How Earth got its moon

full moon
full moon

The story of our moon’s origin does not add up. Most scientists think that that the moon formed in the earliest days of our solar system. That would have been back around 4.5 billion years ago. At that time, some scientists suspect, a Mars-sized rocky object — what they call a protoplanet — smacked into the young Earth. This collision would have sent debris from both worlds hurling into orbit. Some of the rubble eventually would have stuck together, creating our moon.

Or maybe not.

Astronomers refer to that protoplanet as Theia (THAY-ah). Named for the Greek goddess of sight, no one knows if this big rock ever existed — because if it did, it would have died in that violent collision with Earth.

moon Theia
Early Earth and a smaller protoplanet called Theia may have collided long ago, many scientists think. That would have hurled debris from both into space. In this simulation, red particles escaped the system, yellow formed the moon and blue fell to Earth.

And here’s why some astronomers have come to doubt Theia was real: If it smashed into Earth and helped form the moon, then the moon should look like a hybrid of Earth and Theia. Yet studies of lunar rocks show that the chemical composition of Earth and its moon are exactly the same. So that planet-on-planet impact story appears to have some holes in it.

That has prompted some researchers to look for other moon-forming scenarios. One proposal: A string of impacts created mini moons largely from Earth material. Over time, they might have merged to form one big moon.

“Multiple impacts just make more sense,” says Raluca Rufu. She’s a planetary scientist at the Weizmann Institute of Science in Rehovot, Israel. “You don’t need this one special impactor to form the moon.”

But Theia shouldn’t be left on the cutting room floor — at least not yet. Earth and Theia could have been built largely from the same type of material, new research suggests. Then they would have had a similar chemical recipe. There is no sign of “other” material on the moon, this explanation argues, because nothing about Theia was different.

“I’m absolutely on the fence between these two opposing ideas,” says Edward Young. He studies cosmochemistry — the chemistry of the universe — at the University of California, Los Angeles. Determining which story is correct is going to take more research. But the answer could offer profound insights into the evolution of the early solar system, Young says.

Mother of the moon

Earth’s moon is an oddball. Most other moons in our solar system live way out among the gas giants, such as Saturn and Jupiter. The only other terrestrial planet with orbiting moons is Mars. Its moons, Phobos and Deimos, are small. The leading explanation for them is that likely were once asteroids. At some point, they were captured by the Red Planet’s gravity. Earth’s moon is too big for that scenario. If the moon had come in from elsewhere, it probably would have crashed into Earth or escaped and fled into space.

An alternate explanation dates from the 1800s. It suggests that moon-forming material flew off of a fast-spinning young Earth. (Imagine children tossed from an out-of-control merry-go-round.) That idea fell out of favor, though, when scientists calculated the spin speeds required. They were impossibly fast.

In the mid-1970s, planetary scientists proposed the giant-impact hypothesis. (Later, in 2000, they named that mysterious planet-sized body as Theia.) The notion of a big rocky collision made sense. After all, the early solar system was like a game of cosmic billiards. Giant space-rock smash-ups were common.

But a 2001 study of rocks collected during NASA’s Apollo missions to the moon cast doubt on the giant-impact hypothesis. Research showed that Earth and its moon were surprisingly alike. To figure out a rock’s origin, scientists measure the relative abundance of different forms of oxygen. Called isotopes (EYE-so-toaps), they are forms of an element with different masses. (The reason they differ: Although each has the same number of protons in its nuclei, they have different numbers of neutrons.)

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isotope moon
The mix of oxygen isotopes inside moon material and meteorites called enstatite chondrites (shaded yellow) is surprisingly similar to that of Earth rocks (blue line). Other solar system materials are largely composed of different isotopic mixes.

Scientists can use amounts of various isotopes as something like fingerprints at a crime scene. Rocks from Earth and its moon, the scientists found, had seemingly identical mixes of oxygen isotopes. That didn’t make sense if much of the moon’s material came from Theia, not Earth. Rufu and her colleagues modeled the impact on a computer. From that they calculated the chance of a Theia collision yielding a moon with an Earthlike composition. And it was very slim.

Studies have been done of other elements in moon rocks, such as titanium and zirconium. They, too, suggest that Earth and its moon…