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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. But 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 supervillain-origin-story kind of way. Meet Procellarum KREEP,
or just KREEP. It’s a terrain that’s found only on the near side of the Moon, and it’s
defined by high concentrations of potassium, because in chemistry a K stands for potassium,
Rare Earth Elements, and phosphorus. Some of that potassium is radioactive, but KREEP also
contains a substantial amount of thorium and uranium, which are a little more famous for
being unstable little atoms. And if you pack a bunch of radioactivity into one patch of rock,
you can get some interesting effects. For example, KREEP 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 KREEP 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 KREEP is only
on the Moon’s near side, that could explain why almost all of its maria are there, too.
But why is KREEP 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, KREEP-y crust on
one side, and a thicker, KREEP-less 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 and create maria. Meanwhile,
if one hit the far side’s thick, tough crust, it wouldn’t make it through. You’d just get craters,
and the far side definitely has plenty of those. But even without tilted convection on the inside
of the Moon, the temperature difference between the near side and far side could have been enough
to make our Moon two-faced. Astronomers have another hypothesis that looks at how
the asymmetry grew from a baby Moon’s atmosphere which was filled with vaporized rock. It was that
hot. We’re pretty sure the Moon was made about 4.5 billion years ago when a not-quite-finished Earth
had a cosmic collision with another protoplanet. So basically all of the Moon’s rocky bits started
off as a hot liquidy blob until they cooled down. But some bits were hot enough to hover around that
blob as a gas. And over time, some of this gas would make its way over to the cooler far side,
condense into rocky rain, and eventually build up a solid, thick crust. Different
minerals have different temperatures where they become cool enough to condense, so stuff chock
full of aluminum and calcium was more likely to rain down onto the far side. Meanwhile,
the elements in KREEP tend to stay vaporized at lower temperatures. So as with the tilted
convection hypothesis, they end up as the leftovers forming the near side’s crust. But
maybe the Moon looks like it does because it ate a smaller moon? Scientists are pretty sure at least
three bodies collapsed out of the debris cloud created by Earth’s hit and run: the Earth and at
least two moons of different sizes. And it’s also likely these two moons crashed into each other to
make the one we know and love today. Most of the research investigating this hypothesis has the
smaller moon smash into the larger one at high speed, but in 2011, one team proposed it could
have been a slow crash where the smaller moon just sort of blooped into the other. Fine, it’s
technically called “accretion of a companion”, but bloop is way more fun to say. This blooping
would have deposited a bunch of new material onto one side of the Moon, where it would have cooled,
hardened, and become a thick crust. That side wasn’t necessarily the far side at the time. But
it created such a wonky distribution of mass that over many years, the gravitational interactions
with the Earth would have caused it to settle into becoming the Moon’s super thick backside.
But wait, there’s more. The slow crash would also have pushed whatever didn’t solidify on
the far side toward the near side, including all the KREEP. We’ve been trying to figure out why the
Moon is the way it is for a long time. There are even more hypotheses than the ones we’ve covered
here. And one of the biggest hurdles is getting our hands on actual lunar samples. We have some,
but not nearly enough to really dig in and answer all of the questions that we have. But we are
in luck, though. Humans are set to head back up there as part of NASA’s Artemis program, and they
will bring back a plethora of new samples in the coming years. Moon rocks for everyone! Someday,
we’ll solve the mystery of the Moon’s two-faced nature. And in the meantime, we can look up and
admire the one it keeps turned toward us. And in my opinion, that’s definitely its good side.
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