Scientists have known for some time that Phobos, the larger of Mars' two small moons, is a victim of gravity, edging closer toward its parent planet.
But new research shows Phobos already is starting to fall apart.
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Long shallow grooves cut into the moon's surface appear to be stress fractures, according to a study presented at the American Astronomical Society's planetary sciences meeting in Maryland this week.
Previously, scientists thought the grooves were fractures from an asteroid impact that nearly shattered the moon. The impact left a lasting impression in the form of the Stickney crater, a six-mile wide basin that is nearly half the width of Phobos itself. Later analysis showed the cracks are not stemming outward from the crater, but radiate from another point nearby.
That led to a new theory that the grooves are produced by small pieces of debris flying off Mars and smashing into Phobos, which orbits as close as just 5,800 miles above the Martian surface, closer than any other known moon circles its parent planet.