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


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.)