Uranus, Neptune, Pluto, and the Outer Solar System Linda T
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Uranus, Neptune, Pluto, and the Outer Solar System Linda T. Elkins-Tanton Uranus, Neptune, Pluto, and the Outer Solar System 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. Uranus, Neptune, Pluto, and the outer solar system / Linda T.Elkins-Tanton. p. cm. — (The Solar system) Includes bibliographical references and index. ISBN 0-8160-5197-6 (acid-free paper) 1. Uranus (Planet)—Popular works. 2. Neptune (Planet)—Popular works. 3. Pluto (Planet)—Popular works. 4. Solar system—Popular works. I.Title. QB681.E45 2006 523.47—dc22 2005014801 Chelsea House books are available at special discounts when purchased in bulk quantities for businesses, associations, institu- tions, 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 Hermitage 10 9 8 7 6 5 4 3 2 1 This book is printed on acid-free paper. In memory of my brother Thomas Turner Elkins, who, when I was 10 years old, taught me about the Oort cloud, and together we named our pet mouse Oort. Contents Preface . .ix Acknowledgments . .xv Introduction . .xvii PART ONE: URANUS AND NEPTUNE 1. Uranus: Fast Facts about a Planet in Orbit . .3 Fundamental Information about Uranus . .7 What Makes Gravity? . .10 2. The Interior of Uranus . .13 Composition . .13 What Is Pressure? . .15 Internal Temperatures . .17 Elements and Isotopes . .18 Magnetic Field . .20 3. Surface Appearance and Conditions on Uranus . .23 Remote Sensing . .26 4. Rings and Moons of Uranus . .37 Rings . .37 Moons . .39 Why Are There Rings? . .42 Accretion and Heating:Why Are Some Solar System Objects Round and Others Irregular? . .45 What Are Synchronous Orbits and Synchronous Rotation? . .51 5. Neptune: Fast Facts about a Planet in Orbit . .63 Fundamental Information about Neptune . .65 6. The Interior of Neptune . .75 Sabine Stanley and Planetary Magnetic Fields . .78 7. Surface Appearance and Conditions on Neptune . .81 8. Neptune’s Rings and Moons . .85 Rings . .85 Moons . .88 Fossa,Sulci,and Other Terms for Planetary Landforms . .96 PART TWO: PLUTO AND THE KUIPER BELT 9. The Discovery of Pluto and the Kuiper Belt . .101 10. Pluto: Fast Facts about a Dwarf Planet in Orbit . .109 Fundamental Information about Pluto . .110 11. What Little Is Known about Pluto’s Interior and Surface . .113 12. Charon: Pluto’s Moon, or Its Companion Dwarf Planet? . .121 13. The Rest of the Kuiper Belt Population . .125 Numbering and Naming Small Bodies . .128 PART THREE: BEYOND THE KUIPER BELT 14. The Oort Cloud . .145 15. Conclusions: The Known and the Unknown . .151 Appendix 1: Units and Measurements . .159 Fundamental Units . .159 Comparisons among Kelvin, Celsius, and Fahrenheit . .161 Useful Measures of Distance . .163 Definitions for Electricity and Magnetism . .167 Prefixes . .170 Appendix 2: Light,Wavelength, and Radiation . .171 Appendix 3: A List of All Known Moons . .180 Glossary . .182 Bibliography and Further Reading . .190 Internet Resources . .191 Organizations of Interest . .193 Index . .195 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 Uranus, Neptune, Pluto, and the Outer Solar System 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, the dwarf planet 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, 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.