AND THE OUTER SOLAR SYSTEM
Linda T. Elkins-Tanton
An imprint of Infobase Publishing
Uranus, Neptune, Pluto, and the Outer Solar System
Copyright © 2006 by Linda T. Elkins-Tanton
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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
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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.
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
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
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
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
Appendix 2: Light,Wavelength, and Radiation 171
Appendix 3: A List of All Known Moons 180
Bibliography and Further Reading 190
Internet Resources 191
Organizations of Interest 193
The planets Mercury,Venus, Mars, Jupiter, and Saturn—all visible to 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 messenger 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, commanded 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 throughout 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 thunder, replaced Jupiter; and for Friday, Frigg, the goddess of love, replaced Venus.
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 measurements 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 prograde 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 imaginary 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 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 unstable and will make new ring systems as they crash into their hosts. Many moons, like Neptune's giant Triton, orbit their planets backward (clockwise when viewed from the North Pole, the opposite way that the planets orbit the Sun).Triton also has the coldest surface temperature of any moon or planet, including Pluto, which is much farther from the Sun.The solar system is made of bodies in a continuum of sizes and ages, and every rule has an exception.
Obliquity, orbital inclination, and rotation direction are three physical measurements used to describe a rotating, orbiting body.
Obliquity, Inclination, Rotation
Looking down on the north pole of a planet or moon, rotation in this direction is called direct, or prograde. Rotation in the opposite direction is called indirect, or retrograde; Venus, Uranus, and Pluto all have retrograde rotation.
Pluto's orbit is inclined to the ecliptic (the plane of Earth's orbit) by 17.14 degrees; all the other planets have inclinations less than 7 degrees.
Obliquity: / The angle between the planet's equator and its orbital plane is called its obliquity. Pluto's obliquity is 122.5 degrees, Venus's is 177.3 degrees, and Mercury's is 0 degrees,
Obliquity: / The angle between the planet's equator and its orbital plane is called its obliquity. Pluto's obliquity is 122.5 degrees, Venus's is 177.3 degrees, and Mercury's is 0 degrees, inclination:
Pluto's orbit is inclined to the ecliptic (the plane of Earth's orbit) by 17.14 degrees; all the other planets have inclinations less than 7 degrees.
Number of Moons v. AU
Mercury q^» Venus
AU from the Sun
As shown in this graph of number of moons versus planets, the large outer planets have far more moons than the smaller, inner planets or the dwarf planet, Pluto.
Every day new data are streaming back to Earth from space missions to Mars. Early in 2004, scientists proved that there was once standing liquid water on Mars. Another unmanned mission, this time to a comet, determined that the material in a comet's nucleus is as strong as some rocks and not the loose pile of ice and dust expected. Information streams in from space observations and Earth-based experiments, and scientists attempt to explain what they see, producing an equivalent stream of hypotheses about the formation and evolution of the solar system and all its parts.
In this age of constant space missions and discoveries, how can a printed book on the solar system be produced that is not instantly outdated? New hypotheses are typically not accepted immediately by the scientific community. The choice of a leading hypothesis among competing ideas is really a matter of opinion, and arguments can go on for decades. Even when one idea has reached prominence in the scientific community, there will be researchers who disagree with it. At every point along the way, though, there are people writing books about science. Once an explanation reaches the popular press, it is often frozen as perpetual truth and persists for decades, even if the scientific community has long since abandoned that theory.
In this set, some statements will be given as facts: the gravitational acceleration of the Earth, the radius of Mars, the height of prominences from the Sun, for instance. Almost everything else is open to argumentation and change. The numbers of moons known to be orbiting Jupiter and Saturn, for example, are increasing every year as observers are able to detect smaller and dimmer objects. These volumes will present some of the thought processes that have brought people to their conclusions (for example, why scientists state that the Sun is fueled by nuclear reactions), as well as observations of the solar system for which no one has a satisfactory explanation (such as why there is no detectable heat flow out of the gas giant planet Uranus). Science is often taught as a series of facts for memorization—in fact, not until the second half of a doctoral degree do many scientists learn to question all aspects of science, from the accepted theory to the data itself. Readers should feel empowered to question every statement.
The Solar System set explores the vast and enigmatic Sun at the center of the solar system and also covers the planets, examining each and comparing them from the point of view of a planetary scientist. Space missions that produced critical data for the understanding of solar system bodies are introduced in each volume, and their data and images shown and discussed.The volumes The Sun, Mercury, andVenus;The Earth and the Moon; and Mars place emphasis on the areas of unknowns and the results of new space missions. The important fact that the solar system consists of a continuum of sizes and types of bodies is stressed in Asteroids, Meteorites, and Comets. This book discusses the roles of these small bodies as recorders of the formation of the solar system, as well as their threat as impactors of planets. In Jupiter and Saturn, the two largest planets are described and compared. In the final volume, Uranus, Neptune, Pluto, and the Outer Solar System, Pluto is presented not as the final lonely dwarf planet but as the largest known of a extensive population of icy bodies that reach far out toward the closest stars, in effect linking the solar system to the galaxy itself.
In this set we hope to change the familiar litany Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto into a more complex understanding of the many sizes and types of bodies that orbit the Sun. Even a cursory study of each planet shows its uniqueness along with the great areas of knowledge that are unknown.These titles seek to make the familiar strange again.
Foremost, profound thanks to the following organizations for the great science and adventure they provide for humankind and, on a more prosaic note, for allowing the use of their images for these books: the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA), in conjunction with the Jet Propulsion Laboratory (JPL) and Malin Space Science Systems (MSSS). A large number of missions and their teams have provided invaluable data and images, including the Solar and Heliospheric Observer (SOHO), Mars Global Surveyor (MGS), Mars Odyssey, the Mars Exploration Rovers (MERs), Galileo, Stardust, Near-Earth Asteroid Rendezvous (NEAR), and Cassini. Special thanks to Steele Hill, SOHO Media Specialist at NASA, who prepared a number of images from the SOHO mission, to the astronauts who took the photos found at Astronaut Photography of the Earth, and to the providers of the National Space Science Data Center, Great Images in NASA, and the NASA/JPL Planetary Photojournal, all available on the Web (addresses given in the reference section).
Many thanks also to Frank K. Darmstadt, executive editor at Chelsea House; to Jodie Rhodes, literary agent; and to E. Marc Parmentier at Brown University for his generous support.
Introduction aranus, Neptune, Pluto, and the Outer Solar System enters the farthest reaches of the solar system, including the distant gas planets Uranus and Neptune and the regions of asteroids and comets known as the Kuiper belt Oort cloud.These are the areas in the solar system that, in many ways, are the least known. Unlike all the planets closer to the Sun, known since antiquity, the farthest reaches are the discoveries of the modern world: Uranus was discovered in 1781, Neptune in 1846, Pluto in 1930, the Kuiper belt group of objects in 1992, and, though the Oort cloud has been theorized since 1950, its first member was just found in 2004.The discovery of the outer planets made such an impression on the minds of humankind that they were immortalized in the names of newly discovered elements: uranium, neptunium, and plutonium, that astonishingly deadly constituent of atomic bombs.
Scientific theories rely on observations that produce data: temperatures, compositions, densities, sizes, times, patterns, or appearances. There is very little data on these outer solar system bodies, compared to what is known about Earth's neighbors the Moon and Mars, and even Jupiter and Saturn. In the cases of Neptune and Uranus, only the Voyager 2 mission during the mid-1980s attempted close observations. The extreme distance of these bodies from the Earth hinders Earth-based observations. Because there is so little data on Uranus there are fewer scientists conducting research on the planet (they have little data to analyze and to test hypotheses on; see figure on page xviii). Much of the new science on this part of the solar system awaits new space missions to this region. At the moment, there are no planned missions to Uranus and Neptune, and so the wait for reliable new data may be long.The discrepancy in missions to the terrestrial planets compared to the outer planets is shown in the figure on the next page; only Pluto has never had a space mission approach it.
Part One of Uranus, Neptune, Pluto, and the Outer Solar System will discuss what data there are on the distant gas planets and investigate theo-
The approximate number of successful space missions from all nations shows that the Moon is by far the most visited body, only Pluto has had no missions, and Mercury is as neglected as Uranus and Neptune. The definition of a successful mission is arguable, so totals for Mars and the Moon in particular may be disputed.
Successful Space Missions to Solar System Bodies
Op j Mars
.Venus ^ ^Jupiter
I Mercury Saturn Uranus
I Neptune ^Pluto 40
AU from the Sun ries about their formation and evolution. All the gas giant planets, including Uranus and Neptune, are thought to have accumulated as masses of heterogeneous material. The small amount of very dense material available so far out in the nebular cloud of the early solar system fell through self-gravity into the center of each primordial planetary mass, forming whatever rocky or metallic core each planet might now have. The liquid and gaseous material that make up the vast bulk of each planet form layers according to their response to pressure and temperature.Though these planets have relatively low density, the heat of formation may still be influencing their interior circulation today.
Neptune and Uranus are twins in terms of size, internal structure, and color, but they differ in important ways. Uranus, for example, produces virtually no heat internally, while Neptune produces more heat relative to what it receives from the Sun than does any other planet.They both pose special challenges to theories of planet formation. These are huge planets, 15 and 17 times the mass of Earth, respectively, and yet they formed in the outer solar system, where the density of material in the early solar system is thought to have been very low and the orbits are huge, leading to less chance of collisions and collection of material. One theory is that both planets formed in the region of Jupiter and Saturn and were scattered outward in the solar system when Jupiter became huge and acquired its giant gas envelope.
Neptune and Uranus also have huge cores in relation to their overall planetary mass: probably 60 to 80 percent, as compared to Jupiter and Saturn's 3 to 15 percent.The question then is, how did huge cores form and then fail to attract gravitationally the same fractions of gaseous atmospheres that Jupiter and Saturn did? One possibility, in contradiction to the idea that they formed near Jupiter and Saturn and then were thrown further out, is that Uranus and Neptune formed in the orbits they now inhabit. These planets' orbits are so huge and the protoplanetary disk so sparse at those distances from the Sun that the planetary cores would have taken a long time to form, much longer than Jupiter's and Saturn's cores did.
Clearly, very little is known about these remote gas giants, both because data are scarce and because scientists are still forming ideas about how they were created.Yet as remote as these two planets are, there is much remaining material farther out in the solar system still to be studied. Beyond the last two gas giant planets lie fascinating small bodies in closely spaced orbits, the home of Pluto and Charon and the sources of long-period comets. Data on more distant and smaller objects are even more difficult to obtain. The fact that Pluto and its moon Charon are part of a much larger population of icy and rocky bodies now known as the Kuiper belt has been known for only about a decade. In July 2005 a historic announcement was made: A Kuiper belt body larger than Pluto was confirmed by a team of scientists led by Mike Brown, a professor of planetary astronomy at the California Institute of Technology. The discovery of this large body reopened the discussion over the definition of a planet—might this body be considered the solar system's 10th planet? Instead, Pluto was redefined as a dwarf planet. Pluto, Charon, and the other bodies that make up the Kuiper belt are the subject of Part Two of this book; their orbits in relation to those of the others covered in this book are shown in the figure on the next page.
In 2004 the first body orbiting in the distant and until now theoretical Oort cloud was discovered. In Part Three what is known about the Oort cloud is described, along with the reasoning that led to its theorization and the technology that has allowed its discovery. The Kuiper belt and Oort cloud are currently under intense study.
All Orbits: Asteroid Belt, Kuiper Belt, Oort Cloud
Asteroid belt Mercury Venus Earth Mars
Asteroid belt Mercury Venus Earth Mars
This book covers the outermost solar system, including Uranus, Neptune, Pluto, the Kuiper belt, and the Oort cloud, whose orbits and ranges of orbits are highlighted here. All the orbits are far closer to circular than shown in this oblique view, which was chosen to show the inclination of Pluto's orbit to the ecliptic.
Although they are extremely distant and the bodies in them very small, new high-resolution imaging techniques are allowing scientists to discover many new objects in these regions and allowing them to better understand the populations at the edges of the solar system. Uranus, Neptune, Pluto, and the Outer Solar System describes what is now known about the bodies in these distant regions and how they interact with the inner solar system and also with stars entirely outside this solar system.
Part One: Uranus and Neptune
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