Callisto is the outermost of the four Galilean moons of Jupiter (identified as such because they were discovered by Galileo Galilei, and important because they were the first objects discovered that clearly could not be orbiting the earth), and the only one not currently thought to possess a subsurface ocean. Io has been much in the news the last few days with new results supporting the hypothesis of a subsurface magma ocean, while Europa and Ganymede are thought to have water oceans under crusts of water ice. One reason to believe that Callisto does not possess such an ocean is its heavily-cratered and obviously very old surface, unmodified by tectonic processes, such as can be seen on Europa and Ganymede, or volcanism, such as can be seen on Io. A second reason is that Callisto is not subject to tidal forces as strongly as the inner three, so tidal kneading and heating are much weaker.
According to Wikipedia (which is also the source of the image), "It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass." Another way of putting this is that Callisto's density is about a third of Mercury's, a difference due to the latter's suspected massive iron core, and the former's large component of low-density water and ice.
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