Chapter 7
Earth and the Terrestrial Worlds
Mercury
craters
smooth plains,
cliffs
Venus
volcanoes
few craters
Radar view of a twinpeaked volcano
Mars
some craters
volcanoes
riverbeds?
Moon
craters
smooth plains
Earth
volcanoes
craters
mountains
riverbeds
Why have the planets turned out
so differently, when they formed
at the same time from the same
materials?
7.1 Earth as a Planet
Our Goals for Learning
• Why is Earth geologically active?
• What processes shape Earth’s
surface?
• How does Earth’s atmosphere affect
the planet?
Why is Earth geologically active?
Why is Earth geologically active?
Short answer: the Earth is big enough to still have a
hot interior.
So what do we know about the interior of the Earth
and why is it hot?
Internal Structure, by density
Internal structure, by rock
strength
The “lithosphere” is the cool rigid rock that
forms a planet’s outer layer: the crust and
some of the mantle.
The lithosphere floats on the lower layers.
The thickness of the lithosphere
controls many geological processes
Internal Structure of the
Terrestrial Planets
Sources of Internal Heat
1) Gravitational
potential energy of
accreting
planetesimals
2) Differentiation
3) Radioactivity
Why do water and oil separate?
A. Water molecules repel oil molecules
electrically.
B. Water is denser than oil, so oil floats on
water.
C. Oil is more slippery than water, so it slides
to the surface of the water.
D. Oil molecules are bigger than the spaces
between water molecules.
Why do water and oil separate?
A. Water molecules repel oil molecules
electrically.
B. Water is denser than oil, so oil floats on
water.
C. Oil is more slippery than water, so it slides
to the surface of the water.
D. Oil molecules are bigger than the spaces
between water molecules.
Differentiation
• Layers ordered by density
• Highest density on the bottom
• Gravity sorts materials by density.
• Differentiation converts gravitational
potential energy to heat.
What cools off faster?
A. A grande-size cup of Starbucks coffee
B. A teaspoon of capuccino in the same cup.
What cools off faster?
A. A grande-size cup of Starbucks coffee
B. A teaspoon of capuccino in the same
cup.
What cools off faster?
A. A big terrestrial planet.
B. A tiny terrestrial planet.
What cools off faster?
A. A big terrestrial planet.
B. A tiny terrestrial planet.
Why?
What happens to Silly Putty if
you pull it VERY SLOWLY?
A. It stretches.
B. It breaks.
C. Nothing.
What happens to Silly Putty if
you pull it SHARPLY?
A. It stretches.
B. It breaks.
C. Nothing.
Do rocks s-t-r-e-t-c-h?
A. No - rock is rigid and cannot deform
without breaking.
B. Yes - but only if it is molten rock.
C. Yes - rock under strain may slowly
deform.
Do rocks s-t-r-e-t-c-h?
A. No - rock is rigid and cannot deform
without breaking.
B. Yes - but only if it is molten rock.
C. Yes - rock under strain may slowly
deform.
Heat Drives Geological Activity
Convection: hot rock
rises, cool rock falls.
1 cycle takes 100
million years on
Earth.
A large planet…
•
•
•
•
Is still warm inside
Has a convecting mantle
Has a thinner, weaker lithosphere
Has molten rock nearer the surface
which makes it more geologically active
Comparing the Planets
Which planets have the most and least
geological activity?
Planetary magnetic fields
Moving charged particles create magnetic fields.
So can a planet’s interior, if the core it electrically
conducting, convecting, and rotating
Earth’s Magnetosphere
Earth’s magnetic fields protects us from
charged particles from the Sun
The charged particles can create aurorae
(“Northern lights”)
If the planet core is cold, do you
expect it to have magnetic fields?
A. Yes, refrigerator magnets are cold, and
they have magnetic field.
B. No, planetary magnetic fields are
generated by moving charges around, and
if the core is cold, nothing is moving.
If the planet core is cold, do you
expect it to have magnetic fields?
A. Yes, refrigerator magnets are cold, and
they have magnetic field.
B. No, planetary magnetic fields are
generated by moving charges around,
and if the core is cold, nothing is
moving.
How do we know what is inside
the Earth?
A. We can drill deep inside the Earth.
B. We can use optical fibers to see deep
inside the Earth.
C. X-ray machines allow us to view the
inside of the Earth.
D. Seismic waves generated by earthquakes
probe the Earth’s interior.
How do we know what is inside
the Earth?
A. We can drill deep inside the Earth.
B. We can use optical fibers to see deep
inside the Earth.
C. X-ray machines allow us to view the
inside of the Earth.
D. Seismic waves generated by
earthquakes probe the Earth’s interior.
What processes shape Earth’s
surface?
What processes shape Earth’s
surface?
1)
2)
3)
4)
Impact cratering
Volcanism
Tectonics
Erosion
Impact Cratering
Impact Cratering
• Moon must be hit as
often as Earth.
• Where are Earth’s
craters?
• Erased by volcanic
activity and erosion.
The more craters, the
older the surface
Volcanism
Molten rock rises when
it is:
• Less dense than its
surroundings.
• Squeezed by its
surroundings.
• Pushed by expanding
trapped gas (water
vapor, CO2, N2, H2S,
SO2)
Volcanism
• Erases other geological features
• Provided gas for our atmosphere
• Provided water for our oceans
Why doesn’t Mars have as much
volcanic activity as Earth?
A. It’s too far from the Sun, so it cooled off
faster.
B. It’s smaller than the Earth, so it cooled off
faster.
C. It might, we just haven’t seen them erupt
yet.
Why doesn’t Mars have as much
volcanic activity as Earth?
A. It’s too far from the Sun, so it cooled off
faster.
B. It’s smaller than the Earth, so it cooled
off faster.
C. It might, we just haven’t seen them erupt
yet.
Tectonics and Plate Tectonics
Tectonics: any surface reshaping from forces
on the lithosphere
Internal Heat Required!
Plate tectonics: pieces of lithosphere moving
around
Only Earth has plate tectonics.
Erosion
• Wearing down or building up of geological features by
wind, water and ice (weather)
• Important on Earth - why?
How does Earth’s atmosphere
affect the planet ?
How does Earth’s atmosphere
affect Earth?
1) Erosion (already mentioned)
2) Protection from radiation
3) Changes the surface temperature:
greenhouse effect
4) Makes the sky blue!
Radiation Protection
• All X-ray light
absorbed very high in
the atmosphere.
• Ultraviolet light
absorbed by ozone
(O3)
The Greenhouse Effect
How does the greenhouse effect
alter surface temperature?
Earth’s atmosphere absorbs light at most wavelengths.
Greenhouse
effect:
Certain
molecules let
sunlight through
but trap escaping
infrared photons
(H2O, CO2, CH4)
A Greenhouse Gas
• Any gas that absorbs infrared
• Greenhouse gas: molecules with 2 different types
of elements (CO2, H2O, CH4)
• Not a greenhouse gas: molecules with single or 2
atoms of the same element (O2, N2)
Greenhouse Effect: Bad?
The Earth is much warmer because of the
greenhouse effect than it would be without
an atmosphere…but so is Venus.
What have we learned?
• Why is Earth geologically
active?
• 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 have we learned?
• 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.
What have we learned?
• 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 below freezing.
7.2 Mercury and the Moon:
Geologically Dead
• Our Goals for Learning
• Was there ever geological activity on the
Moon or Mercury?
Was there ever geological activity
on the Moon or Mercury?
Moon
• Some volcanic activity 3 billion years ago must have
flooded lunar craters, creating lunar maria.
• The Moon is now geologically dead.
Mercury
• Plenty of craters - including a huge ‘basin’
• Smooth plains from volcanism (recent or long
ago???)
Did Mercury shrink?
Steep long cliffs formed when the core
cooled, shrinking the planet by ~20 km.
Mercury is now geologically dead.
What have we learned?
• Was there ever geological activity on
the Moon or Mercury?
• 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
activity.
7.3 Mars: A Victim of Planetary
Freeze-drying
• Our Goals for Learning
• What geological features tell us that water
once flowed on Mars?
• Why did Mars change?
Mars vs. Earth
•
•
•
•
•
50% Earth’s radius, 10% Earth’s mass
1.5 A.U from the Sun
Axis tilt about the same as Earth.
Similar rotation period.
Orbit is more elliptical than Earth’s: seasons
more extreme in the south than the north.
• Thin CO2 atmosphere: little greenhouse
Main Difference seems to be:
Mars is SMALLER!
What geological features tell us
water once flowed on Mars?
Surface of Mars appears to have ancient river beds
Eroded
crater
Condition of craters indicates surface history
Closeup of eroded crater
Volcanoes…as recent as 180 million years ago…
Past tectonic activity…
Low-lying regions may once have had oceans
Low-lying regions may once have had oceans
Opportunity
Spirit
• 2004 Opportunity Rover provided strong evidence for abundant
liquid water on Mars in the distant past.
•How could Mars have been warmer and wetter in the past?
Today, most water
lies frozen
underground (blue
regions)…
Some scientists
believe accumulated
snowpack melts to
carve gullies even
today
Why did Mars change?
Would “terraforming” Mars
work?
• Yes
• No
What have we learned?
• What geological features tell
us that water once flowed on
Mars?
• Dry river channels, rockstrewn 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.
What have we learned?
• 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.
7.4 Venus: A Hothouse World
• Our Goals for Learning
• Is Venus geologically active?
• Why is Venus so hot?
Is Venus geologically active?
Is Venus geologically active?
Radar images show lots of volcanic &
tectonic features, as expected for a
large terrestrial planet
Why is Venus so hot?
Greenhouse effect.
But why is it so prominent on Venus?
Thick CO2 atmosphere of Venus locks heat in
Where is Earth’s
CO2?
Where is Earth’s CO2?
Rocks - like limestone
(and some in plant life and
in the ocean)
Why did this happen on
Earth and not on Venus?
Venus lacks oceans to
dissolve the carbon
dioxide and lock it away in
rock on the seafloor
And why is that?
We can understand Venus’ history by
thinking about what would happen if
Earth were moved to Venus’ orbit.
How would the initial heating affect
the oceans and the greenhouse effect?
A runaway greenhouse effect would then occur.
Eventually, water molecules would break down & escape to space,
just as apparently happened on Venus
What have we learned?
• 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
7.5 Earth as a Living Planet
• Our Goals for Learning
• What unique features on Earth are
important for human life?
• How might human activity change our
planet?
• What makes a planet habitable?
What unique features of Earth are
important for life?
1)
2)
3)
4)
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability
What unique features of Earth are
important to human life?
1)
2)
3)
4)
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability
Earth’s distance from the
Sun and moderate
greenhouse effect make
liquid water possible
What unique features of Earth are
important to human life?
1)
2)
3)
4)
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability
PHOTOSYNTHESIS
(plant life) is required to
make high concentrations
of O2, which produces the
protective layer of O3.
What unique features of Earth are
important to human life?
1)
2)
3)
4)
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability
Plate tectonics are
an important step
in the carbon
dioxide cycle.
The Carbon Dioxide Cycle
What unique features of Earth are
important to human life?
1)
2)
3)
4)
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability
The CO2 cycle acts like a
thermostat for the Earth’s
temperature.
These unique features are intertwined:
• plate tectonics creates climate stability
• climate stability allows liquid water
• liquid water is necessary for life
• life is necessary for atmospheric oxygen
How many other connections between these
can you think of?
How might human activity affect
Earth’s climate?
Earth’s ice ages end
as oceans freeze over
and volcanoes release
CO2 into the
atmosphere
Human activity is increasing the concentration of
greenhouse gases in the atmosphere, which may
strengthen the greenhouse effect and lead to global
warming.
What makes a planet habitable?
• Located at an optimal distance from the Sun
for liquid water to exist.
What makes a planet habitable?
• Large enough for geological activity to
release & retain water and atmosphere.
Planetary Destiny
Earth is habitable
because it is large
enough to remain
geologically active
and at the right
distance from the
Sun so oceans could
form.
What have we learned?
• What unique features of Earth are important
for life?
• 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
tectonics.
What have we learned?
• 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
What have we learned?
• 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 to 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.