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Mars giant impact
An artist’s conception 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.”

The model also predicts that Phobos is doomed to fall to its destruction in 20 million to 40 million years.

“There could have once been many moons around Mars, the most massive sculpting the system, and the smallest being the last to come down,” Erik Asphaugh, a planetary scientist at Arizona State University, says in a commentary on the research. “Phobos could be the straggler in a series of crashing moonlets, readying its final approach.”

The idea that a giant impact gave rise to Mars’ moons may sound farfetched, but it isn’t. Such a hypothesis is the most widely accepted explanation for the creation of Earth’s moon, as well as the moons of Pluto. In Mars’ case, however, the claim runs counter to an alternative view that Phobos and Deimos are captured asteroids.

A couple of years ago, researchers reported that Phobos’ composition appeared to be similar to that of a D-type asteroid, based on ultraviolet spectral readings. But in a study due to be published by the Astrophysical Journal, a different group of researchers backed the giant-impact hypothesis instead.

Future space probes could flesh out the story behind the origins of Mars’ moons. A Japanese mission known as Mars Moons Exploration, or MMX, would be launched in 2022 to collect samples from Phobos and bring them back to Earth. European and Russian scientists are considering a similar mission, dubbed Phootprint, for launch in 2024.

In addition to Rosenblatt, the authors of “Accretion of Phobos and Deimos in an Extended Debris Disc Stirred by Transient Moons” include Sebastien Charnoz, Kevin Dunseath, Mariko Terao-Dunseath, Antony Trinh, Ryuki Hyodo, Hidenori Genda and Steven Toupin.

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