Solid clues to the Moon’s history

The Apollo 11 astronauts brought home 22 kg of rocks and soil samples. Later missions scooped up more. In just six visits, the Apollo program bagged hundreds of samples of the Moon’s surface, almost 400 kg in total. The unmanned Soviet landers collected material too, about 300 g.

These rocks weren’t souvenirs. They were gathered to answer a hard question: how did the Moon form?

All planets in our solar system except Venus and Mercury have moons. Even dwarf planet Pluto has at least five. Most moons probably formed in the early solar system from discs of gas and dust around their parent planets, NASA says. But our Moon is big compared to our planet. Did it form differently?

One early idea was that the Moon originally wandered free but was captured by Earth’s gravity. Another had it that the Moon was a chunk of the molten early Earth that broke away.

The Giant Impact model, the leading idea since the ‘70s, suggests that a Mars-sized object smashed into the early Earth, and debris from it and the Earth mixed to form the Moon. The Apollo rocks at first lent this model some support, but they also challenge it: many elements in the rocks have isotope ratios the same as Earth’s. There’s no sign of different material from a giant impactor.

Analysing the Apollo rocks

When Apollo 11 returned with its haul of rocks, Australian National University (ANU) geochemist Professor Ross Taylor was one of the first to analyse them.

Visiting the NASA analysis labs after a conference, he showed he knew about the equipment – not surprising, as he’d set up an almost identical lab at the ANU eight years before.

NASA asked him to stay and carry out the first spectrographic analysis of Moon rock, identifying the elements in the rocks.

Taylor received the first samples at 11.45 am on 28 July, 1969. By 4 pm he was delivering preliminary results to a press conference.

For the next few weeks he worked under pressure, in strict quarantine and often around the clock, to deliver results to the press each day.

More Moon rocks, for free

While we’ve learned a lot from the Apollo rocks, they aren’t representative of the whole Moon. They come from six sites on the near side of the Moon, in a region covering less than 5% of the lunar surface. A later orbiting mission, Lunar Prospector, showed that the chemical composition of the near and far sides of the Moon is strikingly different in some ways (for instance, the near side has much more thorium and potassium).

But we do have some rocks that sample more of the Moon’s surface. So-called Lunar meteorites are chunks of rock blasted off the Moon’s surface when other meteorites hit it. Some of the chunks escape the Moon’s gravity and make their way to Earth.

We now know of hundreds of lunar meteorites: one in every thousand newly discovered meteorites comes from the Moon. Most have been picked up in the deserts of North Africa or Antarctica. One comes from Calcalong Creek in Western Australia.

Lunar meteorites weren’t recognised as Moon rocks until 1982, when their similarity to the Apollo rocks was noticed. The Moon has been throwing free samples at us, but we had to visit it before we could realise what treasure it’s handing out.

The Moon’s story is unfinished

We still haven’t settled how the Moon formed. One recent idea is that over millions of years a number of collisions chipped away at Earth. Each collision created debris that formed a tiny satellite – a moonlet – and the moonlets eventually merged to form the Moon.

This model will be worked through and argued over. The Moon rocks remain a hard core of evidence to test it with.