Figure 6-28 The Moon’s Interior and Its Effects on the Moon’s Surface (a) As the Moon’s interior shrank, the surface settled irregularly, creating long lines of cliffs called scarps. (b) Image from the Lunar Reconnaissance Observer showing graben that are about 20 m (22 yd) deep. Inset shows the dynamics that created the graben. (c) Based on seismic experiments left on the Moon by Apollo astronauts and the studies done by Lunar Prospector, we know that the Moon has a crust, a mantle, and a core. The lunar crust ranges in thickness between about 20 and 120 km (12 and 75 mi), with an average thickness of about 60 km (37 mi) on its Earth-facing side and about 72 km (45 mi) on the far side. The crust and solid upper mantle extend inward to about 800 km (500 mi), where the nonrigid inner mantle begins. The Moon’s core has a radius of somewhere between 220 and 450 km (137 and 280 mi). Although the main features of the Moon’s interior are analogous to those of Earth, the proportions and details are quite different. The Clementine spacecraft revealed that the south polar region on the far side of the Moon has a significant basin where the crust was apparently stripped away by an impact. Top inset: A radar image of the Moon’s north polar region. The areas computer-colored in white and light gray are regions where the Sun never shines and that contain water. Bottom inset: A radar image of the Moon’s south polar region, also showing regions of permanent or near-permanent darkness.