Introduction - Introduction to Planetary Geomorphology - page 1

CHAPTER 1

Introduction

The early part of the twenty- rst century saw the comple-
tion of the reconnaissance of the Solar System by space-
craft. With the launch of the New Horizons spacecraft to
Pluto in early 2006 and its expected arrival in 2015, space-
craft will have been sent to every planet, major moon, and
representative asteroid and comet in our Solar System.
With the return of data taken by spacecraft of these objects,
the study of planetary surfaces passed mostly from the
astronomer to the geologist and led to the establishment
of the field of planetary geology. 1 The term geology is
used in the broadest sense to include the study of the solid
parts of planetary objects and includes aspects of geophys-
ics, geochemistry, and cartography. Much of our knowl-
edge of the geologic evolution of planetary surfaces is
derived from remote sensing, in situ surface measurements,
geophysical data, and the analysis of landforms, or their
geomorphology, the primary subject of this topic.
In this chapter, an overview of Solar System objects is
given, the objectives of Solar System exploration are out-
lined, and the strategy for exploration by spacecraft is
discussed. In the following chapters, the approach used
in understanding the geomorphology of planets is pre-
sented, including the types and attributes of various data
sets. The principal geologic processes operating on plan-
ets are then introduced, and the geology and geomorphol-
ogy of each planetary system is described in subsequent
chapters. The topic ends with a discussion of future mis-
sions and trends in Solar System exploration.

comets and asteroids, and tiny bits of dust. Most of the
mass of the Solar System is found within the Sun, a rather
ordinary star that generates energy through nuclear fusion
with the conversion of hydrogen to helium. Coupled with
astrophysical models, analyses of meteorites suggest that
the Solar System began to form at about 4.6 Ga (Ga is the
abbreviation for giga or 10 9 annum, or years).
Planets are relatively large objects that are in orbit
around the Sun. As we learned at a very young age, the
planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, and Neptune. And then there is Pluto! The year
2006 saw an interesting controversy emerge when the
International Astronomical Union (the scienti c group
responsible for formal naming of objects in the heavens)
declared that Pluto was no longer a “ planet ” and demoted
it to a new class of objects called “ dwarf planets. ” This
issue will be discussed later.
All of the planets originally formed through the accre-
tion of dust and smaller objects, making protoplanets.As
the protoplanets grew in size, still more dust and accreted
materials were swept up, a process that continues even
today. For example, it is estimated that more than 10,000
tons of materials are added to Earth each day. Although
this addition is impressive, it is insignificant in compar-
ison with the orders-of-magnitude larger rates of accretion
in the early stages of planetary formation. In the rst 0.5
Ga, so much material was amassed that the heat generated
by impacts probably melted the planets completely, lead-
ing to their differentiation, in which the heavier elements,
such as iron, sank to their interiors to form planetary cores
while the lighter elements floated toward the surface.

1.1 Solar System overview

Our Solar System consists of a fascinating array of
objects, including the Sun, planets and their satellites,

1.1.1 The terrestrial planets

Mercury, Venus, Earth, and Mars are called the terrestrial
planets because they share similar attributes to Earth
(which in Latin is terra). As shown in Fig. 1.1 , these
planets are small in comparison with the other planets

1 Terms when

ned. These terms are given in
the index, where the page number in bold indicates where the term is
dened.

first used are in bold and de

1