This document discusses the geological features and processes that shape the surfaces of terrestrial planets like Mercury, Venus, Earth and Mars. It explains that while all formed from the same materials, their sizes and distances from the Sun led to different levels of geological activity over time. Smaller planets like Mercury and the Moon cooled faster and are now geologically dead, while others like Earth and Venus have evidence of ongoing volcanism, tectonics and erosion continuing to alter their surfaces.

1. © 2007 Pearson Education Inc., publishing as Pearson Addison-Wesley
Today
• Terrestrial Planets
– Earth/moon, Venus, Mars, Mercury
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Homework due now
Exam next Thursday

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Exam Review Session
• This Monday, 6-8 PM, this room
• Completely driven by your questions! The
TAs will not prepare summary slides, but
can go to the lecture slides if needed.
• When your questions are done, the review is
over
• Don’t ask them what will be on the exam;
they don’t know :)
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Earth and the Terrestrial Worlds
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Mercury
craters
smooth plains
cliffs
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Venus
volcanoes
few craters
Radar view of a twin-
peaked volcano
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Mars
some craters
volcanoes
riverbeds?
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Moon
craters
smooth plains
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Earth
volcanoes
craters
mountains
riverbeds
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Why have the planets turned out
so differently, even though they
formed at the same time from the
same materials?
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10. Terrestrial Planets
A.Distance from the Sun
B.Strength of their magnetic field
C. Size/mass of the planets
D.Presence or absence of a large moon
E. I don’t know
The terrestrial planets (including our Moon) have
a wide variety of appearances. What is the main cause
of their geological differences (as opposed to the presence
or absence of life)?
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Earth’s Interior
• Core: Highest
density; nickel
and iron
• Mantle: Moderate
density; silicon,
oxygen, etc.
• Crust: Lowest
density; granite,
basalt, etc.
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Differentiation
• Gravity pulls
high-density
material to center
• Lower-density
material rises to
surface
• Material ends up
separated by
density
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Terrestrial Planet Interiors
• Applying what we have learned about Earth’s
interior to other planets tells us what their interiors
are probably like.
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Heat Drives Geological Activity
Convection: hot rock
rises, cool rock falls.
One convection cycle
takes 100 million
years on Earth.
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Sources of Internal Heat
1. Gravitational
potential energy of
accreting
planetesimals
2. Differentiation
3. Radioactivity
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Heating of Interior over Time
• Accretion and
differentiation
when planets
were young
• Radioactive
decay is most
important heat
source today
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Cooling of Interior
• Convection
transports heat as
hot material rises
and cool material
falls
• Conduction
transfers heat
from hot material
to cool material
• Radiation sends
energy into space
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Role of Size
• Smaller worlds cool off faster and harden earlier.
• Moon and Mercury are now geologically “dead.”
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Earth’s Magnetosphere
Earth’s magnetic fields protects us from
charged particles from the Sun.
The charged particles can create aurorae
(“Northern lights”).
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Special Topic:
How do we know what’s inside a planet?
• P waves push
matter back
and forth.
• S waves
shake matter
side to side.
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Special Topic:
How do we know what’s inside a planet?
• P waves go
through Earth’s
core, but S waves
do not.
• We conclude that
Earth’s core must
have a liquid outer
layer.
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What processes shape a planet’s
surface?
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Geological Processes
• Impact cratering
— Impacts by asteroids or comets
• Volcanism
— Eruption of molten rock onto surface
• Tectonics
— Disruption of a planet’s surface by internal
stresses
• Erosion
— Surface changes made by wind, water, or ice
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Impact Cratering
• Most cratering happened
soon after the solar system
formed.
• Craters are about 10 times
wider than objects that
made them.
• Small craters greatly
outnumber large ones.
• A heavily cratered surface
is normal - a planet needs
active geology or erosion
to erase craters 24

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Impact Craters
Meteor Crater (Arizona) Tycho (Moon)
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Volcanism
• Volcanism happens
when molten rock
(magma) finds a path
through lithosphere to
the surface.
• Molten rock is called
lava after it reaches the
surface.
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Lava and Volcanoes
Runny lava makes flat
lava plains.
Slightly thicker lava
makes broad shield
volcanoes.
Thickest lava makes
steep stratovolcanoes.
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Outgassing
• Volcanism also releases gases from Earth’s interior
into the atmosphere.
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Tectonics
• Convection of the mantle creates stresses in the crust called
tectonic forces.
• Compression forces make mountain ranges.
• A valley can form where the crust is pulled apart.
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Plate Tectonics
• Motion of continents can be measured with
GPS
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Continental Motion
• Idea of
continental drift
was inspired by
puzzle-like fit of
continents
• Mantle material
erupts where
seafloor spreads
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Plate Motions
• Measurements of
plate motions tell
us past and
future layout of
continents
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Erosion
• Erosion is a blanket term for weather-driven
processes that break down or transport rock.
• Processes that cause erosion include
— Glaciers
— Rivers
— Wind
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Erosion by Water
• The Colorado
River continues
to carve the
Grand Canyon.
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Erosion by Ice
• Glaciers carved
the Yosemite
Valley.
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Erosion by Wind
• Wind wears
away rock and
builds up sand
dunes.
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Was there ever geological activity
on the Moon or Mercury?
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Moon
• Some volcanic activity 3 billion years ago must have
flooded lunar craters, creating lunar maria.
• The Moon is now geologically dead.
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Cratering of Mercury
• Mercury has a mixture of heavily cratered and
smooth regions like the Moon.
• The smooth regions are likely ancient lava flows.
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Mars versus Earth
• 50% Earth’s radius, 10% Earth’s mass
• 1.5 AU from the Sun
• Axis tilt about the same as Earth
• Similar rotation period
• Thin CO2 atmosphere: little greenhouse
• Main difference: Mars is SMALLER
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The surface of Mars appears to have ancient riverbeds.
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The condition of craters indicates surface history.
Eroded
crater
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Past tectonic activity… 43

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• 2004 Opportunity Rover provided strong evidence for abundant
liquid water on Mars in the distant past.
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Is Venus geologically active?
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Cratering on Venus
• Impact craters, but fewer
than Moon, Mercury,
Mars
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Volcanoes on Venus
• Many volcanoes,
including both shield
volcanoes and
stratovolcanoes
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Tectonics on Venus
• Fractured and
contorted surface
indicates tectonic
stresses
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Erosion on Venus
• Photos of rocks
taken by lander
show little
erosion
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Does Venus have plate tectonics?
• Most of Earth’s major geological features
can be attributed to plate tectonics, which
gradually remakes Earth’s surface.
• Venus does not appear to have plate
tectonics, but its entire surface seems to
have been “repaved” 750 million years
ago.
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51. © 2007 Pearson Education Inc., publishing as Pearson Addison-Wesley
Terrestrial planet surfaces
• Craters are the normal state
– need geological activity to erase
• e.g.,
– Volcanism
– Erosion
– Plate tectonics
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