Martian moon probably pretty porous

The interior of Mars’ moon Phobos could be as much as 30 percent empty space, new observations suggest. Though it’s still not clear how the object formed, the finding means it is probably not an asteroid that was captured by the Red Planet’s gravity, researchers say.

Scientists have long debated the origin of Phobos, and these new findings narrow down the possibilities, says Tom Andert, a planetary geophysicist at the University of the German Armed Forces in Munich. He and his colleagues report in the May 16 Geophysical Research Letters that Phobos almost certainly isn’t a single solid object.

“Finally we’re drifting away from the idea that the Martian moons are captured asteroids,” says Tom Duxbury, a planetary scientist at George Mason University in Fairfax, Va., who was not part of the new study.

Phobos, the larger of Mars’ two moons, is a cratered, irregularly shaped object just 27 kilometers long. Andert and his colleagues collected the best measurements ever of Phobos’ mass by looking at perturbations in the orbit of Mars Express, a Mars-orbiting spacecraft, caused by the moon’s gravitational tug.

Based on the measurements, the researchers estimate that Phobos contains about 10.7 quadrillion metric tons of material — making the lumpy moon about a billionth the mass of Earth. That, plus the improved volume estimate of the moon gleaned from radar measurements, indicates that Phobos’ overall density is about 1.87 grams per cubic centimeter, much less than the 3 g/cm³ average density of the rocks in Mars’ crust.

The density of Phobos is similar to that of some asteroids. However, Andert says, there aren’t many scenarios that would allow Mars to capture an asteroid in a circular orbit without breaking it to pieces.

It’s also unlikely Phobos is made solely of Mars crust blasted into space by an extraterrestrial impact and then reassembled by gravity, as some studies have suggested, because the spectral characteristics of the moon’s rocks don’t match those of the Red Planet.

The truth of Phobos’ origins might be a blend of those scenarios, the team suggests. Phobos may be the remnants of Martian crust blasted into space, reassembled over time by mutual gravitational attraction, and then struck by a passing asteroid that added enough material to change the moon’s spectral characteristics.

Duxbury, who long studied those moons while at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., is “happy to see that Phobos and Deimos are getting a lot of attention these days.”

In the next couple of years, Duxbury says, analysis of radar data gathered during the recent flybys should help scientists discern whether Phobos is a solid body of relatively light rock or a porous amalgamation of dense rubble.