For years scientists have tried to figure out why the Moon is not perfectly round. In theory it should have been a nice spherical shape due to its rotation as it cooled about 4.4 billion years ago. However, the orb seems to have flattened out a bit, and grown one bulge facing the Earth, and another on its far side. Some say the Moon may slightly resemble a lemon as a result.
A new study shows that most of the Moon’s overall shape can be explained by taking the Earth’s tidal effects into account early in the Moon’s history. Researchers at UC Santa Cruz published their findings concerning the shape of the Moon in the July 30 issue of Nature.
Lead author Ian Garrick-Bethell, assistant professor of Earth and planetary sciences at UC Santa Cruz, explained that as the Moon cooled and solidified more than four billion years ago, the tidal effects and its own rotational forces became frozen in place, creating what is known as a “fossil bulge”.
“If you imagine spinning a water balloon, it will start to flatten at the poles and bulge at the equator,” he said. “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 end of the lemon pointing at the Earth.”
The article detailed further reasons for the Moon’s current shape, including the large impact basins that shape the Moon’s topography. These large basin craters complicate the process of explaining the Moon’s overall shape because the powerful impacts that created them deformed the lunar surface and ejected large amounts of material.
“The Moon has extensive volcanic plaines known as maria,” Garrick- Bethell said. “Those are the dark parts of the Moon when you look at it at night. The other side, which you can’t see without flying over it in a spacecraft, is largely devoid of such volcanism.”
Part of the reason for the volcano placement once the Moon cooled could be the gravitational pull Earth exerts on it. The side facing the planet is more likely to remain hot longer because of the friction between the two bodies, making hot pockets more likely to form there. This is also generally the area where the Moon caves NASA wants astronauts to live in are located.
The Moon itself is believed to have been spawned thanks to an impact between a roving, Mars-sized planet and the Earth, causing a molten lump of rock to break free. Eventually the rock cooled and solidified, ending up becoming an important fixture in our night sky.