Leaving home can change you – and the moon is no exception. As it drifted away from its parent, Earth, the pull of our planet's gravity gave it an odd bulge on each side and a tilted axis. Uncovering the mystery behind its unusual shape is a step towards finding out exactly when and how the moon formed.

Most rocky planets and moons formed from a spinning ball of magma, which gives them a fairly predictable spherical shape.

Earth's moon is thought to have formed when a Mars-sized object smacked into the infant Earth and shot hot rocky material out into space. That should mean normal rules apply, but instead, the moon has a weird bulge on both the near and far side, giving it a shape like a lemon.

There are several ideas for how these bulges formed, but studying them has been difficult because since it formed, the moon has been marred with large basins that mask its original shape. One of them, the South Pole-Aitken basin, is the biggest, deepest impact crater in the solar system.

Maria Zuber at the Massachusetts Institute of Technology and her colleagues made a model that filled in 12 of the largest basins, to see what the moon would have looked like before they formed.

The results suggest the lemon-like bulges formed in the first 200 million years, when Earth's gravity pulled at the moon's magma, building the crust up more on the points closest to and furthest from Earth.

That left the mystery of the moon's puzzling tilt. When the bulge formed, the points of the lemon should have been pointing directly at Earth, but today they are offset by 36 degrees. The researchers suggest that as the moon moved away from the Earth, the density of the cooling crust was uneven. The crust became lopsided and tilted the moon's polar axis to the angle we see today.

Journal reference: Nature, DOI: 10.1038/nature13639

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