Planets and planetary systems

192 Pages·2009·3.054 MB·English

by  Stephen Eales

Planets and Planetary
Systems
Stephen Eales
School of Physics and Astronomy, Cardiff University UK
A John Wiley & Sons, Ltd., Publication
Thiseditionfirstpublished2009
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Eales,Stephen.
Planetsandplanetarysystems/StephenEales.
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ISBN978-0-470-01692-3(cloth) –ISBN978-0-470-01693-0(pbk.) 1.
Planetarytheory. 2. Planets. 3. Solarsystem.I.Title.
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Contents
Preface ix
1. Our planetary system 1
1.1 Diversity in the Solar System 1
1.2 General trends in the properties of the planets 9
1.3 Why are planets round? 12
1.4 When is a planet not a planet? 19
Exercises 21
2. Other planetary systems 23
2.1 The discovery of exoplanets 23
2.2 The implications of the existence of other planetary systems 29
2.3 The future for exoplanet research 32
Exercises 36
Further Reading and Web Sites 37
3. The surfaces of the planets 39
3.1 Rocks 39
3.2 Geological structures 43
3.3 Crater counting 50
3.4 Mercury and Venus 52
3.5 A tourist’s guide to Mars 55
3.6 Recent research on Mars 58
Exercises 64
Further Reading and Web Sites 64
4. The interiors of the planets 65
4.1 What we do and don’t know about planetary interiors 65
vi CONTENTS
4.2 Mercury, Venus, Mars, Saturn, Uranus and Neptune 72
4.3 Why we know so much about the Earth 73
4.4 Why is the surface of the Earth such an interesting place? 78
Exercises 83
Further Reading 84
5. The atmospheres of the planets 85
5.1 The atmosphere of the Earth 85
5.2 The other planets 93
5.3 The weather on the Earth and elsewhere 96
5.4 The origin and evolution of planetary atmospheres 99
Exercises 102
Further Reading and Web Sites 103
6. The dynamics of planetary systems 105
6.1 Laws of planetary motion 105
6.2 Stable and unstable orbits 108
6.3 Tidal forces 111
Exercises 117
7. The small objects in planetary systems 119
7.1 The evidence of the meteorites 119
7.2 The asteroid belt 123
7.3 Comets 126
7.4 The Oort Cloud 132
7.5 The Edgeworth-Kuiper belt 135
Exercises 139
Further Reading and Web Sites 139
8. The origin of planetary systems 141
8.1 Laplace’s big idea 141
8.2 The protoplanetary disc 144
8.3 From dust to planetesimals 147
8.4 From planetesimals to planetary embryos 149
8.5 From planetary embryos to planets 150
CONTENTS vii
8.6 Collisions, the Oort Cloud and planetary migration 152
Exercises 156
Further Reading 157
9. Life in planetary systems 159
9.1 A short history of life on Earth 159
9.2 The evolution of the Solar System as a habitat 164
9.3 The possibility of life elsewhere 167
Exercises 171
Further Reading and Web Sites 171
Answers 173
Appendix A 175
A.1 The epoch of planetary exploration 175
Appendix B 179
B.1 Derivation of Kepler’s first and second laws 179
Index 183
Preface
Theidealplanetaryscientistwouldhaveknowledgeofastronomy,physics,chemistry,
geology, meteorology, oceanography and, because both the atmosphere and the
surface of our planet have clearly been extensively modified by living creatures,
biology.AlthoughIhavegivenacourseonplanetsandplanetarysystemsforthelast
decade,Icanonlyreallyclaimtobeanexpertintwooftheseareas,buttheliberating
thing about writing a book on such a huge rambling interdisciplinary subject is
that nobody else has the perfect credentials for writing one either. As with many
writersoftextbooks,Idecidedtowritethisbook,notbecauseIwantedtosharemy
wisdom withtheworld,butbecauseIneverfoundatextbookthatwas perfectfor
mycourse.Theavailablebookswereeithertoobasicorweregraduate-leveltomes
muchtoobig(andexpensive)foranundergraduatecourse,exacerbatedbythefact
thatplanetaryscienceissuchadynamicareaofresearchthatanytextbookgetsout
ofdateveryquickly.AlthoughIamanastrophysicistratherthananoceanographer
orageologist,Ihavetriedtowriteageneralintroductiontoplanetsandplanetary
systems that uses insights from all the disciplines involved in the study of these
objects.Thebookshouldbesuitableforanystudentstudyingplanetsorplanetary
systemsaspartofanundergraduatesciencedegree,andIhavealsoprovidedaless
mathematical route through the book for any student that does not have a basic
knowledgeofcalculus(anelementaryknowledgeofdifferentiationandintegration).
In such a rapidly changing field, I have tried to make the book as up to the
minute as possible by incorporating results from the most recent planetary space
missions, such as the Cassini mission to Saturn and the many recent missions to
Mars.Ihavealsolistedrecentscientificpapersasfurtherreadingattheendofsome
ofthechapters;sincethesearemostlytakenfromthejournalsScienceandNature,
which, at least in intention, are journals for thenon-specialist reader, they should
becomprehensibletoanyundergraduate.Nevertheless,anybookinsucharapidly
changing subject gets out of date very fast. If you are still interested in planetary
science after finishing this book, there are a number of ways you can learn about
newdiscoveriesinthefield.Thebestplacetolook,ofcourse,istheinternet.Every
spacemissionhasawebsite,andonceyouknowthenameofthespacemission,it
is easy tofind theweb site using astandardsearchengine (Appendix 1contains a
x PREFACE
listofpastspacemissionsandaprovisionallistofupcomingspacemissions).There
arealsotwovaluabledatabasesofscientificpapersontheinternet.Theastrophysics
preprintdatabase(http://xxx.lanl.gov/archive/astro-ph)isanarchiveofastronomy
papers written since April 1992, although unfortunately planetary scientists have
been slower than other groups of astronomers in using the archive. The NASA
Astronomy Data System (http://adsabs.harvard.edu/abstract service.html) is an
archive of all astronomy papers that have ever been published. This is now an
essentialresourceforanyastronomer;itispossible,forexample,touseittofindall
thepapersthathaveeverbeenpublishedonanysubjectinwhichyouareinterested
andtoreadpapersthatwerewrittendecadesagobysomeofthegiantsinthefield,
forexampleOort’soriginalpaperontheOortCloud.
I hope you enjoy the book. Please e-mail me any comments or suggestions for
improvementsforfutureeditions.
StephenEales
[email protected]
PhysicalConstants
Symbol ValueinSIunits Meaning
c 2.9979×108ms−1 Speedoflight
G 6.670×10−11m3kg−1s−2 Gravitationalconstant
h 6.626×10−34Js Planck’sconstant
k 1.381×10−23JK−1 Boltzmann’sconstant
m 9.109×10−31kg Massofelectron
e
m 1.673×10−27kg Massofproton
p
m 1.661×10−27kg Atomicmassunit
amu
N 6.022×1023mol−1 Avogadro’snumber
A
σ 5.667×10−8Wm−2K−4 Stefan–Boltzmannconstant
AstronomicalConstants
Symbol ValueinSIunits Meaning
AU 1.496×1011m Earth-Sundistance
Pc 3.086×1016m Parsec–astronomicalunitofdistance
M(cid:2) 1.989×1030kg Solarmass–astronomicalunitofmass
L(cid:2) 3.827×1026W Solarluminosity–astronomicalunitofluminosity
(a)
(b)
(c)
Figure 1.1 The eight planets in our planetary system. (a) Mercury (Messenger); Venus
(Pioneer Venus Orbiter); Earth (Apollo 8). (b) Mars (Viking Orbiter); Jupiter (Voyager 2);
Saturn(Voyager2).(c)Uranus(Voyager2);Neptune(Voyager2)(courtesy:NASA).
Planets and Planetary Systems Stephen Eales
© 2009 John Wiley & Sons, Ltd. ISBN: 978-0-470-01692-3
(a)
(b)
Figure 1.4 The six largest moons in the solar system (not to scale). (a) the Moon
(Clementine); Io (Voyager 1); Europa (Voyager 1). (b) Ganymede (Voyager 1); Callisto
(Voyager1);Titan(Cassini)(courtesy:NASAandESA).
(a)
(b)
Figure3.12 TwomapsofMars.(a)isageologicalmapshowingthepartsofthesurface
thatwereformedduringthethreeepochsofMartianhistory:Noachian(orange),Hesperian
(green),Amazonian(blue).Thewhiteareasarewheredebrisfromrecentlargeimpactshas
coveredgeologicalstructuresandearliercraters,makingitimpossibletoestimatetheage
ofthesurfacebeneath.(b)isatopographicmapmadebytheMarsOrbiterLaserAltimeter
(see text) (from Solomon et al. 2005, Science, 307, 1214 reprinted with permission from
AAAS)
Figure 3.13 Topographic map of Venus made by Magellan, in a mercator projection
(northisatthetop).Thelowestregionsareshown asblue,thehighestasred(courtesy:
NASA).