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In 1959, humanity learned that the giant gray orb hanging in our sky is two-faced. The near side of the moon, the one facing us, has a thin crust covered with large frozen lava seas called maria. Meanwhile, the far side looks completely different—it has a much thicker crust and almost no maria. Over six decades later, we still haven’t managed to pin down why.
To solve this mystery, scientists have come up with some pretty cool hypotheses, from radioactivity to one moon eating another. Here are a few potential reasons why our natural satellite looks so strange.
For a thing that happens on a microscopic level, radioactivity can have some pretty huge effects—and not just in a super villain origin story kind of way. Meet procellarum creep, or just creep. It’s a terrain that’s found only on the near side of the moon and it’s defined by high concentrations of potassium, rare earth elements, and phosphorus. Some of that potassium is radioactive, but creep also contains a substantial amount of thorium and uranium, which are a little more famous for being unstable little atoms.
If you pack a bunch of radioactivity into one patch of rock, you can get some interesting effects. For example, creep can lower the melting point of the material around it. It’s not quite the same mechanism, but it accomplishes the same effect as throwing salt on an icy sidewalk. Combine this drop in melting point with the fact that radioactive elements also release heat as they decay, and it means the moon’s creep could have helped keep some nearby rock molten. And after that molten rock bubbled up and out of a volcano or flowed onto the surface after a big space rock punched through the crust, it could solidify into maria. Since creep is only on the moon’s near side, that could explain why almost all of its maria are there too.
But why is creep only on the near side and why is the crust on that side so much thinner? Well, back when the moon was young, its insides had these giant currents of churning rock known as convection currents. Warmer rock near the core rose up toward the surface, cooling as it went, then started sinking and warming back up—rinse and repeat. The same thing is happening inside the Earth right now, but sometimes—and it can happen for no apparent reason—these convection currents can go lopsided and produce something called tilted convection.
Some scientists hypothesize that this could have happened to our baby moon. And however the tilt got started, the wonky convection currents may have shifted themselves to match the moon’s orientation relative to Earth. See, the reason why we have this consistent view of the moon is because it’s tidally locked—it’s spinning at just the right speed to keep one side pointed our way at all times. And this locking happened pretty early on in the moon’s history, so the near side was kept hot by a new molten Earth blasting a bunch of heat towards it, while the far side got to cool down. That temperature difference, flowing from near side to far side, could have shifted the lopsided convection currents to match.
And because they’re lopsided, the currents would have started dragging material away from the hot near side, where it was easier for rocks to stay molten, over to the cooler far side, where some of them could solidify. Meanwhile, the stuff that didn’t solidify as easily—including potassium, rare earth metals, and phosphorus—made up the leftovers that could form the crust on the near side. So tilted convection could be one way that the moon ended up with a thin, creepy crust on one side and a thicker, creepless crust on the opposite.
Underneath that solid crust, there was a cooling but still molten mantle. So, on the near side, a meteorite could have punched right through the thin crust and caused magma to bubble up a volcano. And on the far side, the same impact would have been stopped cold by the thicker crust. It’s a neat idea, but it’s still just a hypothesis.