Animation Courtesy of Southwest Research InstituteThe movies show the effect of an outer solar system late heavy bombardment on the interior structure of Callisto (top) and Ganymede (bottom). The left hand side shows the surface of each moon as it suffers repeated impacts that melt its outer layers and clean out rock suspended in its ice. Colors indicate density, with black showing rock, blue showing mixed ice and rock, and lighter shades of blue indicating a decreasing rock fraction. The surfaces are initially blue, indicating a uniform ice/rock mixture. Over time, repeated overlapping impacts remove all of the rock from the moons’ outer layers. The right hand side shows a vertical slice through the spinning globe, allowing us to see the growth of the core (black). Each frame of the movie records 50 new impacts onto each moon. Callisto receives 2,600 impacts, but Ganymede receives 5,200. When the late heavy bombardment on Callisto is complete, the movie of Callisto stops, but Ganymede continues to experience an additional 2,600 impacts (so the movie of its evolution has more frames). The final frame of the movie shows the structure of the moons at the end of the late heavy bombardment. Note that Ganymede's rock core is significantly larger than the core created in Callisto.