440 PART 4^ | THE SOLAR SYSTEM
Summary
▶ Earth is the standard of comparative planetology (p. 425) in the
study of the Terrestrial planets because we know it best and because it
contains all of the phenomena found on the other Terrestrial planets.
▶ Our discussion of the Terrestrial planets considers Earth, the moon,
Mercury, Venus, and Mars. Earth’s moon is included because it is a com-
plex world and makes a striking comparison with Earth.
▶ The Terrestrial worlds differ mainly in size, but they all have low-
density crusts, mantles (p. 426) of dense rock, and metallic cores.
▶ Comparative planetology warns you to expect that cratered surfaces are
old, that heat fl owing out of a planet drives geological activity, and
that the nature of a planet’s atmosphere depends on the size of the
planet and its temperature.
▶ At some point early in its history Earth was hot enough to be completely
molten, which caused it to differentiate into layers of different density.
▶ Earth has passed through four stages as it has evolved: (1) differentia-
tion, (2) cratering, (3) fl ooding by lava and water, and (4) slow surface
evolution. The other Terrestrial planets and the moon also passed
through the same stages, which had different effects and durations
depending on the specifi c properties of each body.
▶ Earth is peculiar in that it has large amounts of liquid water on its
surface, and that water drives strong erosion that alters the surface
geology.
▶ Earth is also peculiar in that it is the only known home for life.
▶ Seismic waves (p. 429) generated by earthquakes can be detected
by seismographs (p. 429) all over the world and can reveal Earth’s
internal structure.
▶ Pressure (P) waves (p. 429) can travel through a liquid, but shear
(S) waves (p. 429) cannot. Observations show that S waves cannot
pass through Earth’s core, and that is evidence that the core is liquid.
Measurements of heat fl owing outward from the interior, combined with
mathematical models, reveal that the core is very hot and composed of
iron and nickel.
▶ Although Earth’s crust is brittle and breaks under stress, the mantle is
plastic (p. 431) and can deform and fl ow under pressure.
▶ Earth’s magnetic fi eld is generated by the dynamo effect in the liquid,
convecting, rotating, conducting core. The magnetic fi eld shields
Earth from the solar wind by producing a bow shock (p. 431) and a
magnetosphere (p. 431) around the planet. Radiation belts called the
Van Allen belts (p. 431), as well as aurora displays, are also produced
by the magnetic fi eld.
▶ (^) Earth is dominated by plate tectonics (p. 434), which breaks the
crust into moving sections. Plate tectonics is driven by heat fl owing
upward from the interior.
▶ (^) Tectonic plates are made of low-density, brittle rock that fl oats on the
hotter plastic upper layers of the mantle. Rift valleys (p. 434) can be
produced where plates begin pulling away from each other.
▶ (^) New crust is formed along midocean rises (p. 434) where molten rock
solidifi es to form basalt (p. 434). Crust is destroyed where it sinks
into the mantle along subduction zones (p. 434). Volcanism and
earthquakes are common along the edge of the plates.
▶ (^) The motion of a plate across a hot spot can produce a chain of volcanic
islands such as the Hawaiian island chain. Hot-spot volcanism is not
related to subduction zones.
▶ (^) The continents are drifting slowly on the plastic mantle, and their
arrangement changes with time. Where they collide, they can form
folded mountain ranges (p. 434).
▶ Most geological features on Earth, such as mountain ranges and the
Grand Canyon, have been formed recently. The fi rst billion years of
Earth’s geology are almost entirely erased by plate tectonics and
erosion.
▶ Because Earth formed hot, it never had a primeval atmosphere
(p. 433) rich in hydrogen and helium that was later replaced by a
secondary atmosphere (p. 433) baked out of the interior.
▶ Because Earth formed in a molten state, its fi rst atmosphere was prob-
ably mostly carbon dioxide, nitrogen, and water vapor. Most of the car-
bon dioxide eventually dissolved in seawater and was added to ocean
sediments. Plant life has added oxygen to the atmosphere.
▶ Ultraviolet photons can break up water molecules in a planet’s
atmosphere, but as soon as Earth had enough oxygen, an ozone layer
(p. 436) could form high in Earth’s atmosphere. The ozone absorbs
ultraviolet photons and protects water molecules.
▶ The albedo (p. 437) of a planet is the fraction of sunlight hitting it
that it refl ects into space. Small changes in the albedo of Earth caused
by changes in clouds and atmospheric currents can have a dramatic
effect on climate.
▶ The greenhouse effect (p. 437) can warm a planet if gases such as
carbon dioxide in the atmosphere are transparent to light but opaque
to infrared. The natural greenhouse effect warms Earth and makes it
comfortable for life, but greenhouse gases added by industrial civiliza-
tion are responsible for global warming (p. 437).
▶ Measurement of carbon isotope ratios and carbon dioxide versus oxygen
abundances make it clear that the CO 2 added to the atmosphere since
1800 is predominantly from burning of fossil fuels. Observations and
model calculations have eliminated other candidate causes for the current
warming such as natural climate cycles or variations in the sun’s output.
▶ (^) The ozone layer high in Earth’s atmosphere protects the surface from
ultraviolet radiation, but certain chemicals called chlorofl uorocarbons
released in industrial processes attack the ozone layer and thin it. This
is allowing more harmful ultraviolet radiation to reach Earth’s surface.
Review Questions
- Why would you include the moon in a comparison of the Terrestrial
planets? - In what ways is Earth peculiar among the Terrestrial planets?
- What are the four stages of planetary development?
- How do you know that Earth differentiated?
- What evidence can you cite that Earth’s metallic core is liquid?
- How are earthquakes in Hawaii different from those in Southern
California? - What characteristics must Earth’s core have in order to generate a
magnetic fi eld? - How do island chains located in the centers of tectonic plates such as
the Hawaiian islands help you understand plate tectonics? - What evidence can you cite that the Atlantic Ocean is growing wider?
- How is your concept of Earth’s fi rst atmosphere related to the speed
with which Earth formed from the solar nebula? - What has produced the oxygen in Earth’s atmosphere?
- How does the increasing abundance of CO 2 in Earth’s atmosphere cause
a rise in Earth’s temperature? - Why would a decrease in the density of the ozone layer cause public
health problems? - How Do We Know? How is deducing the structure of a virus like
fi nding the composition of Earth’s core?