THE CRASH THAT MADE OUR MOON
Understanding how our lunar companion was formed might just explain how we came to be here
It’s the brightest thing in our night sky. Over the course of history it has been revered as a god, trampled across by 12 men and immortalised in poetry. The Moon is our steadfast companion, our only natural satellite as we endlessly orbit the Sun. Yet for an object that has received such scrutiny, arguments rage about where exactly the Moon came from. A suitable explanation needs to take into account what is perhaps the Moon’s greatest oddity: its size. It is the fifthlargest moon in the Solar System, trumping most of the satellites of our much bigger planetary neighbours. If you compare the size of moons to the size of their host planet, ours comes out at the very top. Many of the smaller moons of the Solar System are thought to be captured worlds – bodies that wandered too close to a planet before getting snared in its gravitational pull. Given the size of our Moon, it’s hard to imagine that’s how it ended up circling Earth.
As far back as 1878, George Darwin – the astronomer son of famous naturalist Charles
Darwin – instead proposed that Earth and the Moon were once one body and that the latter formed from material thrown off the spinning Earth. This, he said, would explain why the Moon was moving a little further away from us each year. Supporters of this idea even pointed to the lack of land in the Pacific Ocean – which stretches across half of our planet – as the birthplace of the Moon. However, scientists later realised that any force capable of dislodging such a large amount of Earthly material would likely have destroyed the rest of our planet at the same time.
Attention turned instead to the idea of a giant impact – one that occurred 4.5 billion years ago when a young Earth was still forming. It must have been this early because the rocks brought back from the Moon are that old. Astronomers have long believed that the Solar System had a tempestuous infancy, throwing around huge lumps of rock and metal before eventually calming down. What if one of these objects – perhaps one the size of Mars – hit the young Earth, with the Moon forming out of the hot, spinning debris?
On the face of it this idea makes a lot of sense. We know from the dark patches on the lunar surface that parts of the Moon were once molten. The Moon also has a pretty small iron core – much smaller than Earth’s – and it is less dense than Earth. This also fits, because during an impact the lightest material would have been thrown the furthest, leaving the heavier stuff here on Earth. Astronomers have a name for this proposed Marssized impactor: Theia, named after the Titan who gave birth to the Moon goddess Selene in Greek mythology. Computer modelling has been used to try and figure out what this impact must have been like in order for it to form the modern Moon.
“If the Moon was mostly formed from a smashedapart Theia during a glancing blow with Earth, it should have its own unique oxygen isotope signature. Yet instead it matches Earth’s exactly”
Traditionally the best fit seems to come from a glancing blow – Theia clipping Earth at an angle of about 45 degrees – and at a relatively slow speed. The debris from the impact, mostly formed from the leftover remnants of Theia, would have formed a ring around Earth, which then coalesced into the Moon. But recent analysis of Moon rocks returned to Earth during the Apollo missions appears to throw a spanner in the works.
It is all to do with isotopes. What sets different chemical elements apart is the number of protons present in the nucleus of their atoms. Oxygen, for example, always has eight. Add another proton and you get an entirely different element – fluorine in this case. But several versions of the same element can exist, each with the same number of protons but a differing number of neutrons. Scientists call these different flavours of the same element ‘isotopes’. Oxygen, for example, has three stable isotopes, with eight, nine or ten neutrons.
When it comes to planetary geology, the relative amounts of each of these isotopes present on a celestial object are a key measurement, a bit like a fingerprint. “Each body in the Solar System has a unique oxygen isotope signature,” says Dr Kun Wang, assistant professor of Earth and planetary sciences at Washington University in St Louis. And therein lies the problem. Analysis of the Apollo samples shows that Moon rocks have exactly the same oxygen isotope signature as Earth. If the Moon was mostly formed from a smashed-apart Theia during a glancing blow with Earth, it should have its own unique oxygen isotope signature. Yet instead it matches Earth’s signature exactly.
Scientists first discovered this as far back as 2001, but many believed that this apparent similarity was just an artefact of the precision of the experiments – that one day more accurate analysis would show there to be a tiny difference after all. However, the latest research published found that even with much more precise measurements, the oxygen isotope signature is still identical. Therefore the Moon did not form from Theia alone.
Wang believes this points to a much more violent collision, one which melted the outer layers of both Earth and Theia. This material then mixed together to form a vapour – a cloud of material – stretching from our planet out to 500 Earth radii. The Moon
then condensed from this cloud, explaining why both bodies now have the same oxygen isotopes. “Once they mix together it doesn’t matter what the oxygen isotopes of the two bodies were before,” says Wang. But if the notion of a more catastrophic impact is to be accepted, it needs more than one strand of supporting evidence, so that is exactly what Wang set out to find.
He analysed seven different Moon rock samples from multiple Apollo missions, along with samples of Earth rocks, measuring the different abundances of isotopes of potassium using a technique tentimes more accurate than previously possible. In October 2016, along with his colleague Stein Jacobsen from Harvard University, he published his results. He found that the Moon rocks had a greater abundance of one particular potassium isotope at the level of 0.4 parts per 1,000 more than Earth. “Potassium is a lot more volatile than oxygen, meaning it is more likely to vaporise and be mobile after the collision,” says Helen Williams, an Earth scientist at the University of Cambridge. The potassium was therefore much more likely to end up far away from Earth and become incorporated as part of the Moon. But for potassium to be vaporised in the first place, the collision must have vaporised both Theia and much of Earth’s surface. To Wang that has all the hallmarks of a head-on collision rather than a glancing blow.
“The Moon rocks had a greater abundance of one particular potassium isotope at the level of 0.4 parts per 1,000 more than Earth”
But even if he is correct there are still some outstanding Moon mysteries in need of explanation – none more so than the unusual tilt of the Moon’s orbit around Earth. The Moon would have initially formed in an orbit matching the orientation of Earth’s equator. As it moved further from our planet, the gravitational pull of the Sun would have forced it into line with the orbits of the other planets – a plane known as the ‘ecliptic’. Yet today’s Moon orbits at an angle of five degrees to the ecliptic. “That might not sound like much, but all the other big moons of the Solar System are inclined at less than a degree to their planets
– so the Moon really stands out,” says Douglas Hamilton, professor of astronomy at the University of Maryland. A team led by Hamilton has recently attempted to explain this strange anomaly. They ran many computer simulations of the giant impact, with slightly different parameters each time. The one that gave the closest match to the Moon’s current orbit suggests Theia’s impact was a lot more calamitous for our planet than previous models have suggested.
The almighty wallop from Theia would have sent Earth spinning much faster. More than twice as fast, in fact, as other previous models have suggested. What’s more, Earth would have been knocked over almost on its side, with its axis tilted somewhere between 60 and 80 degrees to the ecliptic – today it is only tilted by 23.4 degrees.
This high inclination affected the Moon as it retreated from Earth, forcing it into an orbit tilted at an angle of around 30 degrees to the ecliptic.
“It then settled down to five degrees over the last 4.5 billion years,” says Hamilton. At the same time Earth’s axis started to straighten up to its present position. It just goes to show that our ideas about the formation of the Moon are still very much in flux. Quite how we came to have such a large Moon on an inclined orbit is a puzzle still occupying teams of astronomers around the world. But it seems we are getting closer.
And that’s very important, because discovering the Moon’s history is a key step in understanding how likely such events are in the wider universe. This in turn might help us answer a much bigger question: whether we are alone in the universe. That’s because many scientists have speculated that the churning of the oceans by a Moon that was much closer to Earth than it is today could have played a key role in the early development of life on Earth. Its gravitational pull also stabilises Earth’s axis, keeping our seasons steady and reliable every year. This flurry of recent research has put us one step closer to understanding how our Moon came to be, and may one day help us understand our place in the universe.
“Earth would have been knocked over almost on its side, with its axis tilted somewhere between 60 and 80 degrees to the ecliptic”