A MYSTERIOUS bulge on the far side of the moon is providing clues to its exotic orbit billions of years ago.
Our moon is thought to have formed after a Mars-sized body smashed into Earth some 4.5 billion years ago. The collision sprayed molten debris into space, which then clumped together and eventually solidified to form the moon at a distance of about 4 Earth radii from the Earth鈥檚 centre. Over billions of years, the moon spiralled outwards to its present nearly circular orbit at about 60 Earth radii. Because its spin period is equal to its orbital period, one side always faces away from us.
Measurements by lunar satellites since the 1960s have shown that the moon鈥檚 gravity is slightly stronger than expected on the axis pointing towards Earth, as if it had a bulge on the far side. More than 200 years ago, the French mathematician Pierre-Simon Laplace figured out the same thing, by analysing subtle wobbles in the moon鈥檚 spin and orbit. 鈥淚t鈥檚 pretty scary how clever that was,鈥� says Ian Garrick-Bethell of the Massachusetts Institute of Technology.
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鈥淭he moon鈥檚 gravity is slightly stronger than expected on the axis pointing towards Earth, as if it had a bulge on the far side鈥�
The bulge is likely to have formed when the moon solidified, and to find out how the moon would have behaved at the time, Garrick-Bethell and his colleagues calculated which combinations of ancient orbits and spin rates could have allowed this to happen. Out of several possible combinations the team found, Garrick-Bethell favours a strongly elongated ancient lunar orbit in which the moon spins 1.5 times per orbit. Mercury has this exact ratio today: it rotates three times for every two revolutions it makes around the sun. 鈥淲e鈥檝e seen in Mercury that this is a plausible and stable orbit,鈥� he says. Such an orbit would have reinforced the bump as the moon solidified, leading to the excess gravity along the Earth-moon axis we see today (Science, vol 313, p 652).
The highly elliptical orbit would have made dramatic viewing 4.3 billion years ago. The moon would have waxed and waned in less than 24 hours, and it would have grown and shrunk as it swung closer and farther from the Earth. It would also have revealed the now hidden far side. 鈥淚t would have been cool to see the other side of the moon,鈥� says Garrick-Bethell.
