||7.1 EARTH AS A PLANET
7.2 MERCURY AND THE MOON: GEOLOGICALLY
Why is Earth
Internal heat drives geological activity,
and Earth retains plenty of internal heat because of its relatively
large size for a terrestrial world. This heat causes mantle convection
and keeps Earth’s lithosphere thin, ensuring active surface geology. It
also keeps part of Earth’s core melted, and the circulation of this
molten metal creates Earth’s magnetic field.
- What processes shape Earth’s surface?
The four major
geological processes are impact cratering, volcanism, tectonics, and
erosion. Earth has experienced many impacts, but most craters have been
erased by other processes. We owe the existence of our atmosphere and
oceans to volcanic outgassing. A special brand of tectonics—plate
tectonics—shapes much of Earth’s surface. Ice, water, and wind drive
rampant erosion on our planet.
How does Earth’s
atmosphere affect the planet?
Two crucial effects are (1)
protecting the surface from dangerous solar radiation—ultraviolet is
absorbed by ozone and X rays are absorbed high in the atmosphere—and (2)
the greenhouse effect, without which the surface temperature would be
7.3 MARS: VICTIM OF PLANETARY FREEZE-DRYING
- Was there ever geological activity on the Moon or
Both the Moon and Mercury had some volcanism and
tectonics when they were young. However, because of their small sizes,
their interiors long ago cooled too much for ongoing geological
7.4 VENUS: A HOTHOUSE
features tell us that water once flowed on Mars?
channels, rock-strewn floodplains, and eroded craters all show that
water once flowed on Mars, though any periods of rainfall seem to have
ended at least 3 billion years ago. Mars today still has water ice
underground and in its polar caps, and could possibly have pockets of
underground liquid water.
- Why did Mars change?
Mars’s atmosphere must once have been
much thicker with a much stronger greenhouse effect, so change must have
occurred due to loss of atmospheric gas. Much of the lost gas probably
was stripped away by the solar wind, which was able to reach the
atmosphere as Mars cooled and lost its magnetic field and protective
magnetosphere. Water was probably also lost because ultraviolet light
could break apart water molecules in the atmosphere, and the lightweight
hydrogen then escaped to space.
7.5 EARTH AS A LIVING PLANET
- Is Venus geologically active?
Venus almost certainly
remains geologically active today. Its surface shows evidence of major
volcanic or tectonic activity in the past billion years, and it should
retain nearly as much internal heat as Earth. However, geological
activity on Venus differs from that on Earth in at least two key ways:
lack of erosion and lack of plate tectonics.
- Why is Venus so hot?
Venus’s extreme surface heat is a
result of its thick, carbon dioxide atmosphere, which creates a very
strong greenhouse effect. The reason Venus has such a thick atmosphere
is its distance from the Sun: It was too close to develop liquid oceans
like those on Earth, where most of the outgassed carbon dioxide
dissolved in water and became locked away in rock. Thus, the carbon
dioxide remained in the atmosphere, creating the strong greenhouse
- What unique features of Earth are important for
Unique features of Earth on which we depend for survival
are (1) surface liquid water, made possible by Earth’s moderate
temperature; (2) atmospheric oxygen, a product of photosynthetic life;
(3) plate tectonics, driven by internal heat; and (4) climate stability,
a result of the carbon dioxide cycle, which in turn requires plate
How might human
activity change our planet?
Ozone depletion can leave surface
life more vulnerable to dangerous solar ultraviolet radiation, and the
high rate of extinctions could have unknown consequences. The human
release of greenhouse gases into the atmosphere may already be causing
global warming and certainly would affect the climate if it continues.
- What makes a planet habitable?
We can trace Earth’s
habitability to its relatively large size and its distance from the Sun.
Its size keeps the internal heat that allowed volcanic outgassing to
lead to our oceans and atmosphere, and also drives the plate tectonics
that helps regulate our climate through the carbon dioxide cycle. Its
distance from the Sun is neither too close nor too far, thereby allowing
liquid water to exist on Earth’s surface.
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