Table Of ContentThe Ringed Planet T
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Cassini’s voyage of discovery at Saturn In
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Joshua Colwell la
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On September 15, 2017, the Cassini spacecraft sent its final transmission to the Earth as it entered the t: C
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atmosphere of Saturn, ending its historic 13 year mission at the ringed planet. This book is a beautifully s
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illustrated journey of discovery through the Saturn system. Cassini’s instruments have revealed never i’s
seen before details, including the only extraterrestrial lakes known in the solar system, and have provided v
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unprecedented views of the rings, moons, and the planet itself. Results from Cassini’s dramatic Grand Finale ya
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of ring-grazing and planet-skimming orbits are included in this expanded and updated second edition. e
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Saturn is the jewel of the solar system. The Cassini spacecraft has been exploring the ringed planet and its is
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moons and rings since 2004 and has helped us solve many of its mysteries while generating a wealth of new v
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questions. Cassini has observed the bizarre mountains of Iapetus, the geysers of Enceladus, the lakes of Titan, ry
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and the dynamic and evolving rings. Along the way, this book explores and explains the fundamental processes t S
that shape not just the Saturn system, but planets and moons in general. a
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Written for the general audience with an emphasis on the fundamental physics of planetary systems, The , 2
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Ringed Planet is a fascinating exploration of the Saturn system that places Saturn in the context of the solar d
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system as a whole. Cassini’s instruments have revealed Enceladus and Titan to have subsurface oceans d
of liquid water. Its cameras have returned stunning images of rings in turmoil, a tumbling moon, the only itio
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extraterrestrial lakes known in the solar system, a hexagon of clouds, some of the highest mountains in the • J
solar system and much more. More than a journey of discovery at Saturn, The Ringed Planet is also an os
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introduction to how planetary systems work. u
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About the Author lw
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Dr. Joshua Colwell is a Planetary Scientist and Professor of Physics at the University of Central Florida with ll
a Ph.D. in Astrophysical, Planetary and Atmospheric Sciences from the University of Colorado. His research
interests are in the origin and evolution of the solar system with a particular emphasis on planet formation,
planetary rings and interplanetary dust. He is a Co-Investigator on the Ultraviolet Imaging Spectrograph on
the Cassini mission. He produces and hosts the astronomy podcast “Walkabout the Galaxy.”
About Concise Physics
Concise Physics publishes short texts on rapidly advancing areas or topics, providing readers with a
snapshot of current research or an introduction to the key principles. These books are aimed at researchers
and students of all levels with an interest in physics and related subject areas.
The Ringed Planet
(Second Edition)
Cassini’s voyage of discovery at Saturn
The Ringed Planet
(Second Edition)
Cassini’s voyage of discovery at Saturn
Joshua Colwell
University of Central Florida, Florida, USA
Morgan & Claypool Publishers
Copyrightª2019Morgan&ClaypoolPublishers
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Mediacontentforthisbookisavailablefromhttps://iopscience.iop.org/book/978-1-64327-714-1.
ISBN 978-1-64327-714-1(ebook)
ISBN 978-1-64327-711-0(print)
ISBN 978-1-64327-712-7(mobi)
DOI 10.1088/2053-2571/ab3783
Version:20191101
IOPConcisePhysics
ISSN2053-2571(online)
ISSN2054-7307(print)
AMorgan&ClaypoolpublicationaspartofIOPConcisePhysics
PublishedbyMorgan&ClaypoolPublishers,1210FifthAvenue,Suite250,SanRafael,CA,
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To my mother, my wife, and my daughter, who see me through.
Contents
Preface to the second edition ix
Acknowledgments xi
Author biography xii
1 Introduction 1-1
2 Seeing Saturn 2-1
Further reading 2-9
3 Why are there moons and rings? 3-1
Further reading 3-11
References 3-11
4 A menagerie of moons 4-1
4.1 The inner mid-sized icy moons 4-1
4.2 Impacts and craters 4-3
4.3 Synchronous rotation 4-9
4.4 Icy satellite tectonics 4-11
4.5 Interactions with the magnetosphere 4-13
4.6 The curious case of Hyperion 4-17
4.7 Iapetus: the two-tone moon 4-21
4.8 Enceladus: Saturn’s old faithful 4-29
4.9 The fabulous five: Cassini’s parting look at five ring-moons 4-35
Further reading 4-38
References 4-38
5 The rings—a dance of gravity 5-1
5.1 Introduction 5-1
5.2 The main rings 5-2
5.3 Self-gravity wakes 5-6
5.4 Satellite wakes and shepherding 5-8
5.5 Density waves and bending waves in the rings 5-13
5.6 The B ring, ballistic transport, and spokes 5-22
5.7 The C ring and Cassini Division 5-29
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TheRingedPlanet(SecondEdition)
5.8 The F ring 5-33
5.9 The dusty rings 5-37
Further reading 5-38
References 5-38
6 Divining the age of the rings 6-1
6.1 The problem 6-1
6.2 The spreading of the rings 6-2
6.3 The mass of the rings 6-4
6.4 The brightness of the rings 6-7
6.5 The erosion of the rings 6-9
6.6 The composition of the rings 6-11
6.7 The age and origin of the rings 6-12
Further reading 6-13
References 6-13
7 Titan: the planet moon 7-1
7.1 The atmosphere 7-2
7.2 Lakes, dunes and mountains 7-7
7.3 Titan’s interior 7-14
Further reading 7-17
References 7-17
8 Saturn’s hidden mysteries 8-1
8.1 Origin and interior 8-1
8.2 Atmosphere 8-6
8.3 Magnetic field 8-13
References 8-20
9 Future exploration of the Saturn system 9-1
9.1 Ocean worlds 9-1
9.2 Rings 9-4
9.3 Saturn 9-6
9.4 Other worlds 9-7
Further reading 9-9
References 9-9
viii
Preface to the second edition
One of the wonderful things about the Saturn system is that there is so much
happeninginitthatitcanalmostbeusedasareviewofplanetarysciencesingeneral.
And that is also the challenge in writing a book about the system. Like the Cassini
mission itself, this book explores Saturn, its rings, moons, and magnetosphere.
That’s a lot of material to cover in a limited space. I emphasize the physical
processesthathaveshapedtheSaturnsystem,usingsomeofthemostdramaticand
intriguing discoveries made by Cassini at Saturn as case studies to illustrate these
processes.Thebookiswrittenforageneralaudience,anditisalsoappropriateasan
introduction to planetary sciences for students.
Cassini’s observations of Saturn, spanning more than 13 years, have revolution-
ized our understanding of the planet, its complex ring system, and many of its
moons,especiallyEnceladusandTitan.Ihavealwaysbeeninterestedinwhythings
are the way they are, and there are many astounding observationsmade by Cassini
that demand an answer to the question ‘why is that?’. We have answers to that
question for many of Cassini’s discoveries, and this book covers a diverse set, from
the origins of dusty ringparticles to the heart of the giant planet itself. At the same
time,therearenewquestionsrevealedbyCassinithatwillrequiremoreresearchand
future missions. I emphasize links between the phenomena at Saturn and planetary
systems in general. And still this only scratches the surface. There are many more
intriguing phenomena and discoveries that I could not include without having the
bookexpandtoseveraltimesitscurrentlength.Ihopethereaderisinspiredtolearn
moreaboutthisfascinatingplanetanditsfamilyofringsandmoons.Theimagesin
this book are also just a sampling of those returned by Cassini to illustrate the
physicsdescribedinthebookandtowhettheappetiteofthereadertodiscovermore
at the public repositories of all Cassini data.
Cassini’s missionat Saturn ended with two daringsequences oforbits, known as
the Ring Grazing Orbits, in early 2017 and the final set of orbits, known as the
ProximalOrbits,thatthreadedthegapbetweentheplanetandtherings.ThisGrand
Finale concluded with the spacecraft entering the atmosphere of the planet on
15 September 2017, exactly one month shy of the 20th anniversary of its launch.
Theseuniqueorbitsprovidednewviewsoftheplanetanditsrings.Inthetimesince
Cassini’s dramatic end, scientists have assembled the jigsaw puzzle-pieces of data
collected over the course of more than a decade from virtually every instrument on
the spacecraft to try to answer one of the biggest questions about Saturn: how old
areitsrings?Whiletheyappeartoberelativenewcomerstothesolarsystematunder
100 million years, there are still some intriguing puzzles that may upend this
conclusion. More surprises were in store for Cassini scientists when they analyzed
thesamedatatounderstandSaturn’sinteriorandmagneticfield.Thesearesomeof
the many revelations from Cassini’s Grand Finale that have been added to this
expanded second edition of The Ringed Planet.
Each chapter has been revised and updated based on the last dramatic year of
Cassini’s mission, and a new chapter devoted to the origin of the rings has been
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