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
“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.)