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THERE have been more than 70 successful missions to the moon: fly-bys, orbiters, landers and of course 12 moonwalkers. After Earth, it is the most studied celestial object in our solar system. These missions have unlocked the moon鈥檚 geological history, determined its internal structure and measured its surface composition. The conclusions of those explorations stretch well beyond the barren lunar surface.

鈥淭he moon has been Earth鈥檚 sister through these last four and a half billion years,鈥� says , a lunar geologist at the University of Manchester, UK. Like all siblings, the moon has secrets to tell.

The same astronomical processes that have influenced Earth have also been felt by the moon. Yet while weathering and the restless shifting of the continents on our planet have largely erased the most ancient events from our geological record, that isn鈥檛 true of moon rocks. 鈥淭he moon is a tape recorder of terrestrial processes,鈥� says Joy.

Decoding the tape began in earnest 50 years ago, when the first moon rocks were collected by Apollo 11鈥榮 Neil Armstrong and Buzz Aldrin. During a 2 hour and 36 minute moonwalk, they pocketed 22 kilograms of the lunar surface, then brought it back to Earth for analysis. Another five Apollo missions added to the tally, returning a total of 2200 samples that collectively weigh 382 kilograms.

The dust and rocks kept at the Johnson Space Center in Houston, Texas, are treated as a priceless scientific and cultural resource. 鈥淥f the 2200 numbered samples, all but six have been looked at in some manner or another,鈥� says Ryan Zeigler, NASA鈥檚 lunar sample curator. About half of each sample is kept in reserve for future study. And for good reason. Over the years, improved instrumentation has allowed us to make ever more sensitive measurements. 鈥淓ach one of those opens up a new avenue of study about the moon,鈥� says Zeigler.

It also allows us to re-examine old questions in light of more precise information. Of these, the biggest question is how the moon formed. 鈥淲e look up in the sky and see the moon and we want to know why it鈥檚 there, and how it got there,鈥� says , an astrophysicist at the University of Bristol, UK. But researchers keep revisiting this question and changing the answer bit by bit.

Astronomers have toyed with many ideas about the moon鈥檚 origin. Perhaps Earth was spinning very fast and a piece broke off? Or maybe the moon was wandering through space and was captured by our gravity? In 1946, Canadian geologist Reginald Aldworth Daly proposed what we now think is the right idea: . In their first investigations of the Apollo moon rocks, geologists found good evidence that this was the case. The moon rocks looked sufficiently similar to Earth rocks to suggest that the pulverised impactor had been mixed with a large portion of Earth debris.

Modern reanalysis shows that the moon rocks are in fact almost identical to Earth鈥檚. In terms of the giant impact formation model, that means just one thing: 鈥淭hey had such a big impact that they were totally, intimately mixed up,鈥� says Leinhardt.

Her colleagues Simon Lock and Sarah Stewart developed a new model in which Earth was hit so hard that it melted, absorbing the impactor and surrounding itself with a doughnut shaped cloud of vaporised rock. They think the moon formed out of this, explaining the similarity of the rocks.

鈥淭he moon rocks have given us a huge amount of information. What we need is to be able to make an entire story,鈥� says Leinhardt. This involves using computers to simulate this cataclysmic event from the moment of impact to the birth of the moon.

At present, the simulations can鈥檛 follow the process from beginning to end. They can simulate the formation of the debris doughnut, called a synestia, but can鈥檛 follow its condensation into the moon. And while they can keep track of the temperature and pressure of the synestia, they don鈥檛 include any chemistry. To make progress, Leinhardt says better computing rather than more exploration is necessary.

Although Leinhardt doesn鈥檛 think new samples from the moon will be helpful for her investigations, what happened to the moon after it formed has got other researchers itching for a return mission.

Even a casual glance at the moon reveals dark markings across its surface. They are thought to have begun forming during a relatively short period called the late heavy bombardment. Evidence came from the Apollo samples, many of which are about 3.9 billion years old. These suggest a period lasting somewhere between about 20 million and 100 million years in which the moon and the rest of the solar system were heavily pummelled by asteroids, creating large impact basins we see on the moon鈥檚 surface.

That may not be entirely accurate. 鈥淥ur ideas are changing as we are reanalysing those samples,鈥� says Joy. Part of that reanalysis is intended to help understand where the moon rocks came from. None of the Apollo samples were bedrock 鈥� rocks sampled in the place where they formed 鈥� and this has robbed geologists of the geographical context needed to fully interpret their results.

It is now thought that most of the Apollo samples could be the debris ejected during the formation of the Imbrium basin, a vast crater formed 3.9 billion years ago. If so, the late heavy bombardment never happened 鈥� we were fooled into believing it did by a single huge event that scattered rocks across the lunar nearside.

鈥淭his could be a massive bias in how we鈥檝e been interpreting these samples,鈥� says Joy. 鈥淔or the next generation of spacecraft, actually picking places where we can go to sample bedrock is going to be really, really important.鈥�

Migrating giants

In this way, the true ages of other basins can be determined. And it will show whether there was a short, sharp late heavy bombardment or a continual rain over a longer period.

It isn鈥檛 just about the Earth and the moon, either. If moon rocks have taught us anything, it is that the entire solar system is connected. Indeed, knowledge about the surfaces of Mercury, Venus and Mars has come from counting craters on the moon and relating that to the ages of the moon rocks. The late heavy bombardment, however it unfolded, was probably caused by gas giant planets including Jupiter migrating through the solar system, knocking asteroids out of their way, some of which sailed towards Earth.

Because our solar system is thought to form in essentially the same way as other planetary systems, we are now realising that the moon can teach us about things beyond the reaches of our star鈥檚 influence. 鈥淭here are lots of giant impacts that happen at the end of a solar system鈥檚 formation. That would happen in extrasolar systems as well,鈥� says Leinhardt. She says that understanding giant impacts is the key to grasping the diversity of those planetary systems, and comparing them with our own. It may even help tell us which exoplanets are likely to be habitable, because in our solar system the moon鈥檚 gravitational pull stops Earth toppling over, keeping its climate stable.

鈥淢oon rocks have told us about so many other places than just the moon,鈥� says Zeigler. And with a return on the cards in the near future, who knows what family secrets our sibling still has to share.