How a crash created many moons for Mars

Image: Mars smash-up
Artwork shows the collision of Mars with another celestial object. The scenario could have given rise to a debris disk, and eventually to Mars’ two present-day moons. (Copyright 2016 Labex UnivEarthS)

Are Mars’ two moons asteroids that were captured by the Red Planet’s gravitational field, or are they the result of an ancient smash-up? Astronomers have now laid out a series of computer simulations to argue in favor of the smash-up hypothesis, and the modeling suggests that Mars should have had a giant moon early in its history.

In a study published today by Nature Geoscience, the researchers say the giant moon would have been created out of the debris from the collision between Mars and another celestial object about a third of Mars’ size. The crash would have occurred sometime between 100 million and 800 million years after Mars’ formation.

Within about 5 million years after the crash, the big moon and a bevy of smaller moons would have broken up and fallen to the surface. But the simulations show that Phobos and Deimos, the two moons we know about today, would have survived all the tumult and ended up in their present-day orbits.

“The proposed scenario can explain why Mars has two small satellites instead of one large moon,” the scientists, led by Pascal Rosenblatt of the Royal Observatory of Belgium, say in their paper. “Our model predicts that Phobos and Deimos are composed of a mixture of material from Mars and the impactor.”

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By Alan Boyle

Mastermind of Cosmic Log, contributing editor at GeekWire, author of "The Case for Pluto: How a Little Planet Made a Big Difference," president of the Council for the Advancement of Science Writing. Check out "About Alan Boyle" for more fun facts.

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