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CHAPTER

10

R I V U X G
Walking on the lunar surface, April 1972.
(John Young, Apollo 16, Science Source)

Our Barren Moon

LEARNING GOALS

By reading the sections of this chapter, you will learn

10–1 The nature of the Moon’s surface
10–2 The story of human exploration of the Moon
10–3 How we have learned about the Moon’s interior
10–4 How Moon rocks differ from rocks on Earth
10–5 Why scientists think the Moon formed as the result of a violent collision between worlds

Other than Earth itself, the most familiar world in the solar system is the Moon. It is so near to us that some of its surface features are visible to the naked eye. But the Moon is also a strange and alien place, with dramatic differences from our own Earth. It has no substantial atmosphere, no global magnetic field, and no liquid water of any kind. On the airless Moon, the sky is black even at midday. And unlike the geologically active Earth, the Moon has a surface that changes very little with time. When Apollo 16 astronauts Charlie Duke (shown in the photograph) and John Young explored the region of the lunar highlands known as the Descartes Formation, the youngest rocks that they found were virtually the same age as the oldest rocks found on Earth.

Because the Moon has changed so little over the eons, it has preserved the early history of the inner solar system and the terrestrial planets. In this chapter we will see evidence that the Moon’s formation and early evolution was violent and chaotic. The Apollo missions revealed that the Moon may have formed some 4.5 billion years ago as the result of a titanic collision between the young Earth and a rogue protoplanet the size of Mars. The young Moon was then pelted incessantly for hundreds of millions of years by large chunks of interplanetary debris, producing a cratered landscape. We will discover that the largest of these impacts formed immense circular basins that then flooded with molten lava. You can see the solidified lava as dark patches that cover much of the face of the Moon. In later chapters we will find other evidence that catastrophic collisions have played an important role in shaping the solar system.

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