Space News Deep Dive: Making a Moon

Article January 25, 2025

There’s more than one way for a planet (or any object for that matter) to get itself a moon. Moons can form in orbit around the object they circle, or they can form somewhere else and get snatched by the gravity of a larger object (the gas giants in particular like to use both of these methods for getting moons).

We’re pretty sure we know how Earth wound up with the Moon. The broad story anyway. The exact details though are a bit harder to pin down. Despite being the most studied object in space, our Moon still has a few mysteries up its sleeve (well, it would if it had sleeves). A new study released this past week may help clear up some of those mysteries. Maybe.

So how did we wind up with our Moon anyway, and what do we still not know? Let’s look at one way to make a moon.

There’s a lot we still don’t know about our Moon. Credit: Gregory H. Revera

Formation or Capture?

For a long time we really had no idea what the Moon’s origin story was. Had it spent its entire existence gravitationally bound to our planet? Or did it form all on its own somewhere else and get captured by Earth’s gravity later?

One of the lunar rocks returned to Earth by Apollo astronauts. Credit: NASA

Observations from Earth show that the Moon is much less dense than Earth is, suggesting it doesn’t necessarily have the same sort of internal makeup that Earth has, a dense iron core and lighter mantle and crust on top. On its own, this might suggest that the Moon has a different origin than Earth and perhaps formed elsewhere.

But wait! We clever humans collected a whole bunch of Moon rocks during the Apollo missions that tell a different story. When we look at the composition of the rocks brought home from the Moon, specifically at isotope ratios, they are almost identical to those found in Earth’s mantle. Coincidence??

No, probably not. Moon rocks resemble Earth rocks in enough ways that it’s hard to imagine that Earth didn’t play some role in the Moon’s formation. In fact, the leading theory is that Earth is one of the Moon’s parents—but also that it had a partner in this little lunar birth scenario.

Enter Theia

An artist’s rendition of the impact between Earth and Theia. Credit: NASA/JPL-Caltech

This leading theory (though not the only theory) is called the Giant Impact Hypothesis. The basic idea is that very early in the solar system’s history, possibly even when Earth was still largely a molten ball of planetary potential, something hit it. Something big.

We’ve named this theoretical object Theia. In Greek mythology Theia is the mother of Selene, the Moon, so the name is appropriate in addition to being poetically lovely. Theia is thought to have been roughly the size of Mars, so we’re not talking any little dinky impact like the one that wiped out the dinos here. We’re talking the kind of hit that can crack a planet in half.

Earth didn’t get split in two by this theoretical impact, but it would have had a bunch of material from its outer layers stripped away and flung into space, along with (theoretically) a bunch of material from Theia. All this knocked-loose stuff would have been trapped in orbit around the recovering proto-Earth. Given time, a couple million years maybe, it would have coalesced into a single object—the Moon.

One Big Hit

There’s several lines of evidence for the idea that a big hit was responsible for the formation of the Moon. For instance, the fact that the Moon lacks a dense iron core like the Earth actually supports the Impact Hypothesis. After all, the Earth stuff that got knocked loose would have been from Earth’s mantle, not its core, so the Moon would have gotten very little of Earth’s dense metals.

A diagram showing the basics of the Giant Impact Hypothesis. Credit: timeanddate.com

There’s also several things that suggest the Moon formed in, shall we say, an energetic environment. For instance, the Apollo rocks suggest that the Moon once had a thick molten outer layer during its formation. Something the size of the Moon forming all on its own isn’t going to have enough energy to liquidize its entire surface to a substantial depth—but something forming out of fiery bits knocked loose by a giant impact would.

The Moon is also lacking a lot of elements with low boiling points like nitrogen or carbon dioxide. A highly energetic formation would have led to these substances boiling off on both the Moon and Earth. Of course, Earth has a lot more gravity than the Moon and would have been able to recapture them. The Moon, on the other hand, would have lost them to space.

Also, we know the early solar system was a rough place. There was a lot of loose material flying around and the planets themselves were on the move. We also know Earth must have suffered at least one huge impact in its history, because the planet has a tilt to its axis. We wouldn’t have seasons otherwise.

Hold On a Second

So that’s the story, right? Well, perhaps not quite. The Giant Impact Hypothesis doesn’t fully explain everything we see in the Earth-Moon system today. For one thing the impact would have had certain effects on the Earth that we don’t see any evidence of. For another, we tend to assume that this scenario, if it occurred, would be a normal one for the early solar system rather than a rare event. In which case, why didn’t it happen to Venus? Venus has no moon.

A big issue though is that Moon rocks look so much like Earth rocks. Oxygen ratios? Virtually identical. Titanium ratios? Identical. Hydrogen? You guessed it. Tungsten? Oh yeah.

If the Moon was formed when Theia hit Earth, where is Theia? There should be some aspects of lunar rocks that look completely different from those of the Earth because they originally came from Theia. The cloud of stuff that formed the Moon should have been at least partly made up of Theia. So one of the biggest problems with the Giant Impact Hypothesis is that the impactor has vanished, apparently without a trace.

Or has it?

Finding a Missing Protoplanet

One possibility is that Theia just looked a whole heckuva lot like Earth. Perhaps they formed in the same region of the solar system and their rocks were very similar to each other. In that case we could very well be looking at Moon rocks that originated with Theia and not even realize it. Is this scenario likely? Nah. But is it possible? Well, yeah.

An artist’s concept showing what might have happened if Theia struck a pre-Earth and both were vaporized. In this depiction, the Moon has already coalesced on the outer edges of the cloud, while the rest of it will eventually coalesce to form the Earth. Credit: Sarah Stewart

Another possibility is that Earth wasn’t a progenitor of this collision but a product of it. In this scenario, Theia hits something that we’ll call the pre-Earth. The impact was so big it essentially vaporized both objects. In their place was a giant, swirling cloud of rock, dust, and gas. Some of that settled down to form the Moon, but the rest of it solidified into the Earth. In this scenario Earth isn’t the parent of the Moon but its sibling, and both are made up of Theia and pre-Earth. Is it likely? Eh, some very specific things would have had to happen on a very specific timeline, so it’s tough to say. But is it possible. Probably.

A new study offers up an alternative theory, that Theia had already suffered a number of big impacts and through them had lost basically everything but its core. In this scenario Theia is more or less a giant metal cannonball careening through space. When it hit the Earth that solid iron ball wouldn’t have shattered like Earth’s mantle did—it would have sunk into the Earth. Very little of it would have gone into the cloud of debris that became the Moon, because Earth consumed it.

Heart of the Earth

Last year scientists announced a pretty major discovery that seems to back this new study up. Basically, we think we found Theia. And, as this latest proposed scenario suggests, we had to look down to find it.

Diagram showing the two massive blobs inside Earth’s mantle now thought to be the remnants of Theia. Credit: Cottaar and Lekic/Live Science

Seismological studies have found two gigantic blobs of material way down deep, sitting on top of Earth’s core. Collectively they are more massive than the Moon and if you brought them to Earth’s surface and melted them they would cover the Earth in a layer sixty miles thick. And they don’t look like the stuff around them. They’re denser, hotter, and have way more iron in them.

The leading hypothesis is that this is Theia. When it struck the Earth, the thinking goes, it fractured into pieces as it sank deep into our planet. Those pieces have remained, embedded at the bottom of Earth’s mantle, for the last several billion years.

If this is true (and there’s plenty of evidence supporting this idea) then Theia never left us, and both of the Moon’s parents have remained with it since the day it came to be. And that, if you ask me, is a delightful conclusion for this particular origin story.