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In the same way that the Moon's gravity causes tides on Earth, Phobos's gravity affects the shape of Mars. The gravity from distorted planets acts back on the moons, causing them to spiral inward or outward. Distant moons, like ours, are pushed outward while close-in moons, like Phobos, are pulled inward. Phobos's orbital decay is expected to last about thirty million years. There are two possible fates: i) Phobos may be pulled apart by strong tidal forces, thereby forming a new martian ring or ii) the moon may remain intact and crash into the Mars at a very shallow angle. Either outcome would be spectacular to witness! For more on Mars's moon Phobos click here.
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