The larger of Mars’s two moons, Phobos, is in the process of being torn apart by its parent planet, according to findingspresented at the annual Meeting of the Division of Planetary Sciences of the American Astronomical Society this week.
Phobos, a small, irregularly shaped moon has has long, shallow grooves across its surface. For years, scientists believed these were the result of an massive collision with an asteroid that created the moon’s six-mile (9.5-kilometer) wide Stickney Crater and nearly shattered the 11–17 mile (18–28 kilometer) diameter moon. However, new analysis of these grooves show that they do not originate from the crater, but rather from another nearby point. Instead of shatterlines, these grooves may be “stretch marks” that occur as Phobos is deformed by tidal forces from Mars—which makes them early signs of structural failure that will eventually destroy the moon.
Just like Earth and its moon, Mars and its moon gravitationally tug on each other, which causes the tidal forces. The closer the two objects, the stronger the pull, and Phobos is closer to Mars than any other moon to its planet—a mere 3,700 miles (6,000 kilometers) compared to the Earth–Moon distance of about 240,000 miles (390,000 kilometers). At that distance, the tidal force inflicted in Phobos generates enough stress to create the grooves.
Phobos’s structure does not help. The moon has a low density, meaning it is not solid rock. Instead, the exterior is thought to be fairly cohesive and elastic while internally it is more of a delicate rubble pile. This kind of interior has little strength and distorts easily, forcing the outer layer to adjust. The outer layer is mostly flexible enough to make this adjustment, but it builds up stress that eventually cause the exterior to fail and fracture the surface.
And Mars’s influence will only get stronger, as it drags the moon about 6.6 feet (two meters) closer every century. This probably means Phobos is rapidly approaching its demise—sometime in the next 30–50 million years. –Elise Ricard