Mars has two moons, but they’re both quite small and unusual. Phobos is the larger of the two, and it orbits close to the planet. Even though mars has less mass than the Earth, a new analysis of Phobos’ geology says that the moon is in the process of being ripped to shreds by Mars‘ gravity. It might not happen tomorrow, but the evidence points to a violent end for this moon.
Phobos is less than 20 miles in diameter, but the smaller Martian moon of Deimos is under 10 miles across. Both objects are believed to be captured asteroids, so their composition is somewhat unusual in that they don’t share their origin with the planet they orbit. Until we took a closer look at Phobos, we always assumed it was a solid piece or rock. However, data from the 2008 Mars Express mission indicates that it’s actually a ball of rubble with a layer of dust 50 to 100 meters thick on the surface holding it together.
Ever since we got a close look at Phobos in the 1970s, scientists have been interested in the a series of parallel grooves that stretch hundreds of meters across the surface. In the past astronomers and geologists believed these were cracks resulting from the impact that created the prominent Stickney crater at one end of the moon. That assertion doesn’t really hold up if Phobos is a giant bag of gravel.
The new analysis of Phobos was conducted by Terry Hurford of NASA’s Goddard Space Flight Center. He says the grooves on the surface match up with the areas of greatest stress from the tidal forces of Mars. They’re essentially fault lines where the moon is beginning to come apart.
Phobos won’t be going to pieces any time soon, though. Hurford estimates it will be several million years before it breaks up, giving humanity plenty of time to land a mission on Phobos. What we know of its composition now will certainly change how we approach such a landing.