Fundamentals Asteroids Moon mercury mars venus earth jupiter saturn uranus Neptune pluto comets

Exploring the Planets v18.0

Endowed with an irrepressible curiosity, the human species has always explored. Humans first explored their immediate surroundings, then neighboring mountains, valleys, and rivers, then adjacent lands, and eventually the vast oceans and faraway continents. Even the night skies were the object of intense scrutiny and wonderment. With time, people would use telescopes to vicariously explore the planets. Eventually, our desire to explore and the capabilities of our technology broke the bonds of Earth's gravity and took us to visit, sometimes through remote eyes, our planetary neighbors.

Thirty-five years ago, we first extended our reach beyond our home planet with a brief visit to the Moon. The exploration of the solar system that followed was just as revolutionary in its way as the exploration of our own planet was in the centuries that preceded this step. In little more than two decades we collected pieces of the Moon, probed the atmosphere of Venus, mapped the huge volcanoes and canyons of Mars, photographed the surface of Mercury, examined comets and asteroids close up, and explored the frigid surfaces of the moons of Jupiter, Saturn, Uranus, and Neptune. Of all the traditional planets, only Pluto has not been visited by spacecraft. There will be other exciting periods of space exploration to be sure, and many great and significant discoveries are waiting to be made, but no other generation will explore for the first time so many worlds in our solar system. Subsequent missions to the planets will continue the flood of data to help us understand the processes that shaped these planetary objects, but the most exciting phase--when we explored the surfaces of these bodies for the first time--is over.

With the development of widespread use of the World Wide Web and completion of so many new missions since the last (paper) edition of this book, it is now an appropriate point to summarize the scientific results of these exploratory missions. We have created these web pages to to share with you some of the knowledge gained from this important period of discovery and to introduce you to the study of the solar system as we consider the fundamentals of how planets originate and evolve.

We hope exPlanet--short for Exploring the Planets--will appeal to all who are curious about planets, moons, and other objects in the solar system. Our approach is nonmathematical, but nonetheless analytical; as such, we hope it will be used by a broad range of interested people, even those with little scientific background. We emphasize the surfaces, internal structures, and histories of the planets from a geological point of view. We assume only what is typically obtained in high school science classes and the intuition gained by experience with our surroundings. The metric system of measurement is used throughout the book; distances are expressed in meters and kilometers, masses in kilograms, and temperatures in degrees Kelvin.


exPlanet could serve as a text for an introductory college science course or as a supplement to college astronomy or geology courses. The chapters are ordered to provide a logical flow of ideas regarding the development of the planets. After considering some fundamental principles needed to understand the planets, we start with a discussion of the smallest and simplest bodies, the asteroids and meteorites, and their role as building blocks of the inner planets. From there we discuss progressively larger terrestrial planets. We proceed from the Moon to larger and more complex planets Venus and Earth. The outer planets are discussed in the order of their occurrence outward from the Sun. The small bodies of the outer solar sytem--Pluto and the Kuiper belt as well as comets and the Oort cloud--are considered next. A closing chapter compares the planets by briefly examining the principal processes that have shaped them. Although Chapters 1 and 2 would be most helpful if studied first, the other chapters generally contain enough background material to stand alone or to be used out of sequence.
We have also incorporated several learning aids to help you in your study of the planets. Each chapter begins with a succinct list of the major concepts that should be understood. A preliminary reading of these items will focus your attention on the fundamental principles. Important new terms are printed in bold type and are reiterated in a list at the end of each chapter. Thought-provoking questions should guide your review and help you to extrapolate beyond simple repetition of observations. Additional readings are also listed at the end of each chapter for those who wish to explore further on their own. A glossary provides short definitions of terms that may be unfamiliar. The definitions are in accordance with the latest edition of the Glossary of Geology published by the American Geological Institute.

About This Edition

The principal author of this book is Eric H. Christiansen, a professor of geology at Brigham Young University. Braxton Spilker joined this effort in 2017 and will provide much needed updates for the chpaters on Mercury and Pluto. Watch for them to be released in the first half of 2018. Much of the original content written back in 1990 was written by W. Kenneth Hamblin--he still inspires me today. This online textbook is open for revision at any time. We welcome your suggestions for improvements. Please contact Dr. Eric Christiansen at with your suggestions.


We sincerely thank all those that have made the exploration of the planets possible--principally the citizens of the United States and the former Soviet Union. Through their desire to know and understand their surroundings, the people of these countries have appropriated funds to construct rockets, spacecraft, and sophisticated instruments that have endowed us with the remote vision we require to explore the planets. Other nations are now joining this endeavor and we hope that the exploration of space will yet become a collective endeavor, shared by all the peoples of Earth.

We are also grateful to the following colleagues who from time to time have reviewed the entire text or various chapters and offered many helpful suggestions: C. J. Casella, Northern Illinois University; J. Cain, Florida State University; Ronald Greeley, Arizona State University; James W. Head III, Brown University; R. Craig Kochel, Southern Illinois University; Jeffrey Moore, Arizona State University; Carleton B. Moore, Arizona State University; Quinn Passey, Exxon Research; Lawrence A. Soderblom, U.S. Geological Survey; Joseph Veverka, Cornell University.

We are especially grateful to Teryl Bodily for producing several original paintings featured in the text.

Chapter 1--Fundamentals of Planetary Science

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