Martian moons have a common ancestor

Mars's two moons, Phobos and Deimos, have puzzled researchers since their discovery in 1877. They are very small: Phobos's diameter of 22 kilometers is 160 times smaller than that of our moon, and Deimos is even smaller, with a diameter of only 12 kilometers. "Our moon is essentially spherical, while the moons of Mars are very irregularly shaped—like potatoes," says Amirhossein Bagheri, a doctoral student at the Institute of Geophysics at ETH Zurich, adding: "Phobos and Deimos look more like asteroids than natural moons."

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