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USGS Astrogeology Research Scientist Greg Vaughan pointing out water on different solar system bodies. The size of the water circles next to each body represents how much water each planetary body would have if you removed it and put it all in a single ball of water. Dr. Vaughan is pointing out how outer solar system bodies like Callisto and Ganymede have so much more water by volume than inner solar system bodies like Earth or Mars. Photo Credit: USGS (public domain).