The Earth and the Moon The Earth and the Moon Linda T. Elkins-Tanton The Earth and the Moon Copyright © 2006 by Linda T.Elkins-Tanton All rights reserved. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechani- cal, including photocopying, recording, or by any information storage or retrieval systems, without permission in writing from the publisher. For information contact: Chelsea House An imprint of Infobase Publishing 132 West 31st Street New York NY 10001 Library of Congress Cataloging-in-Publication Data Elkins-Tanton, Linda T. The Earth and the moon / Linda T.Elkins-Tanton. p. cm. — (Solar system) Includes bibliographical references and index. ISBN 0-8160-5194-1 (acid-free paper) 1. Earth—Popular works. 2. Moon—Popular works. I.Title. QB631.2.E45 2006 525—dc22 2005014189 Chelsea House books are available at special discounts when purchased in bulk quantities for businesses, associations, institutions, or sales promotions. Please call our Special Sales Department in New York at (212) 967-8800 or (800) 322-8755. You can find Chelsea House on the World Wide Web at http://www.chelseahouse.com Text and cover design by Dorothy M. Preston Illustrations by Richard Garratt Printed in the United States of America VB CH 10 9 8 7 6 5 4 3 2 1 This book is printed on acid-free paper. To Tim Grove, Brad Hager, Maria Zuber, Sam Bowring, and my other friends at the Massachusetts Institute of Technology who worked at turning me into a scientist, with special appreciation for the research we did about the Moon. Contents Preface . ix Acknowledgments . xv Introduction . xvii PART ONE: THE EARTH . 1 1. The Earth: Fast Facts about a Planet in Orbit . 3 Fundamental Information about the Earth . 4 2. The Interior of the Earth . 23 Moment of Inertia . 26 The Brittle Crust . 28 The Solid but Moving Mantle . 32 High-Pressure Experiments . 36 Rheology,or How Solids Can Flow . 38 The Solid and Liquid Cores . 43 Magnetic Field . 46 Temperatures, Pressures, and Compositions . 51 What Is Pressure? . 52 3. The Early Evolution of the Planet Earth . 57 Elements and Isotopes . 64 4. The Visible Planet . 71 How Old Is the Earth’s Surface? . 71 Surface Features . 75 Determining Age from Radioactive Isotopes . 76 Volcanism . 87 Earthquakes . 91 Atmosphere, Surface Conditions, and Weather . 95 5. Life on Earth . 105 Dr.Sara Seager and the Search for Extrasolar Planets . 110 PART TWO: THE MOON . 115 6. The Moon: Fast Facts about the Earth’s Moon in Orbit . 117 Formation of the Moon . 118 7. Formation and Evolution of the Moon . 125 Formation and Crystallization of the Lunar Magma Ocean . 128 Highlands . 131 The Late Heavy Meteorite Bombardment . 135 Fossa,Sulci,and Other Terms for Planetary Landforms . 136 Basins and Volcanism . 140 8. The Moon’s Interior and Surface Today . 151 The Interior . 151 Surface Features . 152 Remote Sensing . .156 The Insignificant Magnetic Fields and Atmosphere . 168 Water on the Moon? . 170 9. Missions to the Moon . 173 Planned Missions . 183 10. Conclusions: The Known and the Unknown . 185 Appendix 1: Units and Measurements . 189 Fundamental Units . 189 Comparisons among Kelvin, Celsius, and Fahrenheit . 191 Useful Measures of Distance . 193 Definitions for Electricity and Magnetism . 197 Prefixes . 200 Appendix 2: Light,Wavelength, and Radiation . 201 Appendix 3: A List of All Known Moons . 210 Glossary . 212 Bibliography and Further Reading . 220 Internet Resources . 222 Index . 225 mPreface he planets Mercury,Venus,Mars, Jupiter, and Saturn—all visible Tto the naked eye—were known to ancient peoples. In fact, the Romans gave these planets their names as they are known today. Mercury was named after their god Mercury, the fleet-footed messen- ger of the gods, because the planet seems especially fast moving when viewed from Earth.Venus was named for the beautiful goddess Venus, brighter than anything in the sky except the Sun and Moon.The planet Mars appears red even from Earth and so was named after Mars, the god of war. Jupiter was named for the king of the gods, the biggest and most powerful of all, and Saturn was named for Jupiter’s father. The ancient Chinese and the ancient Jews recognized the planets as well, and the Maya (250–900 C.E., Mexico and environs) and Aztec (ca. 1100–1700 C.E., Mexico and environs) called the planet Venus “Quetzalcoatl,” after their god of good and light. These planets, small and sometimes faint in the night sky, com- manded such importance that days were named after them. The seven-day week originated in Mesopotamia, which was perhaps the world’s first organized civilization (beginning around 3500 B.C.E.in modern-day Iraq). The Romans adopted the seven-day week almost 4,000 years later, around 321 C.E., and the concept spread through- out western Europe. Though there are centuries of translations between their original names and current names, Sunday is still named for the Sun, Monday for the Moon, Tuesday for Mars, Wednesday for Mercury,Thursday for Jupiter, Friday for Venus, and Saturday for Saturn. The Germanic peoples substituted Germanic equivalents for the names of four of the Roman gods: For Tuesday, Tiw, the god of war, replaced Mars; for Wednesday,Woden, the god of wisdom, replaced Mercury; for Thursday,Thor, the god of thun- der, replaced Jupiter; and for Friday, Frigg, the goddess of love, replaced Venus. ix x B The Earth and the Moon More planets, of course, have been discovered by modern man, thanks to advances in technology.Science is often driven forward by the development of new technology, allowing researchers to make meas- urements that were previously impossible.The dawn of the new age in astronomy, the study of the solar system, occurred in 1608, when Hans Lippershey, a Dutch eyeglass-maker, attached a lens to each end of a hollow tube, creating the first telescope. Galileo Galilei, born in Pisa, Italy, in 1564, made his first telescope in 1609 from Lippershey’s model. Galileo soon had noticed that Venus has phases like the Moon and that Saturn appeared to have “handles.”These of course were the edges of Saturn’s rings, though the telescope was not strong enough to resolve the rings correctly. In 1610, Galileo discovered four of Jupiter’s moons, which are still called the Galilean satellites.These four moons were proof that not every heavenly body orbited the Earth, as Ptolemy, a Greek philosopher, had asserted around 140 C.E. Galileo’s discovery was the beginning of the end of the strongly held belief that the Earth is the center of the solar system, as well as a beautiful example of a case where improved technology drove science forward. Most of the science presented in this set comes from the startling- ly rapid developments of the last hundred years, brought about by technological development.The concept of the Earth-centered solar system is long gone, as is the notion that the “heavenly spheres” are unchanging and perfect. Looking down on the solar system from above the Sun’s North Pole, the planets orbiting the Sun can be seen to be orbiting counterclockwise, in the manner of the original proto- planetary disk of material from which they formed. (This is called pro- grade rotation.) This simple statement, though, is almost the end of generalities about the solar system.The notion of planets spinning on their axes and orbiting around the Sun in an orderly way is incorrect: Some planets spin backward compared to the Earth, others planets are tipped over, and others orbit outside the ecliptic plane (the imagi- nary plane that contains the Earth’s orbit) by substantial angles, Pluto in particular (see the accompanying figure on obliquity and orbital inclination). Some planets and moons are hot enough to be volcanic, and some produce silicate lava (for example, the Earth and Jupiter’s moon Io), while others have exotic lavas made of molten ices (for example, Neptune’s moon Triton). Some planets and even moons have atmospheres, with magnetic fields to protect them from the solar wind (for example,Venus, Earth, Mars, Io,Triton, and Saturn’s Preface y xi moon Titan), while other planets have lost both their magnetic fields and their atmospheres and orbit the Sun fully exposed to its radiation and supersonic particles (for example, Mercury). Size can be unexpected in the solar system: Saturn’s moon Titan is larger than the planet Mercury, and Charon, Pluto’s moon, is almost as big as Pluto itself. The figure on page xii shows the number of moons each planet has; large planets have far more than small planets, and every year scientists discover new celestial bodies orbiting the gas giant planets. Many large bodies orbit in the asteroid belt, or the Kuiper belt, and many sizable asteroids cross the orbits of planets as they make their way around the Sun. Some planets’ moons are unsta- ble and will make new ring systems as they crash into their hosts. Many moons, like Neptune’s giant Triton, orbit their planets back- ward (clockwise when viewed from the North Pole, the opposite way Obliquity, orbital inclination, that the planets orbit the Sun).Triton also has the coldest surface tem- and rotation direction are perature of any moon or planet, including Pluto, which is much far- three physical measurements ther from the Sun.The solar system is made of bodies in a continuum used to describe a rotating, of sizes and ages, and every rule has an exception. orbiting body. xii B The Earth and the Moon As shown in this graph of Every day new data are streaming back to Earth from space mis- number of moons versus sions to Mars.