Atmospheres and Ionospheres of the Outer Planets and Their Satellites

232 Pages·1986·8.285 MB·English

by  Professor Sushil K. Atreya (auth.)

Physics and Chemistry in Space Vol. 15
Edited by L. J. Lanzerotti, Murray Hill and
D. SWiller, MUnster
Sushi! K. Atreya
Atmospheres and
Ionospheres of the
Outer Planets and
Their Satellites
With 90 Figures
Professor Sushil K. Atreya
Department of Atmospheric
and Oceanic Science
The University of Michigan
Space Research Building
Ann Arbor, MI 48109, USA
ISBN-13: 978-3-642-71396-5 e-ISBN: 978-3-642-71394-1
001: 10.1007/978-3-642-71394-1
Library of Congress Cataloging-in-Publication Data. Atreya, S. K. Atmospheres
and ionospheres of the outer planets and their satellites. (Physics and chemistry
in space; vol. 15) Bibliography: p. Includes index. I. Outer planets - Atmo
spheres. 2. Outer planets - Ionospheres. 1. Title. II. Series: Physics and chemistry
in space; v. 15. QC80l.P46 vol. 15 530'.0919 s 86-26014 [QB603.A85]
[551.5'0999'2]
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© Springer-Verlag Berlin Heidelberg 1986
Softcover reprint of the hardcover 1s t edtion 1986
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To My Parents~
Evelyn~ and Chloe
Preface
One of the most fundamental discoveries of the solar system was the detection
of four moons in orbit around Jupiter by Galileo Galilei in 1610. The
discovery was significant not only in the context of Jupiter; it gave credence to
and was instrumental in firmly establishing the heliocentric system of
Nicolaus Copernicus. Almost four centuries after Galileo's discovery, exten
sive observations by the two Voyager spacecrafts have once again revolu
tionized our thinking about the major planets, their composition, structure,
origin, and evolution. This book is an attempt at summarizing our present
understanding of the atmospheres and ionospheres in the outer solar system,
with particular emphasis on the relevant physics and chemistry. I was motivat
ed to prepare this manuscript for the following reasons. First, after under
going rapid expansion in the recent past, the subject has finally attained suf
ficient maturity to warrant a monograph of its own. Second, I have felt that as
a result of recent observations, new and challenging problems have arisen
whose resolution requires unconventional analysis and theoretical interpreta
tion of existing data, as well as the collection of new kinds of data. I believe
the time is ripe to put these issues in the appropriate scientific perspective,
with the hope of stimulating novel theoretical, observational, and laboratory
studies. I have highlighted the significant scientific problems throughout the
book, especially at the end of each chapter.
The topics in this book were selected - after much discussion with col
leagues and students - to cover essential aspects of the physical and chemical
processes in the atmospheres and ionospheres of the outer planets and their
satellites. Both measurements and theoretical concepts and formulations have
been included. Because of space restrictions, I have found it necessary to limit
the discussion of meteorology, magnetospheres, interior, and evolution to
only those aspects that are clearly essential to the understanding of atmo
spheric and ionospheric phenomena. A background in physics, chemistry, and
mathematics at the undergraduate level with some basic knowledge of
meteorology, is adequate preparation for understanding the material in the
book and for getting the most out of it.
The book is intended for researchers as well as graduate students in plane
tary science. Recognizing the obvious dichotomy between these two groups, I
have attempted to maintain some balance in discussing the various topics.
Graduate students and other new entrants to the field of planetary atmo
spheres should find the development of conceptual material and the formula-
VIII Preface
tions in the book most beneficial. On the other hand, seasoned researchers
looking for new ideas and insights will be more interested in those parts of the
book that deal with the critical evaluation of latest available information
along with the extensive bibliography for tracking down the source. Because
of the highly interdisciplinary nature of the field of planetary atmospheres, I
have sometimes found it appropriate and convenient to develop the formula
tion of certain topics where they are discussed, rather than separately, e.g., all
at the beginning of the book. The reader, however, should find the Subject
Index most helpful for locating the desired formulation or topic, and for
cross-referencing. Although I have been careful to present a balanced view of
the subject, at times I have found it appropriate to advocate a certain view
point, or include a provocative suggestion. In all such instances of contro
versial ideas, caveats are discussed. It is my sincere hope that in its own way
this book will help to inspire many people to pursue studies in planetary
science and stimulate new research ideas with the ultimate goal of understand
ing the origin and evolution of planetary atmospheres.
Ann Arbor, August 1986 S. K. ATREYA
Acknowledgments
I wish to express my sincere appreciation to many colleagues: those who
critically reviewed one or several chapters of the manuscript, those who sup
plied preprints of their work, and those with whom I have had stimulating dis
cussions during the course of preparing the manuscript. The list of these in
dividuals is long, the following, however, deserve special thanks: Jacques E.
Blamont, Barney J. Conrath, Imke de Pater, Thomas M. Donahue, Von R.
Eshleman, Daniel Gautier, Donald M. Hunten, Louis J. Lanzerotti, Glenn S.
Orton, Tobias C. Owen, Ronald G. Prinn, Bill R. Sandel, Donald E. She
mansky, Edward C. Stone, Darrell F. Strobel, and G. Len Tyler. Parts of the
manuscript were written in 1984 - 85 while I was on a sabbatical leave at Uni
versite de Paris (Pierre et Marie Curie). I am thankful to Claudie Blamont,
whose tireless efforts helped to make my stay in Paris both productive and
pleasant. Provocative inquiries from many of my former and present graduate
students at Michigan and Paris - in particular, J. Hunter Waite, Paul N.
Romani, Robert B. Kerr, and Jean Jacques Ponthieu - were helpful in
clarifying some of the concepts discussed in the manuscript. Very special
thanks are due to Deborah A. Swartz for her efficient and expert help in
preparing several drafts of the manuscript. C. A. Rohn helped with proof
reading and the bibliography. Much of my own research, which is reflected
throughout the book, has been generously supported by the Planetary Atmo
spheres Program of NASA's Solar System Exploration Division.
S. K. ATREYA
Contents
1 Composition .......................................... . 1
1.1 Bulk Atmospheric Composition .......................... . 3
1.2 Outstanding Issues ..................................... . 15
A.1 Appendix ............................................. . 16
A.1.1 Escape and Thermal Velocities ........................... . 16
2 Thermal Structure ...................................... . 18
2.1 Equilibrium and Effective Temperatures, Internal Heat ....... 18
2.2 Radiative Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2.1 Radiative Transfer Equation .............................. 21
2.3 Principle of Solar, Stellar, and Radio Occultations . . . . . . . . . . . . 24
2.3.1 Ultraviolet Solar and Stellar Occultations ................... 24
2.3.2 Visible Stellar Occultation ................................ 27
2.3.3 Radio Occultation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.4 Temperature Measurements - Jupiter and Saturn . . . . . . . . . . . . 30
2.4.1 Lower Atmosphere ...................................... 30
2.4.2 Upper Atmosphere ...................................... 35
2.5 Temperature Measurements - Uranus and Neptune .......... 43
2.6 Outstanding Issues ...................................... 48
A.2 Appendix. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
A.2.1 Great Red Spot ......................................... 48
A.2.2 Equatorial Belts and Zones ............................... 51
A.2.3 Line Broadenings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
A.2.4 Critical Level ........................................... 52
3 Cloud Structure ........................................ . 54
3.1 Lapse Rates, Cloud Densities ............................. . 54
3.2 Thermodynamic Data ................................... . 56
3.2.1 Heat Capacities and Latent Heats ......................... . 56
3.2.2 Vapor Pressures ........................................ . 56
3.3 Cloud Models .......................................... . 61
3.4 Outstanding Issues ..................................... . 65
XII Contents
4 Vertical Mixing 66
4.1 Diffusion Equation ..................................... . 66
4.2 Limiting Flux .......................................... . 68
4.3 Methods for Determining Eddy Diffusion Coefficient ........ . 69
4.3.1 Lyman-Alpha .......................................... . 70
4.3.2 He 584 A .............................................. . 72
4.3.3 Methane .............................................. . 74
4.3.4 Eddy Diffusion on Uranus and Neptune ................... . 76
4.4 Eddy Diffusion in the Homosphere ........................ . 77
4.5 Molecular Diffusion Coefficient .......................... . 77
4.6 Outstanding Issues ..................................... . 79
5 Photochemistry ........................................ . 80
5.1 Hydrogen ............................................. . 80
5.2 Ammonia (NH3) ........................................ . 82
5.3 Phosphine (PH3) ....................................... . 88
5.4 NH3 - PH3 Coupling and NH3 Recycling ................... . 90
5.5 Hydrogen Sulfide (H2S) .................................. . 91
5.6 C - N Coupling: Methylamine (CH3NH2) •••••.•••••••••••••• 93
5.7 C - N Coupling: Hydrogen Cyanide (HCN) ................. . 94
5.8 Carbon Monoxide (CO) .................................. . 95
5.9 Methane (CH4) ...........••....••...•......••..••....••. 98
5.10 Outstanding Issues ..................................... . 105
6 Ionosphere ............................................ . 107
6.1 Measurements ......................................... . 107
6.2 Theoretical Considerations .............................. . 114
6.2.1 Continuity Equation .................................... . 114
6.2.2 Ionization Sources ...................................... . 117
6.2.3 Ion Loss ........................................ ; ..... . 119
6.2.4 Ion Chemistry ......................................... . 121
6.2.5 Model Ionospheres ..................................... . 125
6.2.6 Reconciliation of Models with Measurements ............... . 128
6.2.7 Uranus and Neptune Ionospheres ......................... . 133
6.3 Upper Atmospheric Heating ............................. . 135
6.3.1 Heat Sink ............................................. . 136
6.3.2 Heat Sources .......................................... . 137
6.4 Outstanding Issues ..................................... . 144
7 Satellites .............................................. . 145
7.1 Atmospheres on Satellites ................................ . 145
7.2 10 .................................................... . 145
Contents XIII
7.2.1 S02 Atmosphere ....................................... . 146
7.2.2 Minor Constituents ..................................... . 150
7.2.3 Neutral Cloud and Plasma Torus ......................... . 151
7.2.4 S02 Photochemistry and Model Atmosphere ................ . 155
7.2.5 Ionospheric Measurements ............................... . 160
7.2.6 Ionospheric Models ..................................... . 161
7.3 Europa, Ganymede, Callisto, and Saturn's Rings ............ . 164
7.4 Titan ................................................. . 166
7.4.1 Vo yager Overview ...................................... . 167
7.4.2 Thermal Structure ...................................... . 169
7.4.3 Atmospheric Composition ............................... . 173
7.4.4 Chemistry ............................................. . 179
7.4.5 Ionosphere ............................................ . 183
7.4.6 Origin and Evolution of Titan's Atmosphere ................ . 186
7.5 Triton ................................................ . 190
7.6 Outstanding Issues ..................................... . 193
General Appendix ............................................. 195
A.l Characteristics of the Outer Planets and Comparison with
the Earth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
A.2 Characteristics of the Large Satellites of the Outer Planets and
Comparison with Earth's moon. . . . . . . . . . . . . . . . . . . . . . . . . . .. 196
A.3 Useful Numerical Constants .............................. 197
References ................................................... 199
Subject Index ................................................. 219