428 PART 4^ |^ THE SOLAR SYSTEM
can emphasize some of those stages over others and produce
surprisingly diff erent worlds.
Exceptional Earth
Earth is a good standard for comparative planetology (Celestial
Profi le 2). Every major process on any rocky world in our solar
system is represented in some form on Earth. Nevertheless, Earth
is unusual, if not unique, among the planets in our solar system,
in two ways: the presence of abundant liquid water and the pres-
ence of life.
First, 75 percent of Earth’s surface is covered by liquid water.
No other planet in our solar system has liquid water on its sur-
face, although, as you will learn in later chapters, Mars had sur-
face water long ago, Venus probably did, and some moons in the
outer solar system show evidence of having liquid water under
their surfaces. Water fi lls Earth’s oceans, evaporates into the
atmosphere, forms clouds, and then falls as rain. Water falling on
the continents fl ows downhill to form rivers that fl ow back to the
sea and, in doing so, produces intense erosion. Entire mountain
ranges can literally dissolve and wash away in only a few tens of
millions of years, less than 1 percent of Earth’s total age. You will
not see such rapid erosion on most worlds. Liquid water is, in
fact, a rare material on most planets.
Your home planet is special in a second way. Some of the
matter on the surface of this world is alive, and a small part of
that living matter, including you, is aware. No one is sure how
the presence of living matter has aff ected the evolution of Earth,
but this process seems to be totally missing from other worlds in
our solar system. Furthermore, as you will learn later in this
chapter, the thinking part of the life on Earth, humankind, is
actively altering our planet.
SCIENTIFIC ARGUMENT
Why should you think Earth went through an early stage of
cratering?
When you build a scientifi c argument, take great care to distinguish
between theory and evidence. Recall from the previous chapter that
the planets formed by the accretion of planetesimals from the solar
nebula. The proto-Earth may have been molten as it formed, but
as soon as it grew cool enough to form a solid crust, the remain-
ing planetesimal impacts would have formed craters. So you can
reason from the solar nebula hypothesis that Earth should have
been cratered. But you can’t use a theory as evidence to support
some other theory. To fi nd real observational evidence, you need
only look at the moon. The moon has craters, and so does every
other old surface in our solar system. There must have been a time,
when the solar system was young, during which there were large
numbers of objects striking all the planets and moons and blasting
out craters. If it happened to other worlds in our solar system, it
must have happened to Earth, too.
The best evidence to support your argument would be lots of
craters on Earth, but, of course, there are few craters on Earth.
Extend your argument. How has the presence of liquid water on
Earth affected the appearance of Earth’s crust?Earth’s surface is marked by high continents and low seafl oors, but the crust is
only 10 to 60 km thick. Below that lie a deep mantle and an iron core. (NGDC)Celestial Profi le 2: Earth
Motion:
Average distance from the sun 1.00 AU (1.50 108 km)
Eccentricity of orbit 0.017
Maximum distance from the sun 1.017 AU (1.52 108 km)
Minimum distance from the sun 0.983 AU (1.47 108 km)
Inclination of orbit to ecliptic 0° (by defi nition)
Average orbital velocity 29.8 km/s
Orbital period 1.0000 y (365.25 days)
Period of rotation 24.00 h (with respect to the sun)
Period of rotation 23.93 h (with respect to the stars)
Inclination of equator to orbit 23.4°Characteristics:
Equatorial diameter 1.28 108 km
Mass 5.97 1024 kg
Average density 5.50 g/cm^3 (4.07 g/cm^3 uncompressed)
Surface gravity 1.00 Earth gravity
Escape velocity 11.2 km/s
Surface temperature 90° to 60°C (130° to 140°F)
Average albedo 0.39
Oblateness 0.0034Personality Point:
Earth comes, through Old English eorthe and Greek Eraze, from the
Hebrew erez, which means ground. Terra comes from the Roman goddess
of fertility and growth; thus, Terra Mater, Mother Earth.