Discovered in 1877 by American astronomer Asaph Hall, Mars’ moons Phobos and Deimos were named after the horses that pulled the war god’s chariot.
But much like the wear the body experiences with age, Phobos is under the destructive influence of tidal forces, the mutual gravitational pull of the planet and the moon. Already closer to its planet than any other moon in the solar system (orbiting 3,700 miles away from Mars’ surface), Phobos is drawn closer to Mars by 6.6 ft every hundred years. At this rate, scientists estimate the moon will be ripped apart in 30 to 50 million years.
“We think that Phobos has already started to fail, and the first sign of this failure is the production of these grooves,” said Terry Hurford, of NASA’s Goddard Space Flight Center.
Modeling of the moon shows shallow grooves lining Phobos’ surface. Previously, it was thought the grooves were the result of the collision that created the moon’s largest crater Stickney. However, scientists later determined the grooves’ focal point was not the crater.
Hurford and colleagues discovered the grooves were more like stretch marks.
“The gravitational pull between Mars and Phobos produces these tidal forces,” according to NASA. “Earth and our moon pull on each other in the same way, producing tides in the oceans and making both planet and moon slightly egg-shaped rather than perfectly round.”
In order for the tidal forces to exhibit such fracturing power, scientists believe Phobos’ interior may be a rubble pile surrounded by a regolith layer about 330 ft thick.
“The funny thing about the result is that it shows Phobos has a kind of a mildly cohesive outer fabric,” said Erik Asphaug, of the Univ. of Arizona’s School of Earth and Space Exploration. “This makes sense when you think about powdery materials in microgravity, but it’s quite non-intuitive.”
According to Hurford, planetary bodies falling into their host star can get torn apart in a similar way.