Mars Moons Not Captured Asteroids After All?

Phobos and Deimos likely formed after a big inner moon crashed into Mars, a new study shows.

Image: A view of potato-shaped Phobos from NASA's Mars Reconnaissance Orbiter spacecraft. Credit: NASA/JPL-Caltech/University of Arizona A long-held theory that Mars' two moons, Phobos and Deimos, are captured asteroids is seriously challenged in a new study showing how the pair could have formed in Mars orbit after a previous generation of one or more massive inner moons crashed into the planet.

Phobos and Deimos were never a perfect fit for the captured-asteroid theory.

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"It's like throwing a dart blindfolded at a moving bicycle wheel, trying to get the tip to stick in the rubber, and doing it twice," planetary scientist Erik Asphaug, with Arizona State University, wrote in an email.

If the two were captured asteroids they would have started off with random, highly inclined and egg-shaped orbits around Mars, which then, by some unknown mechanism, shifted into their present circular paths over Mars' midsection.

"You just can't get capture to happen, without a mechanism for damping these random orbital elements down to the equatorial plane of Mars, and circularizing," Asphaug said. "Capture of two ... provides no mechanism for getting them to give up their original wacky orbits."

Intrigued by the mystery, Pascal Rosenblatt at the Royal Observatory of Belgium and colleagues decided to work on a computer simulation that might better fit an alternative explanation for the origin of Phobos and Deimos, that they formed from debris around Mars which had been blasted into space by a giant collision.

"Our first tries were inconclusive," Rosenblatt wrote in an email. "We were able to form five or six small satellites, but not only two."

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The model clicked when the scientists added a giant inner moon -- or multiple moons -- 1,000 times more massive than Phobos and confirmed that such a large moon could indeed form out of debris from an earlier crash between a small planet-like body and Mars.

A similar crash between the hypothesized protoplanet Theia and early Earth is believed to have led to the formation of Earth's moon.

There is some physical evidence for a moon crash on Mars. A previous study showed that Mars' Borealis basin, an asymmetrical region in the planet's northern hemisphere, could be the impact site of a large moon.

"We didn't expect such an agreement between our (study) and their study," Rosenblatt said. "It was really a big surprise," Rosenblatt said.

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More definite proof may come soon. If Phobos and Deimos are captured asteroids, they would have a different chemical composition than Mars.

"If the Martian moons would contain Martian material it would support our scenario," Rosenblatt said. "If they would contain only asteroid material it would support the capture scenario," Rosenblatt said.

The Japanese space agency JAXA plans to launch a Phobos sample return mission in 2022. Europe and Russia are partnering on a similar venture two years later.

However the moons formed, Deimos will one day fly solo around Mars. Phobos, like its proposed ancestor moon, is spiraling toward Mars, though the planet's gravity likely will break the small moon apart before it reaches the atmosphere. The demise of Phobos likely will create a new debris ring, perhaps one day providing building blocks for a next-generation moon.

The research is published in this week's Nature Geoscience.

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