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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 Earthevent.

  • 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 ♪ ]

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