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The Solar System's round moons Planetary Society member Pavel Gabzdyl created this composite image of all the known round moons of the planets in our Solar System and of the dwarf planet Pluto. Top (left to right): Earth's Moon and Jupiter's moons Ganymede, Callisto, Io, and Europa. Bottom (left to right): Saturn's moons Titan, Rhea, Iapetus, Dione, Tethys, Enceladus, and Mimas; Uranus' moons Titania, Oberon, Umbriel, Ariel, and Miranda; Neptune's moon Triton; and Pluto's moon Charon. NASA, JPL, SwRI, MSSS, Kevin M. Gill, Space Science Institute, U.S. Geological Survey, Johns Hopkins University Applied Physics Laboratory, Southwest Research Institute, Alex Parker