There’s a race taking in Mar’s orbit. It’s not between the two moons of the red planet, but between two courses of destruction on the larger moon, Phobos. The proximity and speed of the moon have placed it in a slow death spiral for millions of years, but now scientists are proposing that Phobos is also in the process of being shredded by Mar’s gravitational pull. Both options leave Phobos being destroyed in no less than 50 million years, with the moon possibly being ripped apart shortly before it would have impacted the surface of Mars as a single unit.
Phobos’ spiraling orbit has been known for some time. The moon has a very tight orbit around Mars, taking a full trip around the planet in just over seven hours (faster than the approximately 25 hour martian day.) The gravitational pull on the moon at this altitude has been pulling the moon closer to the planet by about six feet each century, guaranteeing that Mars will be down a moon eventually. The gravitational pull may also be distorting the moon’s surface, yanking apart what may be an unusually fragile satellite.
The most striking feature of Phobos’ surface is a giant impact crater, but that’s a bit of a red herring in this case. With a diameter of 5.6 miles, at a glance the crater appears to have removed a fifth of the small moon’s mass. However, that feature isn’t the indicator of the coming instability. What has caught astronomers’ eyes is the developing series of ridges reaching out of the crater’s perimeter. These ridges are thought to be the result of tidal forces ripping the moon apart asymmetrically. The amount of rippling suggests that Phobos may actually be built around a loose, gravely core, rather than a chunk of dense material like iron. This would leave Phobos without much structural integrity, making it all the more susceptible to the gravitational tugs from Mars below it.
Tidal forces come with the territory
Our own moon has been affected by tidal forces from Earth, but there are some important differences. Rather than ripples from a stretching surface, our Moon has been found to exhibit ridges called scarps from the volume of the satellite decreasing. The tidal influence in that case is affecting the scarps’ orientation, but it’s not creating them. Structurally, scientists are comparing Phobos and its tearing more to asteroids with surfaces wrapping around “rubble-piles” more than the Moon over Earth.
Source: Grooves On Mars' Moon Are Signs That It's Slowly Shattering, NASA Says by Bill Chappell, The two-way