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Where does the moon come from?

Whether we’re trying to find out where it came from, or how to siphon off some of its energy, grappling with the moon is harder than it looks.

New Statesman
A 'Super Moon' rises over Sydney. Image: Getty

When the Apollo astronauts brought back pieces of lunar rock, the samples promised to answer the question of the moon’s origin. It’s a question we are still asking.

The “big splash” theory of a gargantuan collision between two planets is the favourite. The idea is that a Mars-sized object hit the young earth, throwing off a load of matter which coalesced to form the moon.

Scientists became convinced that the big splash theory must be correct because it calls for the stuff in the moon to be lighter than the atoms making up earth. Initial analysis of the relative abundance of various forms of atoms (known as isotopes) in the Apollo samples provided supporting evidence.

However, researchers then thought to take account of the effects of a few billion years of bombardment by high-energy subatomic particles called cosmic rays. Because earth is protected from cosmic rays by its magnetic field, these would change the moon’s isotope abundances only and in particular ways. Unfortunately, all this has been a dampener on the big splash theory.

Other theories are available. The moon could simply have formed independently at the same time as the earth, for instance. Or it could have been a passing body that fell into our planet’s gravitational field and got trapped.

Most planetary scientists remain convinced that the big splash is right but to convince themselves and others they have to work out a consistent story. That’s why they gathered to sift through all the evidence at the Royal Society in London on 23 and 24 September.

Despite the lack of consensus, scientific achievements in this area are astonishing. We are narrowing down the timings of events that occurred 4.5 billion years ago. Some of the research that was presented showed that the moon is roughly 100 million years younger than we had thought. This kind of forensic analysis of vaporised rock is an extraordinary feat.

If only our progress in harvesting lunar energy was as extraordinary. Most experts are convinced that there is a way to profit from the moon’s gravitational pull on the oceans, but the devil is in the detail.

The Scottish government recently gave the go-ahead for the Pentland Firth to host Europe’s largest tidal energy project. It is estimated that the Pentland Firth could eventually meet half of Scotland’s electricity needs, but for now engineers are aiming to have 40 per cent of homes in the Scottish Highlands running off lunar power by 2020.

In many ways, it’s a great leap forward. Yet meeting 40 per cent of the needs of one of the UK’s less inhabited regions also seems a little underwhelming. One of the benefits of being a small island is that Britain has copious tidal and wave power at its disposal: enough to meet a fifth of our electricity needs.

Whether we’re trying to find out where it came from, or how to siphon off some of its energy, grappling with the moon is harder than it looks.