Numerous grooves, cracks and ridges occur on its surface. Several of the 16 small, irregular-shaped satellites orbiting Jupiter are pictured. Some scientists have speculated on the presence of a sub-ice ocean beneath Ganymede's icy crust that might harbor organic molecules and even simple life forms. We can show the surfaces of these satellites best in medium-contrast, black and white images. The darker terrain appears older and may have been part of a crust that after breaking apart was invaded and at times surrounded by the younger material.
Further breakups cause smaller polygons to wedge in a sometimes jumbled patchwork.
We again depict this type of terrain, in which grooved surfaces make up a patchwork of juxtaposed individual segments, each in sharp contrast to its neighbors, in the next pair of images, taken in July of 1998, during Galileo's last, and closest, approach. The first taken from a distance with the shadowed limb in view shows irregular wedges each internal grooving but with different orientations. The second view show parts of three terrains that appear different in relative age. The oldest, Marius Regio, at the bottom is darker. The middle terrain, is Philus Sulcus, with one dominant set of grooves.