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SYFY WIRE Bad Astronomy

Why Do We Have a Two-Faced Moon?

By Phil Plait

Our Moon is weird. It’s pretty big compared with Earth, for one. It has a way lower density than Earth, but seems to have some similarities in chemical composition to our planet. It’s also two-faced: The near side looks a lot different than the far side, as you can see from the Lunar Reconnaissance Orbiter image above. The near side is splattered with dark regions (called maria, the plural of mare, Latin for “sea”). The far side barely has any.

Further study shows an odd anomaly: The crust on the far side is a lot thicker than it is on the near side. That explains the lack of maria; the thicker crust means it was harder for giant impacts to pierce the crust and get darker basaltic lava bubbling up. But why is the far side crust thicker? And why is this so neatly divided by the two hemispheres?

That question has bugged planetary scientists for decades. Recently, two competing hypotheses have come up which can explain it. Both are plausible, but in my mind one has an edge on the other.

Both start with the same premise: Not long after the Earth formed more than 4 billion years ago, a Mars-sized planet was on a collision course with it. This object slammed into Earth on a grazing impact, forceful enough to cause immeasurable damage and remelt Earth. A huge amount of material was blasted into space, which coalesced rapidly (and I do mean rapidly; probably in just a few years) to form the Moon. It originally orbited the Earth very close in but over the eons receded from us, and it is now about 380,000 km away. This idea is called—for obvious reasons—the Giant Impact hypothesis.

It explains why the Moon has some chemicals similar to Earth—it used to be part of the Earth—but also why it has a lower overall density. The Earth was old enough at the time that heavier elements had started to sink to the center of our planet, leaving lighter stuff floating at the surface. The grazing impact skimmed off this lighter stuff, and that’s what formed the Moon.

Then, probably a few tens of millions of years later, there was an episode of heavy bombardment of the inner solar system with big asteroids. This is called the Late Heavy Bombardment, and may have been caused by the outer planets (Jupiter, Saturn, Uranus, and Neptune) shifting slightly in their orbits over time as they interacted gravitationally. This would have disturbed the population of objects past Neptune, dropping them down into the solar system where many collided with planets. The ones that hit the Moon formed the maria.

Got it? That’s where we start. Now let’s take a look at both ideas of why our Moon has such two different faces.

Double Whammy

The first idea, which came out a few years ago, is that the impact which formed the Moon actually formed two moons. One was big, and forms the bulk of the Moon as we know it today. But a smaller moon also coalesced out of the ejected material, and was on a very similar orbit to the bigger moon. After some time, the two collided.

But this wasn’t a high-speed impact. Some orbits allow for a low-speed collision, which would be a lot less explosive. If that were the case, the smaller moon would splash, essentially, touching down on the Moon’s far side and flowing like liquid over it. This would create a lopsided Moon, with a thicker crust on one side than the other, as we see things now.

This second impact hypothesis is an interesting idea, and not as crazy as it sounds. The physics seems to work out. However, there are some problems with it. The chemical composition of the lunar surface doesn’t have a sharp transition between the far and near side as you’d expect from this. Also, to be honest, it’s not parsimonious; it’s an added layer to an already complicated event. That doesn’t mean it’s wrong, but if there’s a simpler explanation for the anomaly then, by Occam’s razor, we can give this one less weight.


The second hypothesis just came out recently, and it is based on two things: After the Moon-forming impact, the Earth and Moon were very close together, and both were very, very hot from the impact.

After it first coalesced, the Moon may have only been 20,000 kilometers away, and would’ve loomed huge in our sky. But the Earth is far larger, and from the Moon the Earth would’ve been huge, 40° across (that’s 80 times wider than the full Moon looks in our sky now, so yikes).

And it would’ve been hot. The surface would have been molten from the energy of impact, and that means the Earth would’ve been around 2,500° C (4,500° F). It would’ve stayed hot for some time, too.

During that time, the Earth and Moon would’ve been heavily affected by their tides on each other. Tides are complicated (read all about them here), but one effect they have is to slow the Moon’s rotation until it spun once for every time it orbited the Earth, just as it does now. That means very quickly after it formed—incredibly, in just a few months—one side of the Moon would always face the Earth, and one side would face away.

The side facing the Earth would have that huge, glowing hot blast furnace radiating away down on it. Volatile chemicals would vaporize, and expand. They could blow around the surface of the Moon, and only solidify where it was cooler … like on the far side, which faced space, away from the fiercely hot Earth. Over time, they built up there, creating the odd anomaly we see today (and more easily created the mixed-up spread of chemicals over the surface I mentioned before).

[UPDATE/CORRECTION (July 6, 2014): Jason Wright, one of the authors of this hypothesis, sent me an email after reading what I wrote above. He notes that it isn't so much that volatile chemicals recondense on the far side of the Moon; it's that refractory chemicals (ones that retain their strength when heated) would have gotten mixed in all over the Moon's surface, but would have preferentially condensed on the far side of the Moon, where it was colder due to the lack of heating from the Earth. These would have formed feldspars (in this case, minerals that contain aluminum and calcium) that would have caused the newly formed crust to be thicker on the far side.]

I admit I like this idea better than the first one. It’s simpler, and it doesn’t need special circumstances. We know the Earth was hot, we know the Moon spun down rapidly, and we know vaporized material would flow from the hot side of the Moon to the cool side, forming minerals there that eventually turned into crust. It all fits together nicely.

This doesn’t mean it’s right, of course. But in my opinion (and bear in mind I’m no selenologist) it seems to be an excellent step in the right direction. One of the authors of this idea, Jason Wright, has a great series of blog posts with details (here are Parts 1, 2, and 3).

And let me add how much I love all this! The Moon is our nearest neighbor in the Universe, the brightest object in the sky other than the Sun, and the only other celestial body humans have set foot on. Yet we still don’t really understand how it was formed. But we’re clever, we space-faring apes, and we’ll figure it out. We have imagination, and when that’s backed by science, solving problems is what we do best.

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