ASTRONOMY AND ASTROPHYSICS LIBRARY Eugene F.Milone William J.F.Wilson Solar System Astrophysics Planetary Atmospheres and the Outer Solar System SeriesEditors: G.Bo¨rner,Garching,Germany A.Burkert,Mu¨nchen,Germany W.B.Burton,Charlottesville,VA,USAand Leiden,TheNetherlands M.A.Dopita,Canberra,Australia A.Eckart,Ko¨ln,Germany T.Encrenaz,Meudon,France E.K.Grebel,Heidelberg,Germany B.Leibundgut,Garching,Germany J.Lequeux,Paris,France A.Maeder,Sauverny,Switzerland V.Trimble,CollegePark,MD,andIrvine,CA,USA Eugene F. Milone and William J. F. Wilson Solar System Astrophysics Planetary Atmospheres and the Outer Solar System 123 EugeneF.Milone WillamJ.F.Wilson Dept.Physics&Astronomy Dept.Physics&Astronomy UniversityofCalgary,Calgary UniversityofCalgary,Calgary 2500UniversityDriveNW 2500UniversityDriveNW Calgary,AlbertaT2N1N4 Calgary,AlbertaT2N1N4 Canada Canada [email protected] [email protected] CoverIllustration:“MysteriumCosmographicum”byDavidMouritsen LibraryofCongressControlNumber:2007933183 AstronomyandAstrophysicsLibrarySeriesISSN0941-7834 ISBN:978-0-387-73156-8 e-ISBN:978-0-387-73157-5 ISBN:978-0-387-73153-7(set) Printedonacid-freepaper. ©2008SpringerScience+BusinessMedia,LLC Allrightsreserved.Thisworkmaynotbetranslatedorcopiedinwholeorinpartwithoutthewritten permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY10013,USA),exceptforbriefexcerptsinconnectionwithreviewsorscholarlyanalysis.Usein connectionwithanyformofinformationstorageandretrieval,electronicadaptation,computersoftware, orbysimilarordissimilarmethodologynowknownorhereafterdevelopedisforbidden. Theuseinthispublicationoftradenames,trademarks,servicemarks,andsimilarterms,evenifthey arenotidentifiedassuch,isnottobetakenasanexpressionofopinionastowhetherornottheyare subjecttoproprietaryrights. ObservingtheSun,alongwithafewotheraspectsofastronomy,canbedangerous.Neitherthepublisher northeauthoracceptsanylegalresponsibilityorliabilityforpersonallossorinjurycaused,oralleged tohavebeencaused,byanyinformationorrecommendationcontainedinthisbook. 987654321 springer.com Preface Thisworkisappearingintwopartsbecauseitsmassistheresultofcombining detailed exposition and recent scholarship. Book I, dealing mainly with the inner solar system, and Book II, mainly on the outer solar system, represent the combined, annually updated, course notes of E. F. Milone and W. J. F. Wilson for the undergraduate course in solar system astrophysics that has beentaughtaspartoftheAstrophysicsProgramattheUniversityofCalgary since the 1970s. The course, and so the book, assumes an initial course in astronomy and first-year courses in mathematics and physics. The relevant concepts of mathematics, geology, and chemistry that are required for the course are introduced within the text itself. SolarSystemAstrophysics isintendedforusebysecond-andthird-yearastro- physics majors, but other science students have also found the course notes rewarding.Wethereforeexpectthatstudentsandinstructorsfromotherdisci- plines will also find the text a useful treatment. Finally, we think the work will be a suitable resource for amateurs with some background in science or mathematics. Most of the mathematical formulae presented in the text are derived in logical sequences. This makes for large numbers of equations, but italsomakesforrelativelyclearderivations.Thederivationsarefoundmainly inChapters2–6inBookI,subtitled,BackgroundScienceandtheInnerSolar System,andinChapters10and11inBookIIsubtitled,PlanetaryAtmospheres and the Outer Solar System. Equations are found in the other chapters as well but these contain more expository material and recent scholarship than someoftheearlierchapters.Thus,Chapters8and9,and12–16containsome usefulderivations,butalsomuchimageryandresultsofmodernstudies. Thefirstvolumestartswithadescriptionofhistoricalperceptionsofthesolar system and universe, in narrowing perspective over the centuries, reflecting the history (until the present century, when extra-solar planets again have begun to broaden our focus). The second chapter treats the basic concepts in the geometry of the circle and of the sphere, reviewing and extending material from introductory astronomy courses, such as spherical coordinate transformations. The third chapter then reviews basic mechanics and two- body systems, orbital description, and the computations of ephemerides, thenprogressestotherestrictedthree-bodyandn-bodycases,andconcludes with a discussion of perturbations. The fourth chapter treats the core of the solar system, the Sun, and is not a bad introduction to solar or stellar astrophysics; the place of the Sun in the galaxy and in the context of other VI Preface starsisdescribed,andradiativetransport,opticaldepth,andlimb-darkening are introduced. In Chapter 5, the structure and composition of the Earth are discussed, the Adams–Williamson equation is derived, and its use for determining the march of pressure and density with radius described. In Chapter6,thethermalstructureandenergytransportthroughtheEarthare treated,andinthischapterthebasicideasofthermodynamicsareputtouse. ExtendingthediscussionoftheEarth’sinterior,Chapter7describestherocks andmineralsintheEarthandtheircrystallinestructure.Chapter8treatsthe Moon, its structures, and its origins, making use of the developments of the precedingchapters.InChapter9,thesurfacesoftheotherterrestrialplanets are described, beginning with Mercury. In each of the three sections of this chapter, a brief historical discussion is followed by descriptions of modern ground-basedandspacemissionresults,withsomeofthespectacularimagery ofVenusandMars.Thechapterconcludeswithadescriptionoftheevidence for water and surface modification on Mars. This concludes the discussion of the inner solar system. The second volume begins in Chapter 10 with an extensive treatment of the physics and chemistry of the atmosphere and ionosphere of the Earth and an introduction to meteorology, and this discussion is extended to the atmospheres of Venus and Mars. Chapter 11 treats the magnetospheres of the inner planets, after a brief exposition of electromagnetic theory. In Chapter 12, we begin to treat the outer solar system, beginning with the gas giants. The structure, composition, and particle environments around these planets are discussed, and this is continued in Chapter 13, where the natural satellites and rings of these objects are treated in detail, with abundant use made of the missions to the outer planets. In Chapter 14, we discuss comets, beginning with an historical introduction that highlights the importance of comet studies to the development of modern astronomy. It summarizes the ground- and space-based imagery and discoveries, but makes use of earlier derivations to discuss cometary orbits. This chapter ends with the demise of cometsandthephysicsofmeteors.Chapter15treatsthestudyofmeteorites and the remaining small bodies of the solar system, the asteroids (aka minor planets, planetoids), and the outer solar system “Kuiper Belt” objects, and the closely related objects known as centaurs, plutinos, cubewanos, and others,allofwhicharenumberedasasteroids.Thechapterendswithdiscus- sions of the origin of the solar system and of debris disks around other stars, which point to widespread evidence of the birth of other planetary systems. Finally, in Chapter 16, we discuss the methods and results of extra-solar planetsearches,thedistinctionsamongstars,browndwarfs,andplanets,and we explore the origins of planetary systems in this wider context. At the end of each chapter we have a series of challenges. Instructors may use these as homework assignments, each due two weeks after the material from that chapter were discussed in class; we did! The general reader may find them helpful as focusing aids. January 22, 2008 E. F. Milone & W. J. F. Wilson Acknowledgments These volumes owe their origin to more than 30 years of solar system classes in the Astrophysics Program at the University of Calgary, called, at various times, Geophysics 375, Astrophysics 301, 309, and 409. Therefore, we acknowledge, first, the students who took these courses and provided feedback. It is also a pleasure to thank the following people for their contri- butions: David Mouritsen, formerly of Calgary and now in Toronto, provided for Chapter 1 and our covers an image of his original work of art, an inter- pretation of Kepler’s Mysterium Cosmographicum, in which the orbits of the planets are inscribed within solid geometric figures. In Chapter 3, the latest version of David Bradstreet’s software package, Binary Maker 3 wasusedtocreateanimagetoillustraterestrictedthree- body solutions. University of Calgary Professor Emeritus Alan Clark gave us an image of an active region and detailed comments on the solar physics material of Chapter 4; Dr Rouppe van der Voort of the University of Oslo provided high-quality images of two other active region figures, for Chapter 4; the late Dr Richard Tousey of the US Naval Research Laboratory provided slides of some of the images, subsequently scanned for Chapter 4; limb- darkened spectral distribution plots were provided by Dr Robert L. Kurucz, of the Harvard-Smithsonian Center for Astrophysics; Dr. Charles Wolff, of Goddard Space Flight Center, NASA, reviewed the solar oscilla- tions sections and provided helpful suggestions. Dr. D. J. Stevenson provided helpful criticism of our lunar origins figures, and Dr. Robin Canup kindly prepared panels of her lunar simulations for our Fig. 8.10. Dr Andrew Yau provided excellent notes as a guest lecturer in Asph 409 on the Martian atmosphere and its evolution, which contributed to our knowledge of the material presented in Chapters 9, 10, and 11; similarly, lectures by Professor J. S. Murphree of the University of Calgary illumi- nated the magnetospheric material described in Chapter 11. Dr. H. Nair was kind enough to provide both permission and data for Fig. 11.3. NASA’s online photo gallery provided many of the images in Chapters 8, 9, 12, 13, 14, and some of those in Chapter 15; additional images were provided by the Naval Research Laboratory (of both the Sun and the VIII Acknowledgments Moon). Some of these and other images involved work by other institu- tions, such as the U.S. Geological Survey, the Jet Propulsion Laboratory, Arizona State Univ., Cornell, the European Space Agency, the Italian Space Agency (ASI), CalTech, Univ. of Arizona, Space Science Institute, Boulder,theGermanAirandSpaceCenter(DLR),BrownUniversity,the Voyagers and the Cassini Imaging Teams, CICLOPS, the Hubble Space Telescope, University of Maryland, the Minor Planet Center, Applied Physics Laboratory of the Johns Hopkins University, and the many individualsources,whethercitedincaptionsornot,whocontributedtheir talents to producing these images. Dr. John Trauger provided a high resolution UV image of Saturn and its auroras for Chapter 12. Dr William Reach, Caltech, provided an infrared mosaic image of Comet Schwassmann–Wachmann 3, and Mr John Mirtle of Calgary provided many of the comet images for Chapter 14, including those of Comets 109P/Swift–Tuttle, C/1995 O1 (Hale–Bopp), C/Hyakutake, Lee, C/Ikeya–Zhang,Brorsen–Metcalfe,andMachholz;Dr.MichaelJ.Mumma ofNASA’sGSFCprovidedanimportantcorrection,andProfessorMichael F. A’Hearn of the University of Maryland critiqued the comet content of Chapter 14. Dr Allan Treiman, of the Lunar and Planetary Institute, Houston, was kind enough to provide background material on the debate over the ALH84001 organic life question, mentioned in Chapter 15. Mr Matthias Busch made available his Easy-Sky images of asteroid family distributions, and the Minor Planet Center provided the high-resolution figures of the distribu- tions of the minor planets in the inner and outer solar system. Dr Charles Lineweaver, University of New South Wales, provided a convincing illustration for the brown dwarf desert, illustrated in Chapter 16; University of Calgary graduate student Michael Williams provided several figures from his MSc thesis for Chapter 16. Mr Alexander Jack assisted in updating and improving the readability of equations and text in some of the early chapters, and he and Ms Veronica Jack assisted in developing the tables of the extra-solar planets and their host stars for Chapter 16. In addition, we thank the many authors, journals, and publishers who have given us permission to use their figures and tabular material or adaptations thereof,freely.Finally,itisalsoapleasuretothankSpringereditorsDrHans Koelsch,DrHarryBlom,andtheirassociate,ChristopherCoughlin,fortheir support for this project. Contents Planetary Atmospheres and the Outer Solar System 10. Planetary Atmospheres ................................. 1 10.1. Atmospheric Constituents................................. 1 10.2. Atmospheric Structure.................................... 4 10.2.1. Pressure Variation with Height..................... 4 10.2.2. Temperature Variation with Height................ 7 10.3. Circulation in the Atmosphere............................. 11 10.3.1. Centrifugal and Coriolis Forces..................... 11 10.3.2. Physical Effects of the Centrifugal and Coriolis Forces............................................. 13 10.3.2.1. The Centrifugal Force................... 13 10.3.2.2. The Coriolis Force....................... 15 10.3.3. Pressure Gradient Force............................ 16 10.3.4. Friction............................................ 17 10.3.5. Geostrophic Balance and Geostrophic Winds....... 18 10.3.6. Thermal Effects.................................... 19 10.3.6.1. Thermal Circulation..................... 19 10.3.6.2. The Thermal Wind...................... 20 10.3.7. Global Circulation................................. 22 10.3.7.1. The Observed Surface Pattern........... 22 10.3.7.2. The Hadley Cell......................... 24 10.3.7.3. The Ferrel and Polar Cells............... 25 10.3.7.4. Eddie Motions in the Westerlies......... 26 10.3.7.5. Air Masses and Fronts................... 27 10.3.7.6. Jet Streams............................. 27 10.4. Atmospheric Effects on the Heat Budget................... 30 10.4.1. The Earth......................................... 30 10.4.1.1. Troposphere of the Earth................ 31 10.4.1.2. Stratosphere and Mesosphere............ 32 10.4.1.3. Thermosphere........................... 33 10.4.1.4. Exosphere............................... 34 10.4.2. Mars............................................... 34 10.4.2.1. Troposphere............................. 36 X Contents 10.4.2.2. Stratomesosphere........................ 36 10.4.2.3. Thermosphere........................... 36 10.4.3. Venus.............................................. 36 10.4.3.1. Troposphere............................. 37 10.4.3.2. Stratomesosphere........................ 37 10.4.3.3. Thermosphere........................... 37 10.5. Planetary Circulation Effects.............................. 38 10.5.1. Circulation and the Coriolis Force.................. 38 10.5.2. Meridional (N–S) Circulation....................... 39 10.5.3. Zonal (E–W) Circulation........................... 39 10.5.3.1. Mars.................................... 40 10.5.3.2. Venus................................... 41 10.5.3.2.1. Atmospheric Superrotation............... 41 10.5.3.2.2. Cyclostrophic Balance....... 41 10.5.3.2.3. Atmospheric Angular Momentum.................. 42 10.5.3.2.4. Superrotation vs. Other Circulation Patterns......... 43 10.5.4. Other Considerations.............................. 44 10.5.4.1. Latent Heat............................. 44 10.5.4.2. Thermal Inertia......................... 44 10.5.4.3. Brunt-Va¨is¨ala¨ Frequency:................ 44 10.5.4.4. Diffusion and Mixing in Planetary Atmospheres............................ 45 10.5.4.4.1. Diffusion.................... 45 10.5.4.4.2. Diffusion vs. Mixing......... 47 10.5.4.4.3. The Homopause............. 48 10.5.5. Chemical Cycles................................... 48 10.5.5.1. Carbon Cycle (Earth)................... 48 10.5.5.2. Oxygen Cycle (Earth)................... 48 10.5.5.3. Nitrogen Cycle (Earth).................. 49 10.5.5.4. Sulfur Cycle (Earth)..................... 51 10.5.5.5. Sulfur Cycle (Venus)..................... 52 10.5.5.6. Thermospheric Chemistry of Neutrals (Earth).................................. 53 10.5.6. Excess Radiation................................... 54 11. Planetary Ionospheres and Magnetospheres ............. 57 11.1. Earth: Ionospheric Layers................................. 57 11.1.1. The F Layer....................................... 57 11.1.1.1. Atoms and Ions in the F Layer.......... 57 11.1.1.2. Production Mechanisms................. 58