A paper published in this week's issue of the journal Nature presents evidence that the Earth might once have had a second moon, based on a puzzling problem with the geology of the Moon's surface. Even to the naked eye, it's possible to differentiate the Moon's surface into dark flat volcanic planes, the seas, and much lighter coloured rougher mountainous regions, the highlands. The visible shape of the boundary between these regions is what some have romantically called "The Man in the Moon".
What's puzzling is that on a body as large as the Moon, you'd expect the flat planes and the mountainous highlands to be randomly distributed over its surface. In fact, the seas are tightly clustered together on one side of the Moon, whilst the other side is much rougher. Such a large inhomogeneity seems beyond what could be expected from random processes alone.
In this week's issue of Nature, this puzzle has led Martin Jutzi and his colleagues at the University of California reconsider the traditional view of how the Moon formed. The standard view is that around 4.5-billion years ago, a planet around the size of Mars, sometimes called Theia, collided with the Earth. Both planets were entirely melted by the energy of the impact, and a small globule of this molten rock separated from the rest to become the Moon.
Jutzi considers what would have happened if, instead of a single moon forming out of this collision, in fact two moons had been formed. His conclusion is that two moons could have co-existed in orbit around the Earth for around a hundred million years, but they would eventually have collided. Could such a collision have led to the lop-sided shape of the Moon that we see today?
One obvious problem with that idea is described rather well by Jutzi in his paper: when astronomical bodies collide, the result is usually to make holes in things -- craters and basins. In this case, something made mountains on one side of the Moon. So, Jutzi sets out to investigate whether a collision could ever deposit mountains onto the surface of a body. And he argues that if the relative speed of the two bodies is small enough, there isn't enough energy to excavate a large crater, and instead the result is a large pile of rubble rather resembling mountains.
An interesting further outcome of Jutzi's computational models is that, in this scenario, the molten magma in the interior of the Moon is pushed towards the Moon's opposite hemisphere by the force of the impact, which might in turn lead to increased volcanism around the point on the Moon opposite to the point of impact, and so the formation of volcanic planes, as we in fact see.