeds exceeding Mars's escape velocity without completely melting them.
Today. Several hundred Martian meteorites are known, including ALH 84001 and NWA 7034. Their oxygen isotopes form a distinct group, providing a further fingerprint.
These meteorites remain our only samples of Mars until samples are returned by spacecraft.