Callisto [kah-LISS-toe] is the second largest moon of Jupiter, the third largest in the solar system, and is about the same size as Mercury. It orbits just beyond Jupiter's main radiation belt. Callisto is the most heavily cratered satellite in the solar system. Its crust is very ancient and dates back 4 billion years, just shortly after the solar system was formed.
Callisto lacks any large mountains. This is probably due to the icy nature of its surface. Impact craters and associated concentric rings are about the only features to be found on Callisto. The largest craters have been erased by the flow of the icy crust over geologic time. Two enormous concentric ring, impact basins are found on Callisto. The largest impact basin is Valhalla. It has a bright central region that is 600 kilometers in diameter, and its rings extend to 3000 kilometers in diameter. The second impact basin is Asgard. It measures about 1600 kilometers in diameter.
Callisto has the lowest density (1.86 gm/cm3) of the Galilean satellites. From recent observations made by the Galileo spacecraft, Callisto appears to be composed of a crust about 200 kilometers (124 miles) thick. Beneath the crust is a possible salty ocean more than 10 kilometers (6 miles) thick. Beneath the ocean, is an unusual interior that is not entirely uniform nor does it vary dramatically. Prior to Galileo, scientists believed that Callisto's interior was totally undifferentiated, but Galileo data suggests that the interior is composed of compressed rock and ice with the percentage of rock increasing as depth increases. Meteorites have punctured holes in Callisto's crust, causing water to spread over the surface and forming bright rays and rings around the crater. Callisto has no known atmosphere.
When the Galileo spacecraft flew by Callisto, it made measurements which showed that Callisto was made almost entirely of the same thing. This means that Callisto never separated into layers, but is probably formed of a unusual material which is made of rock and ice combined.
At the same time, there were some measurements that suggested that there might be a thin salty-slushy layer of watery-ice right under the solid surface of Callisto that creates an electric current. This watery-ice layer is thought to be on Ganymede and Europa as well.
Callisto has a faint ionosphere. Callisto’s ionosphere is more powerful than would be explained as an interaction between its thin carbon dioxide atmosphere and Jupiter’s magnetic field. Magnetic resonance imaging shows that there is a layer below the surface that conducts Jupiter’s magnetic field. If it were an ocean, it would be very salty and lie about 100 to 200 kilometers below the crust. If the layer is comprised entirely of ice, it would be up to 300 kilometers thick. Due to the lack of plate tectonics and volcanic activity, Callisto has probably never had liquid water under its surface and has been frozen to its core since its creation. There are astronomers who theorize that Callisto has an ocean at least 150 kilometers below its surface, but there is less evidence for this than the other Jovian moons.