Mimas is composed mostly of water ice with only a small amount of rock. Due to the tidal forces acting on it, the moon is not perfectly spherical; its longest axis is about 10% longer than the shortest.
Mimas’ most distinctive feature is a colossal impact crater 130 km across, named Herschel after the moon’s discoverer. Herschel’s diameter is almost a third of the moon’s own diameter; its walls are approximately 2 miles high, parts of its floor measure 6 miles deep, and its central peak rises 4 miles above the crater floor. If there were a crater of an equivalent scale on Earth it would be over 2400 miles in diameter. The impact that made this crater must have nearly shattered Mimas: fractures can be seen on the opposite side of Mimas that may have been created by shock waves from the impact travelling through the moon’s body.
The surface is saturated with smaller impact craters, but no others are anywhere near the size of Herschel. Although Mimas is heavily cratered, the cratering is not uniform. Most of the surface is covered with craters greater than 25 miles in diameter, but in the south polar region, craters greater than 12 miles are generally lacking. This suggests that some process removed the larger craters from these areas, or that something prevented larger stellar bodies from hitting the south polar region.