Theia and the Collision That Gave Birth to the Moon

06.12.2023 - 23:13 / atlasobscura.com

Not long after the light first burst from the furnace at the heart of the realm, chaos reigned. Cosmic islands were careening into one another, fracturing and shattering, then melding and building. The most colossal orbs emerging from the pandemonium often had companions of rock or ice dancing around them. One stood largely alone, its fiery surface bubbling away, as another, Theia, approached at breathtaking speed. Soon a white-hot, soundless blast sent an eruption of diamantine matter out to the stars. It took an eon to see the outcome of this massive collision. In the end: a world of green and blue, and its pale companion, Selene.

As far as scientists can tell, this 4.5-billion-year-old creation story is no myth. Not long after the Sun formed, and the planets were beginning to take shape in the cloud of rock, dust, and gas around it, a protoplanetary object roughly the size of Mars (about half the size of Earth) slammed into our still-cooling, mostly molten planet. The collision jettisoned matter into space, mostly from Earth, as the protoplanet was almost entirely vaporized. The wreckage fell into orbit and, after a few million years or so, clumped together.

Scientists refer to the rogue impactor as Theia—an ancient deific entity from Greek mythology. Theia is the mother of Selene, known in Roman mythology as Luna, goddess of the Moon. And this story wasn’t haphazardly assembled by guessing astronomers. They put it together from fragments they could observe.

Lunar rocks have a chemical signature similar to many of Earth’s own volcanic rocks, suggesting a shared origin story. Radioactive matter within those rocks, which acts like a stopwatch that started running when the rocks formed, show that the Moon emerged very shortly after Earth did. And computer simulations demonstrate that you can get the Moon we know and love today if you slam a Mars-size projectile into a magma ocean–smothered Earth.

The one issue with this scientifically grounded saga is what happened to the culprit, Theia. To really establish the hypothesis, some of Theia would ideally be found—but it has long been thought that any surviving fragments were sent screaming into the neverending void billions of years ago. Part of the Moon is almost surely made of Theia matter, but it’s so thoroughly mixed into lunar chemistry that identifying it is all but impossible.

Perhaps, this last part of the scientific story needs to be updated.

On the fringes of Earth’s core are two giant blobs—one beneath Africa, the other below the Pacific. Nobody is quite sure what they are. Seismic waves that pass deep through the planet reveal their existence, and their expansive size—continental, about 30 percent of the core-mantle boundary. But their