How do we know a meteorite came from the Moon?
Lunar meteorites match the rocks and soils brought back by the Apollo and Luna missions in their minerals, chemistry and oxygen isotopes. The first was recognised in 1982 from a stone found in Antarctica, ALH 81005.
We have a huge advantage with the Moon: we already hold samples of it. Between 1969 and 1972 the Apollo astronauts brought back about 382 kilograms of lunar rock and soil, and the Soviet Luna probes returned smaller amounts. Those samples gave scientists a detailed reference library of what Moon rocks look like and what they are made of.
In January 1982 an American team in the Allan Hills region of Antarctica collected a small greenish-grey stone of about 31 grams, later catalogued as ALH 81005. When researchers examined it, they saw it was a breccia, a rock made of broken fragments, rich in white anorthosite, a calcium-rich feldspar rock typical of the lunar highlands. Its mineral chemistry and composition matched Apollo highland samples, and later in 1982 it became the first meteorite recognised as coming from the Moon. That discovery also proved that impacts could launch rocks from a large body to Earth, which strengthened the case for Martian meteorites. Earlier Japanese Antarctic finds, such as Yamato 791197, were then also shown to be lunar.
Several tests are used today. Lunar meteorites have the same oxygen isotope composition as Earth, which rules out most asteroid groups, but they lack the water-bearing minerals and oxidised iron common in Earth rocks. Ratios of elements such as iron to manganese in their minerals match lunar samples. Many contain gases implanted by the solar wind, which strikes the airless lunar surface directly.
Lunar meteorites are now valued because they come from random places on the Moon, including the far side, which Apollo never sampled. China's Chang'e 5 and Chang'e 6 missions have since returned new samples, the latter in 2024 from the far side, adding to the reference library.