Scientists have long wondered where Mars' two moons, Phobos and Deimos, came from. The leading theories: They're asteroids snagged by Mars' gravity from the outer part of the main asteroid belt which lies between Mars and Jupiter; they formed from debris that settled into orbit around Mars after an asteroid or comet smashed into the planet; or they formed from the remnants of a prior moon that had been ripped apart by tidal forces.
New evidence suggests you can kiss the captured-asteroid theory good-bye, say astronomers who presented a compositional analysis on Phobos drawn from data collected by two Mars-orbiting science probes.
The scientists say materials in Phobos, the larger of the two moons, don't match up with the carbon-rich materials found in meteorites that are tied to asteroids from the middle part of the asteroid belt. Instead, they found a type of mineral known as phyllosilicates on the moon's surface, concentrations of which are particularly high northeast of the moon's largest impact crater.
"This is very intriguing as it implies the interaction of silicate materials with liquid water on the parent body prior to incorporation into Phobos," Marco Giuranna, with Istituto Nazionale di Astrofisica in Rome, said in a statement.
The mineral could have formed on Phobos, but that would mean the moon had enough heat to keep liquid water stable, he added.
The scientists also found other minerals on Phobos that appear to match the types of minerals found on Mars. And, they determined that Phobos, which orbits about 3,700 miles from the planet's surface, is rather spongy, unlike denser material from meteorites that are associated with asteroids. A porous asteroid probably wouldn't have survived getting captured by Mars, the astronomers point out.
The research was presented this week at the European Planetary Science Congress in Rome.
Phobos is the target of a joint Russian-European sample return mission scheduled for launch next year.
(Image: Phobos, a chip off the home world? Credit: NASA/JPL/University of Arizona.)