Gravitational heating and pulling explains how the Moon got its lumps, scientists say

The Moon in the sky may look like a pizza pie, but it turns out that its not perfectly round, and now we may know why.

New findings published this week in the journal Nature reveals that the Earth’s lone natural companion is actually more lemon-shaped, with weird lumps on its sides and even an off-kilter axis.

And the reason for these unusually lunar properties appears tied directly to titanic tidal and rotational forces that rocked the moon billions of years ago when it had just formed.

Current accepted theories point to the Moon being born out of a colossal collision between a primordial Earth and a Mars-sized object some 4.5 billion years ago. The resulting giant splatter of hot magma that shot into space went on to form our rocky satellite. However, these same theories don’t really account for the bulges on the front and far-side faces of the moon.

Using new topography maps recently produced by NASA Lunar Reconnaissance Orbiter, combined with a recent gravity map produced by the space agency’s other lunar mission called GRAIL, the research team was able to piece together a working simulation model that suggests the Moon got its lemon shape about 200 million years after its violent birth.

When the moon was still new, it was much closer to Earth and had a stronger gravitational pull than it does today, so at the time its newly-formed crust was still floating on a magma ocean heated by the strong tidal forces.

“The heating caused thickness differences throughout the crust that remained until today, and cause much of the Moon's topography we see now,” said Ian Garrick-Bethell, lead author of the new study and planetary scientist at University of California at Santa Cruz.

The same tides that stretched the Moon, as well as the rotation that the Moon experienced at the time, froze into the Moon when it cooled.

“You can imagine this by picturing high tide at the beach, and imagining the ocean freezing in place. We believe the same thing happened on the Moon, except with rock,” he added.

But this gravitational heating and pulling is not unique to our own moon. The same process can be seen going on right now with Jupiter’s moon, Europa, which has a floating shell of ice thought to hide a salty ocean of liquid thanks to the intense tidal forces of its giant planet. Garrick-Bethell admits that this is what inspired his team to try and explain our own moon’s early evolution.

This new lunar theory may actually also impact our understanding of Earth’s own evolution as well, he says, since we don’t have much information on Earth’s physical makeup going back more than 4 billion years ago.

“I believe [our work] helps us understand how quickly the Moon was moving away from the Earth very early in Earth-Moon history – and it turns out that this rate depends on the bulk physical properties of the Earth, and whether or not it had an ocean,” he explained.

Next up for the research team is to try to examine possible origins of the differences in height between the two sides of the Moon.

The hope is that by resolving the mystery behind the moon’s lumpy shape, it may actually help understand some of the most fundamental processes that go into forming other moons in our solar system and beyond.

“The Moon affords us a chance to study these processes at a location that is very easy to reach with spacecraft, compared to other places in the solar system,” said Garrick-Bethell.

“It has always been, and remains a great laboratory for studying planetary processes everywhere.”

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