Callisto (bottom left), Jupiter (top right) and Europa (below and left of Jupiter's Great Red Spot) as viewed by Cassini-Huygens. Image source: [1]
Callisto is the third largest moon of the Solar System, and the second of Jupiter, behind Ganymedes. It is the same size as Mercury, the planet closest to the Sun, and is composed of equal parts of ice and rocks. Almost in all probability it has a nucleus composed of siliceous, and may an ocean of water beneath the surface. This satellite is one of the few that, like the Moon, has a synchronous rotation, that is, it always shows the same face towards Jupiter. Its surface shows no evidence of geological activity, although it has the highest crater impacts of the Solar System.
Callisto's anti-Jovian hemisphere imaged in 2001. Image source: [2]
This image was taken in May 2001 by NASA's Galileo satellite, which studied the Jupiter system from 1995 to 2003. Looking similar to a golf ball, Calisto's surface is covered with cracks and craters. In fact Callisto is the body with the most craters of the entire solar system. It is believed that the brightest areas are mostly water ice, while the darker spots are more eroded regions of rocky material with little ice.
Callisto possesses a magnetic field when, according to the best models of planetary composition, he should not have it. The existence of a magnetic field would normally mean that a moon has a distinct nucleus, but in Callisto this can not be. Most likely, the Callisto field is "induced", which is believed to orbit the much more powerful magnetic field of Jupiter itself. However, for this to happen, Callisto should have a conductive fluid layer inside it and the properties of the field suggest that it is an ocean of salt water beneath the bark.
More details here: Callisto
Size comparison of Earth, Moonand Callisto. Image source: [3]