Introduction to Planetary Science

preface xix
structure, their geologic history and present activity, and the presence
or absence of water or other volatile compounds in solid, liquid, or
gaseous form. We emphasize that the present surfaces of the terrestrial
planets are shaped by the geological processes that have occurred in
the past or are still occurring at the present time.
The so-called gaseous planets and their satellites are alien worlds
with properties that are radically different from those of the Earth and
the other terrestrial planets. We pay particular attention to Jupiter and
its Galilean satellites because they exemplify the bizarre properties
of Saturn, Uranus, and Neptune and their major satellites. Each one
of these giant planets is unique and yet all of these planets and their
satellites fit the mold provided by the theory of the solar system.
Until quite recently, very little information was available about
the Pluto-Charon system on the outer fringes of the solar system.
Pluto is now known to be a member of a newly-defined group of
objects composed primarily of ices of several volatile compounds. The
largest members of this group, including Pluto, are now classified as
dwarf planets. In addition, the entire solar system is surrounded by
a “cloud” of icy bodies whose highly eccentric orbits take them far
into interstellar space. When these small solar-system bodies return to
the inner solar system, they may either collide with the gaseous and
terrestrial planets or they may swing around the Sun as comets.
The discussion of comets returns our attention to the inner solar
system and thus to the Earth, which is the only planet we know well.
Therefore, the next to last chapter contains a review of the dominant
planetary processes and of the way these processes are manifested
on Earth. The principal point of these comparisons is to recognize
the uniqueness of the Earth as “an abode fitted for life”. In the final
chapter we take note of the existence of planets in orbit around other
stars in our galactic neighborhood. Most of the planets that have been
discovered so far are more massive than Jupiter and some of them orbit
close to their central star. Although none of the 160 extrasolar planets
known at the present time are likely to harbor life, we are beginning
to consider the possibility that alien organisms may occur on other
planets in the galaxy. We are at a critical juncture in the history of
humankind because our scouts are about to leave the “cradle of life”
to venture forth to other planets of the solar system and beyond.
The teaching aids used in this textbook include black-and-white
diagrams that illustrate and clarify the text. We also provide numerous
and carefully chosen color pictures of planetary surfaces and extraterrestrial
objects. All of the diagrams and color pictures have extensive
explanatory captions that complement the information provided in the
text. This textbook also includes end-of-chapter problems by means
of which students can test their understanding of the principles of
planetary science and can verify statements made in the text. In
addition, we provide subject and author indexes, and appendices
containing relevant formulae and lists of the properties of the planets
and their satellites. In conclusion, we reiterate our basic premise that
the Earth scientists in training today will benefit from a capstone course
that unifies what they have learned about the Earth and that broadens
their intellectual horizon to include the solar system in which we live.