"Later on, those tides warped the outside of the moon while it was cooling, and it froze in that warped shape," Garrick-Bethell told the Agence France-Presse.

Scientists have known for some time that the moon is not perfectly round. But it's been difficult to determine exactly how the bulges formed, since deep craters on the lunar surface obscure the moon's original shape.

So a team from the Massachusetts Institute of Technology, which also contributed to the study, created a topographical model that sealed many of the moon's crevices in order to paint a more complete picture of what the moon looked like billions of years ago.

"If you imagine spinning a water balloon, it will start to flatten at the poles and bulge at the equator," Garrick-Bethell said in a written statement. "On top of that you have tides due to the gravitational pull of the Earth, and that creates sort of a lemon shape with the long axis of the lemon pointing at the Earth."