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Based on new evidence from Jupiters moon Europa, astronomers hypothesize that chloride salts bubble up from the icy moons global liquid ocean and reach the frozen surface where they are bombarded with sulfur from volcanoes on Jupiters moon Io. credit : NASA/JPL-CalTech
Callisto, 2nd largest of Jupiter's moons and 3rd largest moon in the Solar System. There is thought to be a liquid ocean under the surface, which could potentially host microbial life, though it's less likely than Europa to support life. The crust of the moon is extremely old and cratered nearly to saturation (any new craters will erase old craters). Callisto's rotation is tidally locked, so the same side always faces Jupiter, which sits motionless in the Callistan sky.
Ganymede is Jupiter's largest moon, and is the largest moon in the Solar System, even slightly larger than the planet Mercury. It has a liquid iron core, a thick icy crust, and layers of silicate rock in between. There is also thought to be a saltwater ocean about 124 miles below the surface. It was discovered in the 1990s that Ganymede has a very thin oxygen atmosphere and an ozone layer.