Callisto is the outermost of the four Galilean moons. It is a large moon, 1.5 times larger than our moon (4840 km). This moon orbits Jupiter at a distance of 1.9 million km with a period of 16.6 days. Callisto's density is much lower than Earth's moon however, being only 1.8 g/cm3. This low density indicates that the moon is probably composed of rocky and icy materials in equal proportions.

     Images of this world show a heavily cratered surface blasted into the ice. It is easy to tell the newest craters, the ejecta around them is much lighter in color. Most of the craters are very old however, it is suggested that Callisto's surface is as ancient as the solar system itself.

     The most obvious, and certainly most interesting feature on Callisto is a large formation called Valhalla. It is a huge impact site with about 10 concentric rings surrounding it. These rings are probably ripples from the initial impact that froze into place. It is estimated that Valhalla formed about 4 billion years ago judging by the number of impact craters within it.