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As a SciShow viewer, you can keep building   your STEM skills with a 30 day free trial  and 20% off an annual premium subscription   at Brilliant.org/SciShow. [♪ INTRO]  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.  In preparation for those exciting Artemis  findings, you can take the Brilliant course:   Real Engineering to learn all about rocket  launches, orbits, and material science that sends   missions like that to the Moon and back.

Brilliant  is an interactive online learning platform with   thousands of lessons in science, computer  science, and math. And we’re so happy to have   their continued support for yet another SciShow  video! Rocket science is awesome because it helps   us answer these moon mysteries, but also because  it sits at the intersection of math, chemistry,   engineering, physics, and pretty much all of the  stuff that Brilliant helps you learn.

In this   course, the brilliant people at Brilliant have  taken one of the most complicated topics ever,   I mean it is rocket science, and distilled it into  just six lessons. And you can probably complete   six lessons in less than a month, which would make  this course free for SciShow viewers like you,   because Brilliant is giving you your first 30  days for free! After that, you can take 20% off an   annual premium Brilliant subscription by clicking  the link in the description down below or going to   Brilliant.org/SciShow. [♪ OUTRO]