Introduction - Introduction to Planetary Geomorphology - page 2

Figure 1.1. A family portrait of the
planets imaged by spacecraft. The
inner planets (Mercury, Venus, Earth
and Moon, and Mars) are shown to
scale with each other and are enlarged
relative to the giant planets (Jupiter,
Saturn, Uranus, and Neptune), which
are shown to scale with each other.
Earth is somewhat smaller than the
Giant Red Spot (indicated by the
arrow) of Jupiter (NASA PIA01341).

and are found closest to the Sun, leading to their alter-
native description as the inner planets. They are com-
posed primarily of rocky material and have solid surfaces.
In planetary geology, Earth

magnesium; they include olivine, pyroxene, hornblende,
and biotite mica. Silicate minerals are the basic building
blocks of most rocks in the crusts of Earth and the Moon,
and they are thought to make up most of the rocks on
Mercury, Venus, and Mars.
Venus, Earth, andMars all have signi cant atmospheres
composed of gasses that are gravitationally bound to the
planets ( Table 1.1 ). Mercury and the Moon are too small
to retain anything but the most tenuous atmospheres,
measurable only by very sensitive instruments. Although
some gasses were accumulated by all of the terrestrial
protoplanets during their initial formation, these primary
atmospheres were lost to space. Secondary atmo-
spheres were later released as gasses escaped from the
interior and interacted with the surface. Earth ' s atmo-
sphere may be termed a tertiary atmosphere because it
has been greatly modi ed by biologic processes.

is Moon is typically included
with the terrestrial planets because of its large size and
similar characteristics.
As the terrestrial planets began to cool and form crusts,
elements combined and crystallized into rocks and min-
erals. For the most part, these elements are silicon, oxy-
gen, iron, magnesium, sodium, calcium, potassium, and
aluminum in various combinations that collectively make
up the silicate minerals. The most important silicate
minerals fall into two groups. Light-colored silicate min-
erals are common in continental rocks on Earth and
include quartz, orthoclase feldspar, plagioclase feldspar,
and muscovite mica. Dark-colored silicate minerals are
common on Earth ' s sea floor and are rich in iron and

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