Mariner 10 images of Mercury, the innermost planet, revealed a heavily cratered landscape similar to the Moon, but also revealed prominent topographic scarps. One of these scarps, Discovery Rupes, is shown here. This scarp is roughly 650 kilometers long and 2 kilometers high. Discovery Rupes crosses the floors of two old impact craters, the largest of which is 70 kilometers across. Both of these craters have been shortened in diameter, thereby indicating that Discovery Rupes is a thrust fault and that the crust of Mercury has been compressed. These scarps indicate that Mercury shrank a little early in its history. This shrinkage was probably due to global cooling of the interior, causing the outer surface to shrink and crumple like the skin of a dried apple.
10 images 27399, 166619.