Mars' moons, Phobos and Deimos, are unusual on many counts. In addition to being among the darkest objects in the Solar System, Phobos and Deimos are thought to be recently captured asteroids rather than satellites that formed along with, or from, the planet they circle.
Most unusually, Phobos orbits faster than Mars spins, so while Deimos appears to a Martian observer to rise in the east and set in the west, like the Sun and stars, Phobos does the reverse. This extraordinary behavior is an effect of orbiting just 6,000 kilometers (3,700 miles) above the surface of Mars, and 9,400 kilometers (5,900 miles) from Mars' center. This makes it the closest natural satellite to its planet in the Solar System, although some twin asteroids orbit each other at much smaller distances.
Even around a planet with gravity as weak as Mars this is a dangerous place to be. Tidal interactions push our own Moon away from the Earth, but it has been known for decades that Phobos is creeping towards Mars at the rate of 1.8 meters (6 feet) a century.
This pulls Phobos closer and closer to the Roche limit, the place where gravitational forces will pull any solid orbiting object apart. This occurs because the gravity on the planet-facing side is so much larger than on the other side it can't stay in one piece. Eventually Phobos will be destroyed, forming a smaller version of Saturn's rings.
At the 47th Annual Meeting of the American Astronomical Society's Division of Planetary Sciences this week, Dr Terry Hurford presented evidence that this is underway. “We think that Phobos has already started to fail, and the first sign of this failure is the production of [grooves on the surface],” Hurford said in a statement.
Phobos has been through a lot, including the collision that caused the crater Stickney. At nine kilometers (5.6 miles) across, it is almost half as wide as the moon itself. The grooves Hurford referred to run from Phobos' leading apex and are 100 to 200 meters (330 to 650 feet) wide and up to 30 meters (100 feet) deep. They were thought to be remnants of the impact that caused Stickney or encounters with smaller objects thrown up by Mars.
Hurford is reviving an old theory. When the Viking missions returned images of Phobos' grooves in the 1970s, astrophysicists wondered if these were early signs of its impending demise. However, calculations indicated that the forces on Phobos are not large enough to produce these effects on a solid object.
Since then, however, we have learned that many asteroids are “rubble piles,” barely held together by their own gravity. It is now thought that Phobos is like this on the inside, cased in a layer of powdery material. Hurford pointed out that the tidal forces Mars exerts could produce surface grooves on something this fragile.
Previous estimates give Phobos 30 to 50 million years, but if Hurford is right it might break up even sooner.