Callisto Full Face . ... Credit Copyright: Galileo Project , Voyager Project , JPL , NASA Explanation: Callisto 's surface shows its age. ... Io's surface is young, shows practically no impact craters, and is continually being repaved by the lava exploding from its many large volcanoes. Callisto 's surface is old, shows the highest density of impact craters in the Solar System , and harbors no volcanoes or even any large mountains. ... NASA Web Site Statements, Warnings, and Disclaimers . ...