Meteorites from the Moon and Mars

A few hundred meteorites are pieces of the Moon or Mars. How do we know, and what can they tell us?

Escaping a world

To throw a rock off the Moon or Mars into space, it must be launched faster than the escape velocity: about 2.4 kilometres per second on the Moon and about 5 kilometres per second on Mars. Only a large asteroid impact provides that kind of energy.

For a long time, scientists doubted that rocks could be launched without being melted or crushed to powder. But studies of impacts showed that rocks near the surface, at the edge of the impact zone, can be accelerated quickly while being shocked relatively little.

After escape, these rocks orbit the Sun. Some reach Earth within a few thousand years, while others take millions. Lunar meteorites usually arrive much sooner than Martian ones, because the Moon is so close.

Lunar meteorites

The first meteorite recognised as coming from the Moon was found in Antarctica in 1982 and named Allan Hills 81005. Scientists compared it with the samples collected by the Apollo astronauts and Soviet Luna missions and found a clear match in its minerals, chemistry and oxygen isotopes.

Lunar meteorites are especially valuable because the Apollo and Luna samples came from a small area of the near side. Meteorites come from random places all over the Moon, including the far side. Many are breccias from the bright highlands, rich in a pale mineral called plagioclase feldspar. Others are dark basalts from the lunar seas, the maria.

Lunar meteorites often barely react to a magnet, because they contain very little metal. Several hundred lunar meteorites are now known, though many are paired pieces of the same fall.

Martian meteorites

Martian meteorites were once called SNC meteorites, after three examples: Shergotty, which fell in India in 1865; Nakhla, which fell in Egypt in 1911; and Chassigny, which fell in France in 1815.

The proof that they come from Mars arrived in the early 1980s. Scientists found gas trapped in shock-melted glass in the Antarctic meteorite Elephant Moraine 79001. The gas had the same unusual mix of noble gases and nitrogen that the Viking landers had measured in Mars's atmosphere in 1976.

Martian meteorites are mostly volcanic rocks. Many are much younger than asteroid meteorites, showing that Mars was volcanically active long after small asteroids had cooled. They are our only samples of Mars until missions return rocks from its surface.

Did you know?