Chapter 10

Page 1

6268_e10_p235-254

12/20/06

2:42 PM

Page 235

10 Our Barren Moon

O

WE

0.1

ther than the 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 INK 1 RI V UXG Moon is also a strange and alien place, with BL dramatic differences from our own Earth. It has Walking on the lunar surface, April 1972. (John Young, Apollo 16, NASA/Science Photo no substantial atmosphere, no global magnetic Library) field, and no liquid water of any kind. On the airless Moon, the sky is black even at midday. 10-1 The Moon’s airless, dry surface is And unlike the geologically active Earth, the Moon has a surface that changes very little with time. When Apollo 16 astronauts covered with plains and craters Charlie Duke (shown in the photograph) and John Young explored the region of the lunar highlands known as the Descartes Compared with the distances between planets, which are meaFormation, the youngest rocks that they found were virtually the sured in tens or hundreds of millions of kilometers, the average same age as the oldest rocks found on Earth. distance from the Earth to the Moon is just 384,400 km (238,900 Because the Moon has changed so little over the eons, it has mi). Despite being so close, the Moon has an angular diameter of preserved the early history of the inner solar system and the terjust 1⁄2°. This tells us that it is a rather small world. The diamerestrial planets. In this chapter we will see evidence that the ter of the Moon is 3476 km (2160 mi), just 27% of the Earth’s Moon’s formation and early evolution was violent and chaotic. diameter and about the same as the distance from Las Vegas to The Apollo missions revealed that the Moon may have formed Philadelphia or from London to Cairo (Figure 10-1). Table 10-1 some 4.5 billion years ago as the result of a titanic collision belists some basic data about the Moon. tween the young Earth and a rogue protoplanet the size of Mars. The young Moon was then pelted incessantly for hundreds of milMotions of the Earth-Moon System lions of years by large chunks of interplanetary debris, producing a cratered landscape. We will discover that the largest of these imAlthough the Moon has a small mass (just 1.23% of the Earth’s pacts formed immense circular basins that then flooded with mass), Newton’s third law tells us that the Moon exerts just as molten lava. You can see the solidified lava as dark patches that much gravitational force on the Earth as the Earth does on the cover much of the face of the Moon. In later chapters we will find Moon (see Section 4-6). Hence, it is not strictly correct to say that other evidence that catastrophic collisions have played an importhe Moon orbits the Earth; rather, they both orbit around a point tant role in shaping the solar system. between their centers called the center of mass of the Earth-Moon

Learning Goals By reading the sections of this chapter, you will learn

10-4 How Moon rocks differ from rocks on Earth

10-1 The nature of the Moon’s surface

10-5 Why scientists think the Moon formed as the result of a violent collision between worlds

10-2 The story of human exploration of the Moon 10-3 How we have learned about the Moon’s interior

235

APUS/AMU - Property of WH Freeman - 0-7167-8584-6- Copyright 2009


Turn static files into dynamic content formats.

Create a flipbook
Issuu converts static files into: digital portfolios, online yearbooks, online catalogs, digital photo albums and more. Sign up and create your flipbook.