The days are numbered for one of Mars' two moons, and the culprit turns out to be Mars itself. It seems that the two are engaged in what Discovery describes as a gravitational tug-of-war, and Mars is going to win. It will take a while, about 30 million to 50 million years, but eventually Phobos will literally fall apart. For now, Mars is drawing the moon toward it at a rate of about 6 feet per century, though the two won't actually collide. Someday, Phobos will drop low enough to reach what's known as the Roche Limit, explains Universe Today, and it will be "torn apart" by the planet's tidal forces. Researchers came to the conclusion by studying the moon's distinctive grooves, marks that were once thought to be created by asteroid strikes. Now, however, they liken them to "stretch marks" formed by this tug-of-war, the scientists say in a post at Science Daily.
"We think that Phobos has already started to fail, and the first sign of this failure is the production of these grooves," says Terry Hurford of NASA's Goddard Space Flight Center. "Eventually, Phobos will be ripped apart before it reaches Mars’ surface." Phobos already holds the distinction of being the moon closest to its planet in our solar system, orbiting at a relatively scant distance of 3,700 miles. While the idea that the moon's grooves were formed by tidal forces, not impacts, was first floated years ago, it was generally rejected because Phobos was thought to be too solid to succumb to such forces. The new thinking is that Phobos' interior is more like a "rubble pile, barely holding together," according to NASA. (Mars may be doing a number on Phobos, but the sun is doing a number on Mars.)