Callisto Voyager computerenhanced photograph of Callisto the fourth and faintest Galilean satellite of Jupiter The spacecraft was million kilometres from Callisto when the photograph was taken Callisto is an airless body where surface temperatures are never higher than centigrade It is believed that the moon has a large rocky core and a crust that consists of a mixture of water ice and rocky material The only surface features evident on Callisto are numerous impact craters formed by meteorites and a number of bright concentric ring systems Otherwise Callisto is extremely smooth with no mountains or valleys