Callisto is the farthest from Jupiter of the Galilean satellites. It is the third largest satellite in the Solar System, about the same size as the Earth’s Moon. It’s darker in color than any of Jupiter’s other large satellites, although it’s still twice as bright as our Moon, due to the high albedo of its surface ice. Callisto was discovered by Galileo and names for one of Zeus' many lovers.
Jupiter and its moons Callisto and Europa. Click here for original source URL
Jupiter's moon Callisto. Click here for original source URL.
Four billion years of bombardment by interplanetary debris has scarred the surface of Callisto. It’s the most heavily cratered, and the oldest, landscape in the solar system. Impact craters appear bright against the surrounding surface, because the impacts uncover underlying brighter ice with less dark, carbonaceous material mixed in. Some craters may also be flooded with purer ice, like Valhalla, one of the largest impact structures on any world. It stretches 4,000 kilometers (2,500 miles) across the face of Callisto, and the center is filled with bright ice that flooded the original crater. The bright center is surrounded by over 25 concentric rings & mdash truly a multi-ring basin! The unusually high number of rings may be due to a thin, icy crust, which cracked easily upon impact. Because ice relaxes slowly over time, Callisto no longer has high mountains or deep craters. It’s very smooth, like Europa and the other icy satellites.
The Galileo mission to the Jupiter system found that there are less small craters on Callisto’s battered surface than researchers expected to find. It’s possible that small craters are filled in by the dark carbonaceous material that covers the surface, or there may be less small impactors than we expected in the neighborhood of Jupiter. But some photos revealed craters with ragged rims, as if they are being broken up. The best explanation for this is erosion: if volatiles in the icy surface are sublimating away into space, they would leave behind just the rocky component of the surface. Over time, perhaps this mechanism is erasing the small craters.
Callisto’s low density (only twice that of water ice) can only be explained by a composition of roughly equal parts ice and rock, with no metallic core like Ganymede’s. It could be that the rock is concentrated more towards the center of the satellite, or it may be mixed uniformly throughout. Available evidence suggests that Callisto has a subsurface oceans at depths below 100 kilometers, making it another of the Solar System's "water worlds."
There is no evidence on Callisto for tectonic activity or any other resurfacing process, like on Ganymede. Why are these two satellites so different? They are nearly the same size, and should have had similar starting compositions. Something must have occurred differently in their pasts. By studying the relatively simple Callisto, perhaps someday we will be able to explain the complicated geology of other large satellites. The European Space Agency's Icy moon Explorer will make several close fly-bys of this fascinating moon sometime after 2020