Sage: How did one of Earth’s oldest rocks end up on the moon? In 2019, scientists discovered something amazing inside a moon rock: a tiny, embedded, two-centimeter chip. Just a little clump of rock, right? But the scientists figured out that that tiny chip was around 4 billion years old and contained minerals like quartz, feldspar, and zircon crystals— in forms that can only be found on Earth.

So, what happened? And what can that tell us about our planet? Hi, I’m Sage, and this is Crash Course Geology.

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[0:36] Sage: Rocks—on Earth and in space—hold a lot of clues about our planet’s beginnings. And, if you haven’t already noticed, Earth is pretty special. As far as we know, it’s the only planet that sustains life. What can I say?

We’re kind of a big deal. So, knowing how Earth came to be is also a big deal, for understanding its past and its future. Everybody loves a good origin story.

But it’s tricky, because there isn’t much physical evidence left from Earth’s formation. See, forces like weather and plate tectonics cause Earth’s materials to  constantly move and change. So, as far as we know, all of the rocks that were around when Earth was born just… don’t exist anymore.

But a lot of physics research has led to some compelling theories about how we went from nothing to a pretty special something. This is Earth’s birth story—or, our best guess at it. Sometime around 4.6 billion years ago, our solar system was a  swirling cloud of gas and dust.

Then something—maybe a nearby exploding star called a supernova—made that swirling cloud collapse into a solar nebula, a condensed spinning disk that eventually formed the solar system we now know and love. First, gravity pulled together most of the stuff swirling in that cloud, causing nuclear fusion between the hydrogen and helium, which created our sun. Then, what was left spread out into a slowly rotating disk like a spinning record—what’s called a protoplanetary disk.

The stuff in that disk started to clump and smash together, break apart, collide again, and create new objects. The largest of these planetesimals became baby planets, or protoplanets. Then, once their gravity pulled all the matter in the surrounding area toward them, they finally got big enough to be considered bona fide planets.

But that doesn’t mean our solar system immediately looked the way it does today—it still had a lot of growing up to do. During Earth’s toddler stage, it was less “pale-blue dot” and more “fiery hellscape.” Lasting for hundreds of millions of years, this period is known as the Hadean eon, after the Greek god Hades, who ruled the underworld. How’s that for “terrible twos”?

Things did eventually start to cool down, but the surface wasn't exactly quiet. The end of the Hadean was the start of a pretty violent period for our planet, known as the Late Heavy Bombardment, when Earth was pummeled by asteroids. What’s that, Dwayne?

Ah yeah, that would make me cranky, too. And this era might explain how a rock from Earth ended up on the moon. The moon was closer to Earth in those days, so an asteroid impact could have ejected chunks of Earth all the  way out to our lunar buddy.

And since there’s a lot less weather and geologic activity on the moon, the rock could have stayed relatively undisturbed for literal ages. If that’s what happened, then our little mystery rock can offer us clues to what was happening  on Earth at that time. Its mineral mixture hints that continents were already starting to form 4 billion years ago.

And there’s other evidence of early continents in 4.4 billion-year-old zircons found in Australia. They even hint at the existence of early oceans! So, asteroid impacts might be the reason why we found an Earth rock on the moon.

But what about the moon itself? How did it become our planet’s BFF? Like with Earth, we’re not exactly sure.

But we do have some pretty good theories. The most widely accepted one is known as the giant-impact theory. The story goes that back when Earth was just a wee protoplanet, it collided with another  protoplanet, known as Theia.

The collision created a bunch of space debris in Earth’s orbit, which eventually came together to become Earth’s moon. But there are other theories, too. Like, that some material broke off of Earth and began to orbit the planet as a separate body.

Or that Earth and the moon  formed at the same time. Or maybe the moon formed somewhere else, moved towards Earth, and was captured by its gravity. We know that the last one, known as the capture theory, is possible because it’s  happened with other objects.

Like, in 2024 Earth had a “mini-moon” for two months! An asteroid had fallen towards Earth and was temporarily pulled into its orbit. The moon was like “No big deal.

Earth can have other friends, I don’t care.” But these theories don’t explain everything. Like, Earth and the moon have very similar isotopic signatures — combinations of atoms that act like fingerprints or a genetic code. But if the moon was a separate body that formed elsewhere in the solar system, its isotopes would be way different from Earth’s.

And if the moon simply broke away from Earth, or formed from the same material, they should be even more similar. So, what do we do with that? Let’s learn about a different theory with one of my geology rock stars… In Sarah Stewart’s lab, planetary geologists shot cannons at rocks, recreated the conditions of planetary formation, and used computer models to simulate planetary collisions.

They tried different speeds and different sizes to replicate the moon’s formation, but in every scenario, the earth and the moon didn’t wind up with that shared genetic code. But one day, Stewart and her team had a realization: sometimes a planet stops being a planet. When two giant, celestial objects collide, their surfaces get partially vaporized.

Which means that proto-Earth could have gotten so hot playing Bumpers Cars— and spun out so fast from the impact— it turned into a doughnut-shaped  mass of rock and vapor. As Stewart put it in her 2019 TED Talk: “I discovered a new type of astronomical object.” She called it a synestia, “syn-”—Greek for “together”—and “-estia” after Hestia, the Greek goddess of the hearth and home, and older sister of fiery Hades. Eventually, a synestia cools and turns back into a planet. Stewart thinks that when Earth was a synestia, the moon may have formed inside it. Then, it orbited there for years before Earth began to cool and shrink, dividing the synestia into two separate bodies with nearly identical isotopes.

She finally developed a theory that could explain everything. Well… almost everything. A more recent computer simulation suggests another version of giant-impact theory: Earth’s moon formed within a few hours of the collision between Earth and Theia.

So, some scientists think there’s still more to the story. Fun fact: as a kid, I once sent a fan email to a NASA astronaut who was on a TV special about how the moon formed. Shout-out to Michelle Thaller for the very kind response and also for being smart and cool.

Okay, so we’ve got some pretty decent ideas for how Earth and the moon formed. But how did we get from asteroid-pummeled planet to life-sustaining paradise? I got two words for you: H2O, baby.

Water didn’t just appear one day out of the blue. Wait, what? That might be exactly what happened?

I mean, okay, space. Give me that chaotic energy I guess. According to some scientists, a space object containing water happened to collide with Earth like a cosmic water balloon, and voilà: the Blue Planet was born.

We know that might be possible because the Ryugu and Bennu asteroids orbiting in our solar system have shown evidence of the same type of water that exists in Earth’s oceans. So it’s possible that asteroids are responsible for bringing water to the planet. Or there may have been water on Earth before any asteroid strikes.

Like, our mystery moon rock and those 4.4-billion-year-old Australian zircons show evidence of water in their isotopes. Which means that there was liquid water on Earth’s surface soon after the planet’s birth. And one 2023 study suggests that, once Earth cooled, it always had water, thanks to hydrogen present in the protoplanetary disk that formed it.

Any of these theories could be correct— or maybe all of them are. But Earth’s water has stuck around because of one thing: its atmosphere. Without it, any liquid water on the surface would’ve evaporated into space.

So… where did it come from? Remember all those volcanoes on that hot, baby Earth? One theory is that volcanic eruptions released gases like carbon dioxide and methane, which made up Earth’s early atmosphere.

Those volcanoes could have also released nitrogen and water vapor, which built up in it. Then, as Earth cooled, the water vapor condensed, forming the oceans. Eventually, early plants started performing photosynthesis and releasing oxygen gas: that kind of important molecule that lets us breathe.

And… ta da! A grown-up Earth ready to take on the universe. I’m so proud.

We may not have a crystal-clear understanding of how our planet formed. That baby photo album is still a work in progress. But if finding an Earth rock on the moon is any indication, there are still a lot of clues out there, just waiting for us to discover them.

When you think about it, the fact that we got here is pretty amazing. After all, Earth could still be a raging, erupting toddler. Next time, we’ll journey below Earth’s surface.

See you then. Thanks for watching this episode of Crash Course Geology, which was filmed at our studio in Indianapolis, Indiana and was made with the help of all these dope people. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.