Small, bright patches of pure water ice are intermixed with patches of a rock–ice mixture and extended dark areas made of a non-ice material.
It was found that Callisto responds to Jupiter's varying background magnetic field like a perfectly conducting sphere; that is, the field cannot penetrate inside Callisto, suggesting a layer of highly conductive fluid within it with a thickness of at least 10 km.
The density and moment of inertia are compatible with the existence of a small silicate core in the center of Callisto.
The radius of any such core cannot exceed 600 km, and the density may lie between 3.1 and 3.6 g/cm In fact, the crater density is close to saturation: any new crater will tend to erase an older one.
Galileo orbiter data—0.3549 ± 0.0042—determined during close flybys) suggest that its interior is composed of compressed rocks and ices, with the amount of rock increasing with depth due to partial settling of its constituents.
In other words, Callisto is only partially differentiated.
The slow convection in the interior of Callisto, which commenced soon after formation, led to partial differentiation and possibly to the formation of a subsurface ocean at a depth of 100–150 km and a small, rocky core.
Various space probes from Pioneers 10 and 11 to Galileo and Cassini have studied Callisto.
Callisto's surface can be divided into several geologically different parts: cratered plains, light plains, bright and dark smooth plains, and various units associated with particular multi-ring structures and impact craters.Callisto's anti-Jovian hemisphere imaged in 2001 by NASA's Galileo spacecraft. The large impact structure Asgard is on the limb at upper right.The prominent rayed crater below and just right of center is Bran.This is thought to result from the sublimation-driven degradation of small landforms, which is supported by the general deficit of small impact craters and the presence of numerous small knobs, considered to be their remnants.Callisto's gradual accretion and the lack of tidal heating meant that not enough heat was available for rapid differentiation.