The "Big Splash" that formed the Moon was ten times bigger than we thought

Astronomers think that the Moon was formed when a Mars-sized planet smashed into the still-growing Earth some 4.5 billion years ago--a titanic collision often called the "Big Splash."

New high-precision comparisons of the ratio of potassium isotopes in rocks from the Earth and Moon make that collision even more catastrophic than previously thought—releasing so much energy that all of the impactor and most of Earth's mantle were splashed into orbit, forming a hot, dense rapidly-rotating disc from which the Moon quickly condensed.

Artist's conception of Moon-forming impact

Credit: NASA/JPL-Caltech

“Our results provide the first hard evidence that the impact really did—largely--vaporize Earth,” says Kun Wang, a geoscientist at Washington University in St. Louis, Missouri. He and Harvard University researcher Stein Jacobsen detailed their findings in a Nature article published today.

Wang and Jacobsen developed analytic techniques that let them measure minute differences between Earth and Moon rocks for the first time. It turns out that Moon rocks have significantly more heavy isotopes of potassium than Earth rocks, which is best explained by partial condensation from a superheated disc with a high internal pressure. That, in turn, requires a collision ten times more powerful than previously estimated.

Old and new collision models: In the lower-energy collision (top), the Moon has no more heavy potassium than Earth. In the higher-energy collision (bottom), the Moon has more heavy potassium than Earth, as the new study found. Credit: Kun Wang

What was earlier hypothesized to be a Mars-sized object--named Theia after the mythological mother of Selena, the Moon--crashing into the proto-Earth might have been hurtling through space much more rapidly than thought, or it might have been much more massive, perhaps as large as Earth. “It does not have to be Mars-sized anymore,” says Wang.

Many scientists think that our unusually large Moon—one quarter of Earth's diameter—has played a vital role by stabilizing and slowing Earth's rotation, making it easier for life to develop here. If they're right, we may owe our existence to a 4-billion-year-old smashup that almost vaporized Earth.