Asaph Hall's discovery of Mars's two small moons in 1878 is one of the greatest success stories of observational astronomy. For nearly a century thereafter, however, Phobos and Deimos were little more than dynamical curiosities that traveled oh-so-close to their parent planet.
With the Space Age came the opportunity to see these objects at closer range, and a succession of spacecraft (beginning with Mariner 9 in 1971) has revealed Deimos and especially Phobos with ever-greater detail. Some particularly dramatic views have come from the HiRISE camera aboard Mars Reconnaissance Orbiter.
Yet, despite decades of careful scrutiny from space and from ground-based telescopes, astronomers still debate where these little worlds came from. They're very dark, implying a carbon-rich composition akin to bodies at the outer margins of the asteroid belt. Both also have a low density, suggesting they're riddled with internal cavities. All this evidence points to the notion that Mars somehow captured a couple of wandering objects that happened to stray too close by. (One big "gotcha": both moons have extremely circular orbits very close to Mars's equatorial plane — not exactly the outcome you'd get from two random encounters.)
Now a team of observers has stirred up the pot of possibilities. Yesterday, at the European Planetary Science Congress in Rome, hometown researcher Marco Giuranna (IFSI/INAF) argued that far-infrared spectra of Phobos acquired with the Mars Express orbiter don't match the composition of any known chondritic asteroid or meteorite type. (Chondrites are rocky bodies that have remained unaltered since the solar system's formation.)
What he and others have found instead is a distinct composition rich in dark ultramafic minerals (having lots of iron and magnesium) and clay minerals, called phyllosilicates. The clay signature appears strongest near the large crater Stickney, hinting that deposits were dredged up from deeper down.
Moreover, Martin Pätzold (Köln University) also announced at the ESPC that Phobos has a density of just 1.86 ±0.02 g/cm3. To be so low, the moon's interior must be incredibly porous, with voids taking up 25% to 35% of the total volume. The new density value is based on careful tracking of Mars Express during close flybys of Phobos, including a brush-by just 42 miles (67 km) away last March 3rd.
That mix of clay and ultramafic minerals might be rare among the asteroids, but it's likely a lot more common on the Martian surface down below. The upshot, Giuranna suggests, is that Phobos wasn't captured but more likely formed in place from debris blasted off the surface of Mars during a large, long-ago collision. Most of the debris would have escaped to interplanetary space, but enough of it (11 trillion tons, more or less) hung around to recollect into the two Martian moons.
It's a dramatic hypothesis, to be sure, but is it the correct one? First, as Giuranna's team points out, the new compositional clues don't rule out that Phobos (and Deimos) were captured. Observers have found plenty of asteroids with clay minerals on their surfaces, and ultramafic meteorites (achondrites) aren't exactly rare. Second, the moons' spongy interiors could have resulted if a single body broke apart during its capture by Mars and then then reassembled into a pair of not-quite-solid satellites. Finally, someone needs to run a computer simulation to see if this smash-and-dash scenario makes dynamical sense (according to impact modeler Robin Canup, no one's done it yet).
In any case, we might not have to wait too long to learn the Martian moons' pedigree. A Russian mission called Phobos-Grunt is being readied for launch late next year. The effort has encountered technical problems — it was supposed to head off toward Mars a year ago — but if it succeeds, scientists will have 100 to 200 grams of Phobos to analyze when the sample-return capsule lands in July 2014.