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The Moon is a little bit chubby. It might look like a perfect circle when there’s a full moon, but the Moon is really a couple of kilometers wider than it is tall.

And scientists have spent more than two hundred years trying to unravel exactly why. Now, new calculations indicate that the Moon’s wide waist is a remnant of its earliest dances with young Earth -- back when Earth might’ve been covered in a single, global ice sheet. Any spinning sphere will bulge out around its equator, just due to its inertia.

But the Moon’s equatorial bulge is a bit of a mystery, because it’s about twenty times larger than it should be based on its rotation alone. And even though we’ve learned a lot about the Moon in the centuries since the mismatch was discovered, scientists still haven’t had much luck solving the mystery. Faster rotation leads to larger equatorial bulges, so we’ve always just kind of assumed that the Moon spun faster when it was very young, back when it was closer to Earth and still partially molten.

Then, as the Moon cooled, the larger bulge must have frozen in place, too. Still, this idea has been hard to conclusively prove. There’s just so much that we don’t know about the early interactions between the Earth and the Moon.

A new study published this month in the journal Geophysical Research Letters is a first step in changing all of that. To try and crack the case, the team used computer models to look back in time. Specifically, they looked back over four billion years, around the time the Moon first formed.

The authors modeled the interactions between the Earth and the young, solidifying Moon with lots of different initial conditions -- things like how fast both bodies might’ve spun in those first billion years. What they discovered was a great combination of what everyone expected to find and what no one had ever even really guessed. Unsurprisingly, they found that, to produce an equatorial bulge like the one we see today, the young, partially molten Moon must’ve been spinning a lot more quickly.

Then, as the molten layers froze, the bulge would’ve frozen in place. But the more unexpected results have a lot more to teach us about the Earth than about the Moon. The models indicate that it took a few hundred million years to form the Moon’s equatorial bulges, which is longer than scientists had guessed.

They also showed that, when the bulges were first formed, the Moon must’ve still been a lot closer to Earth than scientists had thought. That actually says a lot about our planet. The Moon has been moving away from Earth ever since it formed.

And that recession is driven by complex interactions between the Moon and Earth’s tides. One consequence of these interactions is that the water on Earth sloshes around and rubs against landforms and ocean floors. This friction causes the Earth to spin more slowly.

And in turn, that puts more energy in the Moon’s orbit, so the Moon moves further away from us. So if the young Moon stayed closer to Earth for longer than people thought, that means that, among other things, those tidal effects couldn’t have been nearly as strong as they are today. The authors point out a couple possible solutions for this.

One is that the Earth could’ve been covered in a roughly global ocean with very smooth sea floors. In that case, there wouldn’t have been as many features for the water to slosh against, so there wouldn’t have been as much friction. But it’s hard to see why Earth would’ve been smooth enough for this to work, so it’s not a great explanation.

Still, the other solution the researchers proposed seems a lot more plausible. And it’s pretty cool. See, there’s already a lot of evidence that the Sun was about 30% cooler four billion years ago, and that Earth probably didn’t have enough greenhouse gases to trap much heat.

So, according to the paper’s authors, all of Earth’s surface water could’ve been locked up in a worldwide ice sheet -- known among scientists as a “Snowball Earth” event. If all the water were frozen, there would be almost none of those complex tidal interactions between the Earth and the Moon. So the Moon wouldn’t have moved away as quickly.

We have evidence for a Snowball Earth around 650 million years ago, but this is the first evidence for something like it happening very early on -- around four billion years ago. The paper’s model is one of the first of its kind, so it’ll almost definitely be refined over time. But for now, it suggests that we should be ready to rethink some of our ideas about the early history of both the Moon and the Earth.

And look -- all this happened because we were curious why the Moon had a little bit of extra bulge. And if we hadn’t had the moon, we wouldn’t have this amazing glimpse into the early history of our planet. Thanks Moon!

And thank you for watching this episode of SciShow Space! If you would like to learn more about Moon research, you can check our video about three more Moon mysteries we’re still trying to figure out. [ ♪ Outro ♪ ].