Mars’ potato-shaped moons may be the remains of a shattered asteroid

The moons of Mars may be the remnants of an asteroid with a rogue star that came too close to the Red Planet.

A fragmented asteroid origin may help explain the mysterious features of small, oddly shaped moons, scientists suggest in the January issue. Icarus.

Where most moons are big round balls, Mars’ Phobos and Deimos are little bumpy potatoes.

There are two main ideas about how the moons formed. One is that the moons were actually asteroids that were captured by the gravity of Mars. But this idea does not explain the moon’s circular and stable orbits around the equator of Mars.

The other is that Phobos and Deimos formed like our moon, from the debris of a giant impact (SN: 7/4/16). “This is one of the few ways to make a large disk from which to grow moons,” says planetary scientist Jacob Kegerreis at NASA’s Ames Research Center in Mountain View, California.

Kegerreis and colleagues suggest something in between: Maybe Mars captured an asteroid, but the planet’s gravity tore the rock apart. The debris could have formed a short-lived ring around the Red Planet. Potato moons can form from there, with circular orbits already established.

To test the idea, Kegerreis and colleagues ran hundreds of computer simulations of doomed asteroids in the vicinity of Mars. The different sizes, speeds, and spins of the asteroids made a big difference in how the rings formed—but the rings did form.

“What we were pleased to find is that, under a variety of conditions, you get a lot of material that can form a disc,” says Kegerreis.

A future mission may help to show which idea is correct. The Japan Space Agency’s Lunar Exploration Mission to Mars is scheduled to launch in 2026 with plans to collect surface material from Phobos and return it to Earth.

If those samples have a composition similar to Mars, that supports the giant impact hypothesis, Kegerreis says. If they are more asteroid-like or have more water and other compounds that would evaporate in the heat of an impact, then the fragmented asteroid hypothesis seems more likely.

Studying these moons can also help to understand the moons around extrasolar planets (SN: 1/13/22).

“Even if this isn’t how the moons of Mars formed in particular, it could be how moons form around another planet,” says Kegerreis. “Now that we’re finding all these exoplanets and hopefully exomoons, it’s worth exploring how these things might happen in different solar systems, even if not in this one.