(Background) The Apollo 15 mission to the Moon explored a geological fault, called Hadley Rille, where molten lava once flowed. In the large photograph printed here, the rille—a system of valleys—runs along the base of the Apennine Mountains (lower right) at the edge of Mare Imbrium (to the left). For scale, the lower of the two large craters, Autolycus, spans 40 km. The shadow-sided, most prominent peak at lower right, Mount Hadley, rises almost 5 km high.
(Inset A) An astronaut embarks from his lunar rover, preparing to explore Hadley Rille in the distance. The width of the rille is about 1.5 km and its depth averages 300 m.
(Inset B) Photograph of an astronaut and a lunar rover in front of Mount Hadley.
(Inset C) An astronaut collects samples at the edge of Hadley Rille.
(Inset D) This photographic mosaic of a small part of an interior wall of Hadley Rille shows evidence for subsurface horizontal layering. Each distinct layer (denoted A, B, C) is a few meters deep and presumably represents successive lava flows that helped form the extensive lava plain called Mare Imbrium.
Studying this chapter will enable you to:
Specify the general characteristics of the Moon and Mercury, and compare them with those of Earth.
Explain how the Moon's rotation is influenced by its orbit around Earth, and Mercury's by its orbit around the Sun.
Describe the surface features of the Moon and Mercury, and recount how they were formed by dynamic events early in their history.
Explain how observations of cratering can be used to estimate the age of a body's surface.
Compare the Moon's interior structure with that of Mercury.
Summarize the various theories for the formation of the Moon, and indicate which is presently considered most likely.
Discuss how astronomers have pieced together the story of the Moon's evolution, and compare its evolutionary history with that of Mercury.
The Moon is Earth's only natural satellite. Mercury, the smallest terrestrial world, is the planet closest to the Sun. Despite their different environments, however, these two bodies have many similarities—indeed, at first glance, you might even mistake one for the other. Both have heavily cratered, ancient surfaces, littered with boulders and pulverized dust. With no atmosphere to moderate variations in solar heating, each experiences wild temperature swings from day to night. Both are geologically dead. In short, the Moon and Mercury differ greatly from Earth, but it is precisely those differences that make these desolate worlds so interesting to planetary scientists. Why is the Moon so unlike our own planet, despite its nearness to us, and why does planet Mercury apparently have so much more in common with Earth's Moon than with Earth itself? In this chapter we explore the properties of these two worlds as we begin our comparative study of the planets and moons that make up our solar system.
