Allan Hills 81005

A golf-ball-sized rock found in Antarctica in 1982 – the first meteorite recognised as coming from the Moon.

Key facts

Type

Lunar meteorite (anorthositic regolith breccia)

Fell or found

Found

Year

1982 (18 January)

Country

Antarctica (Allan Hills)

Mass

about 31 g

Official name

Allan Hills A81005

On 18 January 1982, a member of a United States meteorite-collecting team was searching the blue ice of the Allan Hills in Antarctica when he spotted a small, dark rock. It weighed only about 31 grams, roughly the size of a golf ball. It was collected, bagged and labelled ALH 81005, meaning that it was found in the Allan Hills during the 1981 to 1982 field season. When scientists examined it, they realised that it was unlike any meteorite found before. It was a piece of the Moon.

Recognising a lunar rock

At first glance, ALH 81005 looked like a breccia, a rock made of broken fragments cemented together. Inside were white and grey fragments in a darker matrix. Scientists compared it with the lunar samples brought back by the Apollo astronauts between 1969 and 1972, and with samples collected by Soviet robotic missions. The resemblance was striking. Its minerals, its textures and its chemistry matched those of rocks from the Moon's highlands, the bright, ancient regions of the lunar surface.

Crucially, its oxygen isotopes, the proportions of different forms of oxygen atoms, matched those of the Moon and Earth. Its chemistry, however, was clearly lunar, not terrestrial. It contained gases implanted by the solar wind, showing that its grains had once lain on the surface of an airless body. Brian Mason of the Smithsonian Institution recognised its likely lunar origin in 1982, and studies by many scientists, including the geologist Ursula Marvin, confirmed it in 1983.

A piece of the highlands

ALH 81005 is an anorthositic regolith breccia. Anorthosite is a rock made mainly of the mineral plagioclase feldspar, which is pale in colour. The Moon's highlands are largely made of anorthosite, which formed as light crystals floated to the top of a global ocean of molten rock early in the Moon's history. Regolith is the layer of broken rock and dust covering the Moon's surface, created by billions of years of impacts. ALH 81005 is a sample of that ancient, battered highland soil.

Its chemistry differs from the highland samples collected by Apollo, which came from a limited area on the side of the Moon facing Earth. Scientists think ALH 81005 may come from a different region, possibly far from the Apollo landing sites.

How did it reach Earth?

The discovery of ALH 81005 proved that impacts on the Moon can launch rocks fast enough to escape the Moon's gravity and eventually land on Earth. Escaping the Moon is much easier than escaping Mars, because the Moon's gravity is weaker. This finding helped persuade scientists that rocks could also be blasted off Mars, supporting the idea that the SNC meteorites, such as Shergotty, Nakhla and Chassigny, are Martian.

After ALH 81005 was identified, scientists re-examined other Antarctic meteorites and realised that some, found earlier by Japanese teams, were also lunar. Today hundreds of lunar meteorites are known, from Antarctica and from hot deserts.

Why it matters

ALH 81005 opened a new chapter in planetary science. It showed that the Moon delivers samples to Earth for free, offering material from regions that astronauts have never visited. Lunar meteorites now provide a much broader picture of the Moon's crust than the Apollo samples alone.

It is also a reminder of how important Antarctica is for meteorite science. On its blue ice, a small dark rock can be spotted easily, collected cleanly and studied with care. Without the Antarctic search programmes, ALH 81005 might never have been found. The United States programme that collected it, which began in the 1970s, has since recovered tens of thousands of meteorites, all carefully curated and made available to researchers around the world.

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Allan Hills A81005 in the catalogue