Scientists Think Earth Once Had 2 Moons

A new theory by planetary scientists says the Earth may once have been orbited by two moons, which formed early in the history of the solar system and collided slowly into one another to form a single Moon.

Collision course

Scientists believe the Moon was formed when a Mars-sized object crashed into Earth some 4.5 billion years ago and the debris accreted, or came together to form the Moon.

But to Erik Asphaug a planetary scientist at the University of California Santa Cruz, that does not address what astronomers call the lunar dichotomy between the visible side of the moon, marked by craters and a thin crust, and the far side of the moon, known to contain a thicker crust and mountain ranges topping 3,000 meters.

Using a computer model, Asphaug and Martin Jutzi of the University of Bern in Switzerland devised a scenario of what might have caused the geographic dichotomy between the visible and far sides of the Moon. They believe it was caused by a slow-moving collision, less than 2.5 kilometers per second, by a nearby smaller moon.

“It turns out when the impact that is striking the moon comes from right next door to the moon, it doesn’t have a lot of velocity - it’s a pretty slow collision," said Asphaug. "And by the time it hits the moon, it doesn’t have enough energy to excavate a big crater. All it has the velocity to do is squash itself flat as a big pancake. And that was a real surprising finding of the result, and when we saw that in the renderings of the computer simulations, we knew we were on to something pretty interesting.”