Why does the Moon look like that?

Added: 2026-05-12

[00:00:00]So, we went to the moon again. The most recent mission as of this video was the Aremis 2 orbiting all the way around the moon during which this image of the Earth's setting was taken. And this bit is the dark side of the moon, a perspective we never see from Earth. And if you're like me, you may think it looks a little odd compared to the moon we're used to. Well, we have a full map of the moon. So, let's zoom out a little bit and take a better look.

[00:00:36]Ah, there we go. Much better. We can see the whole dark side of the moon. And why does it look like that? Where is everything? Where are the dark spots? I guess we must be pretty lucky to have the interesting side facing us, right?

[00:00:54]But the secret is that's not luck at all. We Earth are the reason the moon even has a good side like it does.

[00:01:06]First, why does the same side of the moon always face us anyways? Aren't space rock supposed to, you know, spin?

[00:01:15]I could be a pedantic [ __ ] and say, "Oh, it does spin just in sync with its rotation." But we all know that's not what you're asking. What happened to the moon is called tidal locking, where over the lifetime of a natural satellite like a moon, the small differences in weight and density between its different sides get pulled on with different levels of force from its mother planet. The rotation gradually slowing down, down, down as whichever side is a little bit heavier is pulled more strongly towards its planet until that one side is always facing down at you. This can happen to any stellar object orbiting a bigger one, but for many reasons, this usually happens with moons. Tidal locking typically only happens when objects in space are close to each other, like a planet and moon. At least relatively speaking, space is still really big. A planet close enough to its star to get tidily locked would usually get a tiny bit burned to a crisp unless it's a particularly cool star like a red dwarf.

[00:02:28]Once you get to the size of gas planets and stars, tidily locking stops being possible since they're so fluid, they don't really have sides like a rocky planet would. Ah, you might think those dark spots must be heavy. That's why it faces this way and not another. And you would be almost right. But here's the kicker. Those dark spots, they formed after the moon had already become tidily locked and turned to always face one side to us, making how the moon's facing affect its geology. So, what are these dark spots anyway that I've been talking about this whole time? And how did they happen? These are called Maria because we used to think they were bodies of water on the moon. They are not, of course, but they did used to be liquid.

[00:03:21]Hello, just editing professor here.

[00:03:24]Maria is Latin for seas, like bodies of water. So, we called them Maria because we thought they were water, so we called them seas.

[00:03:34]volcanic rock to be precise. These now cooled basins of stone are not just dense enough to where their gravity needs to be accounted for when landing probes, but they are also incredibly old. Billions of years old at their youngest estimate, long predating any feature on the surface of the Earth today. In fact, they're so old they formed when the moon was a lot hotter.

[00:04:01]The moon may be quite cool and mostly dead in our modern day, but in the early years of the solar system, it was liquid and hot. Liquids, of course, flow.

[00:04:14]Planets, moons, and stars as we know them flow such that the densest materials settle towards the middle since that's the closest thing to down or a bottom that can really exist in space. And being so fluid, spinning around allows all the gravity fields near you and the deformationations they cause to be spread out and evenly distributed such that nothing can really distort the circular shape of your layers and their positioning inside you.

[00:04:43]Much like a turning object in a microwave doesn't get too hot, at least in theory. [ __ ] those things. How do they make the plate hotter than the food? Like it doesn't it doesn't have water in it. I I don't understand. But when the moon is tidily locked, things tend to settle. When the moon is tidily locked and no longer spinning to even out the lopsidedness of gravity, the moon warps, not on the outside, but on the inside. The heavier materials of the moon are pulled closer to the near side of the moon, making the molten rock closer to the surface in only one direction, giving one side a dangerously thin crust and the other one so thick nothing molten could penetrate. Now, this next bit is disputed, but both interpretations end up with the same result. So, let's talk about them both.

[00:05:42]The first is the idea that with one side of the moon having molten material so close to its surface, meteorite impacts could crack the surface and let the magma spill out. Alternatively, the moon most likely once had volcanoes that could only form on one side again due to crust thickness spewing out their molten eluvia. The truth probably lies somewhere in between as there's evidence for both volcanoes and impacts in the moon's past. The end result is the same.

[00:06:15]Ignous moon rock spilling out and collecting only on one side, which if you were paying attention at the beginning, explains why the basins, the maria, are so full of dense material.

[00:06:28]It's literally core material that had a rare chance to spill out and make those dark basins and creating that classic man in the moon.

[00:06:40]A lunar face staring down at humanity in the night from the darkness of space can be a little bit creepy. Literally, the combination of elements, metals, and other minerals that make up the dense plastic flow on the lunar surface is called creep for potassium, rare earth elements, and phosphorus, which includes the not so nice uranium, thorium, and potassium 40. All of which are, of course, radioactive. A thought that should make you feel nice and warm inside, just like it does in Earth's interior. The decay of radioactive elements helping keep our planet warm with plenty of internal heat from uranium, thorium, and potassium 40. Wait a minute. How similar is the moon's interior to Earth? How did all that radioactive stuff get in both places?

[00:07:34]There's too much for it to just be moon rocks. What is going on here? To understand the moon, we have to understand how the moon formed. And oh no, there's a second space geology video inside the first space geology video.

[00:07:51]No, you can't leave. I've locked the door, so get comfortable. Besides, if you didn't leave the moment I said space geology, I know you're in it for the long haul, so you may as well subscribe as well. See, the moon, ours, is not like most other moons in our solar system. Not only is it the biggest moon relative to its host planet out of any of them, unless you count Pluto, which I do not and you should not because it is not a planet, but it also formed differently. See, most moons formed the way the planets did out of the protolanetary disc in the early solar system and were later captured by their host planets gravity. Mercury and Venus have no moons. Earth is a special case, so we'll get back to her. Mars has the two. Jupiter has 115. Big jump there.

[00:08:38]Saturn has almost 300. Uranus has 29 and Neptune, you get the picture. Earth did not capture a moon in the early solar system. It captured an entire planet.

[00:08:50]This planet being a tad bit larger than any moon crashed into our planet with its rather large force of attraction.

[00:08:58]Kaboom. This collision of two molten planet masses predictably resulted in a humongous splash with the lighter crust material being jettisoned off into orbit and knocking the Earth off its rotation, giving us both a moon and seasons. So when you look up at the moon, smile. The moon isn't just our closest companion in this big and dark universe. It is also our only child. But as all children must, the moon will eventually leave the nest. Every year, the moon gets 38 mm, about an inch and a half further away.

[00:09:37]If that fills your heart with a sense of loss, I will leave you with this. The time you're living in is the only time in Earth's history where the sun and moon appear the same size in the sky.

[00:09:49]So, we have the rare treat to see eclipses, a privilege creatures in the past and the distant future will not have. The man in the moon made sure to be in just the right place at the right time to put on a performance with the sun just for you.

[00:10:08]Anyway, thanks for watching and I'll see you next time.