Topic Science Subtopic & Mathematics Astronomy A Field Guide to the Planets Course Guidebook Professor Sabine Stanley Johns Hopkins University Published by THE GREAT COURSES Corporate Headquarters 4840 Westfields Boulevard | Suite 500 | Chantilly, Virginia | 20151‑2299 [phone] 1.800.832.2412 | [fax] 703.378.3819 | [web] www.thegreatcourses.com Copyright © The Teaching Company, 2019 Printed in the United States of America This book is in copyright. All rights reserved. Without limiting the rights under copyright reserved above, no part of this publication may be reproduced, stored in or introduced into a retrieval system, or transmitted, in any form, or by any means (electronic, mechanical, photocopying, recording, or otherwise), without the prior written permission of The Teaching Company. Sabine Stanley, PhD BloomBerg DistinguisheD Professor Johns hoPkins university Sabine Stanley is a Bloomberg Distinguished Professor in the Morton K. Blaustein Department of Earth and Planetary Sciences at Johns Hopkins University. She received a HBSc degree in Physics and Astronomy from the University of Toronto and then completed MA and PhD degrees in Geophysics from Harvard University. Prior to joining Johns Hopkins, Professor Stanley was a postdoctoral researcher at the Massachusetts Institute of Technology and a professor at the University of Toronto. At Johns Hopkins, she also holds appointments in the Henry A. Rowland Department of Physics and Astronomy, the Applied Physics Laboratory’s Space Exploration Sector, and the Hopkins Extreme Materials Institute. i PROFESSOR BIOGRAPHY Professor Stanley has received several honors and awards for both her research and teaching. Her research honors include the American Geophysical Union’s William Gilbert Award for her major theoretical contributions to the study of planetary magnetism, a Sloan Research Fellowship as an early career scientist of outstanding promise, and a Canada Research Chair. Her teaching awards include the Ranjini Ghosh Excellence in Teaching Award and the Dean’s Outstanding Teaching Award, both from the University of Toronto. Professor Stanley’s research involves investigations of planetary interior processes and evolution. She focuses on studying planetary magnetic fields and uses high-performance computing to study the dynamo process, in which convection in electrically conducting fluid regions inside planets generate self-sustaining magnetic fields. Professor Stanley’s research includes studies of the magnetic fields of Earth, Mercury, Mars, Saturn, Uranus, Neptune, and exoplanets. She is also a coinvestigator on NASA’s Mars InSight mission. In addition to her research work, Professor Stanley has written articles for Bloomberg View and Eos. She has also appeared several times on Canada’s CBC Radio show Quirks & Quarks. Professor Stanley has served as the editor of the Journal of Geophysical Research: Planets and has chaired the Women in Physics Canada Conference. She also tweets about science and life as a scientist as @PlanetSabine. ii Table of Contents introDuction Professor Biography . . . . . . . . . . . . . . . . . . . . . . . . . . . . i Course Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 GuiDeS 1 How the Solar System Family Is Organized. . . . . . . . . . . . . . 4 2 Mercury, the Extreme Little Planet. . . . . . . . . . . . . . . . . . . . 15 3 Venus, the Veiled Greenhouse Planet . . . . . . . . . . . . . . . . . 27 4 Earth: How Plate Tectonics Sets Up Life. . . . . . . . . . . . . . . . 38 5 Orbiting Earth: Up through the Atmosphere . . . . . . . . . . . . . 50 6 Exploring the Earth-Moon System . . . . . . . . . . . . . . . . . . . 62 QUIZ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 7 Humans on the Moon: A Never-Ending Story . . . . . . . . . . . 76 8 Exploring Mars from Space and the Ground. . . . . . . . . . . . 88 9 Water on Mars and Prospects for Life. . . . . . . . . . . . . . . . . 99 10 Near-Earth Asteroids and the Asteroid Belt . . . . . . . . . . . . 110 11 Mighty Jupiter, The Ruling Gas Giant . . . . . . . . . . . . . . . . 122 12 Jupiter’s Planetlike System of Moons. . . . . . . . . . . . . . . . . 133 QUIZ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 iii TABLE OF CONTENTS 13 Saturn and the Rings: Gravity’s Masterpiece . . . . . . . . . . . 151 14 Saturn’s Moons: Titan to Enceladus . . . . . . . . . . . . . . . . . 163 15 Uranus: A Water World on Its Side . . . . . . . . . . . . . . . . . 174 16 Neptune: Windy with the Wildest Moon . . . . . . . . . . . . . 185 17 Pluto and Charon: The Binary Worlds. . . . . . . . . . . . . . . . 196 18 Comets, the Kuiper Belt, and the Oort Cloud. . . . . . . . . . . 207 QUIZ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 19 How Our Sun Defines Our Solar System. . . . . . . . . . . . . . 222 20 A Solar System Time Machine and Meteorites. . . . . . . . . . 234 21 What the Biggest Exoplanets Reveal . . . . . . . . . . . . . . . . 247 22 Closing in on Earthlike Exoplanets . . . . . . . . . . . . . . . . . . 257 23 Planets Migrated in Our Early Solar System! . . . . . . . . . . . 269 24 Human Futures in the Solar System. . . . . . . . . . . . . . . . . . 280 QUIZ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 SuPPlementary material Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 NAVIGATION TIP To go back to the page you came from, press Alt + ← on a PC or ⌘ + ← on a Mac. On a tablet, use the bookmarks panel. iv COURSE SCOPE A Field Guide to the Planets This course provides a tour of all the planets and other smaller planetary objects in our solar system and beyond. If you were suddenly transported to Mars, what would you experience? What’s it like to float through Saturn’s rings? Through experiential examples, you’ll discover the wonders of our solar system while also learning about the planetary science behind these wonders. The tour involves trips to all the planets in the solar system, from Mercury to Neptune. What are the differences between worlds that are close to the Sun and those that are far from the Sun? Why do planets have the surface features they do? How were planets different in the past? Why are planets’ atmospheres so different? The course ventures through the atmospheres, over the surfaces, and deep into the interiors of the rocky and giant planets in our solar system to answer these questions. But you’ll visit more than just planets. You’ll travel to the asteroid belt between Mars and Jupiter; to the extensive moon systems of the giant planets; to the Kuiper belt, home of Pluto and other dwarf planets; and to comets coming from the far reaches of the solar system. You’ll also investigate the influence of the Sun in defining and shaping what we call the solar system—from gravity, to beneficial heat and light, to hazardous radiation and high-energy particles. 1 COURSE SCOPE You’ll also examine the origins and evolution of our solar system, so you’ll also venture backward in time, to the era when our solar system was just forming, to see how the system we take for granted all began. You’ll also travel to so-called exoplanets—the planets that orbit other stars— where there are worlds that are vastly different from those in our own solar system as well as some solar system analogs. Have we found Earth 2.0? Lessons from exoplanets will have us rethink our understanding of how our own solar system formed and lead us to explore a solar system conundrum known as the late heavy bombardment, when the number of impactors in the early solar system increased 1000-fold some 500 million years after the solar system “finished” forming. There are some common themes throughout the lectures in this course: 1 Water seems to be ubiquitous in the solar system, appearing on world surfaces, like Earth, as well as in the deep interiors of Uranus and Neptune and in the permanently shadowed craters of Mercury and the Moon. 2 How planets cool affects their surface and atmospheric features. Volcanoes, canyons, earthquakes, magnetic fields, and giant storms are all related to the fact that planetary bodies are cooling to space. 3 Collisions shape the planets. Giant collisions are responsible for everything from the formation of the planets, to Earth’s large Moon, to impact craters strewn over planetary surfaces. They are also responsible for bringing water to Earth and may be responsible for the axial tilts of some of the planets. 2 COURSE SCOPE In this course, you’ll explore some of the surprising, even bizarre, features of our solar system, such as pancake volcanoes on Venus, a giant hexagonal storm on Saturn, and the icy “heart” on Pluto. And you’ll discover why the Sun sometimes changes direction in Mercury’s sky, why we always see the same side of the Moon from Earth, and why Neptune and Pluto never collide even though their orbits cross. In addition to exploring the features found on planetary bodies, you’ll also learn about the fundamental physical, chemical, and geological processes that are responsible for generating the observed features. These lessons will be scientifically detailed but accessible to nonexperts. No previous expertise in math, physics, chemistry, or geology is needed, but important principles in these fields will be taught when they are relevant. Throughout this course, lectures incorporate the latest discoveries from planetary spacecraft missions. You’ll rove over Mars with the Curiosity rover, explore the moons of Saturn with the Cassini mission, and even reach interstellar space with Voyagers I and II. In the end, you’ll focus on the next big ideas for exploring and understanding our solar system by considering the future of spacecraft propulsion, the potential terraforming of Mars, and the exploration of subsurface oceans in outer solar system moons to look for life. 3 LECTURE 1 How the Solar System Family Is Organized 4