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Moon and Companions

January 30, 2016

Two bright lights flank the Moon at dawn tomorrow. The star Spica stands to the right of the Moon. And the planet Mars looks like a bright orange star to the lower left of the Moon.

Mars has two moons of its own. But Phobos and Deimos aren’t very impressive. Each looks like an asteroid: dark, potato-shaped, and scarred by countless craters. And since Mars resides next to the asteroid belt, scientists have long thought that the moons really were asteroids that Mars grabbed for itself. But some recent computer simulations have bolstered another idea: that the moons formed much as Earth’s moon did.

Our moon probably formed when another planet slammed into the young Earth. The impact blasted debris into orbit. Much of this material coalesced to form the Moon.

Likewise, some scientists have suggested that Phobos and Deimos formed after a large asteroid hit Mars. But the proposal is so controversial that it took two decades for a paper advocating the idea to get published.

Inspired by that work, other scientists have conducted simulations that indicate that such an impact could indeed create moons like Phobos and Deimos.

But the simulations don’t prove that an impact created the moons. Instead, solving the issue probably requires a visit by a spacecraft. If the moons formed after an impact, they would have been too hot to have any ice. So if a spacecraft finds ice inside the Martian moons, they probably really are captured asteroids.

More about Phobos tomorrow.


Script by Ken Croswell, Copyright 2015


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