The average diameters of Deimos and Phobos are 12 and 22 km respectively. Both moons are misshapen because their low gravity (1/1000th of Earth's) is insufficient to pull them into spheres.
Hypotheses of the formation of Phobos and Deimos range from Mars impact remnants to captured asteroids (due to their similar size and composition to many asteroids).
Deimos - Credit: NASA/JPL/University of Arizona
Phobos - Credit: NASA/JPL/University of Arizona
Phobos' Stickney Impact Crater - Credit: NASA/JPL/University of Arizona
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