In the 1420s Jan van Eyck began to paint something no one in the West had thought to depict before, as far as we know, though all had seen it: the face of the Moon. It hangs, realistically detailed, in the sky behind the crosses of the crucifixion. It sits above the armies of the knights of Christ and over John the Baptist. Pairing close observation with imagination, van Eyck gave the face of the Moon its rightful place, as part of the human world.
Two centuries later, telescopes gave that face new detail. People started to think of the Moon not as part of their world but as a world of its own—a place with mountains and seas, bays and storms, with an atmosphere (a term used of the Moon before it was applied to the Earth) and, quite possibly, humans. In the years that followed, the more imaginative of those additions drained away. By the time people actually got there it was known to be deeply dissimilar to the Earth, a dead, drab, alien counterpart to our planet’s richness.
Science, though, thrives on finding similarities between apparently disparate things. A dolphin looks like a shark—but as a mammal and a social hunter it is more like a wolf. The coasts of Uruguay and Namibia appear quite different—but the rocks of which they are made are identical, laid down together in the same ancient sea before the opening of the South Atlantic pulled them apart. Perhaps most famously, the fall of an apple in a Lincolnshire garden, and the monthly swing of the Moon around the Earth, are manifestations of the same gravitational attraction. And in the 1970s, chemical analysis of Moon rocks showed that though the Moon looked nothing like the Earth, its crust was made of the same mixture of elements, in strikingly similar proportions.
The idea that the Earth and Moon might be made of the same stuff seems sensible at first glance; compared with most objects in the solar system, they are remarkably near each other. In the 19th and early 20th centuries there were various theories that they were formed together and subsequently separated. Unfortunately such arguments had a problem. The conservation of angular momentum means that spinning things go slower when pushed apart, faster when closer together (this explains why ice skaters spin so fast when they tuck their elbows in). The Earth-Moon system has a great deal of angular momentum, so if its two parts had been joined when they were created they would have had to spin as fast as an ice skater turning into a drill bit and boring her way down through the rink.
Hence suggestions that the Moon was created elsewhere and captured by the Earth’s gravity. But once the Apollo programme had shown how chemically earthlike the moon was, they seemed implausible too. In 1974 scientists hit on a new idea: that a wandering planet the size of Mars had struck the Earth a glancing blow. This, they suggested, caused the cores of each body to merge and part of their rocky mantles to be thrown into orbit, where the molten mess eventually condensed into the Moon. The catastrophic arrival of the impactor explained both why Moon rocks look earthly and why the Earth-Moon system has a lot of angular momentum—if one skater grabs another’s outstretched arm as he passes, the two will be set a-spinning.
It was a simple idea, but radical. And it had profound implications. The fact that the Earth has a big moon stops its axis from wobbling about too much—Mars, which has titchy moons, swings its poles around like a drunken drum majorette. A stable alignment of the poles may have kept the Earth’s climate stable too, and thus hospitable towards life. So an accidental collision in the early solar system could have been crucial to the emergence of complex life on Earth. We could be the result of a planetary fluke.
Now this account too is under attack. The more people look at Moon rocks, the more spookily similar to Earth rocks they seem. Yet computer models of the impact that might have created the Moon show that it would have ended up with much material from the body that hit it. This suggests either that the impactor was chemically similar to the Earth (which Mars and the asteroid belt are not), or that the rocks of the impactor and the Earth that it hit were mixed together with spectacular thoroughness by some as yet unidentified process.
Or that there is some other explanation. Robin Canup, whose work shows that much of the impactor would end up in the Moon, suggests that studying rocks from Venus—the closest planet to Earth—could provide clues. But sampling them would be immensely pricey. My less informed hopes lie at the other end of the solar system. In 2015 a spacecraft called New Horizons will provide the first close-up pictures of the dwarf planet Pluto and its moon Charon.
Pluto is utterly unearthlike. It is made more of ice than rock; if it has an ocean it is deeply buried. But there is a real chance that it and Charon are also the products of a cosmic collision. Might new insights into that collision help to explain the Earth’s earliest history? Perhaps not. But part of the fun of science is not knowing what is similar until you look. And what an enticing prospect: that in the 15 hours the spaceship spends as close to Pluto as the Moon is to Earth, it could find answers to a millennia-old mystery.