The shape of the Moon might encode subtle clues about its orbit billions of years ago, say planetary scientists in the US. Their work has turned up one possible solution to a lunar mystery first identified in revolutionary France, two centuries ago.
Astronomers think the Moon formed when a Mars-sized body – about half the diameter of Earth – walloped into our planet about 4.5 billion years ago, spraying molten debris into space. The debris quickly clumped together to form the Moon at a distance of about 4 Earth radii from the Earth's centre, and solidified 100 million to 200 million years later.
Over billions of years, the Moon gradually spiralled outwards to its current, near-circular orbit at about 60 Earth radii. Tidal forces (gravitational effects) from the Earth have made the Moon's spin period equal to its orbital period, so one side permanently faces the Earth.
Imagine instead that someone magically created the Moon today by placing a perfectly uniform ball of rock in the Moon's current position. Immediately, it would no longer be uniform. The Earth's tidal forces would stretch the barren satellite from the near to the far side, for instance, while the forces of the Moon's rotation would make it bulge out a little around its equator. That means its gravity along different axes would be different.