Chapter 9 - Planetary Geology
of Earth and the other Terrestrial Worlds
9.1 Planetary Interiors and Surfaces
• Learning Goals:
– Common processes on “solid” worlds
• How do these things work?
When you visit a planet, it ceases to be an astronomical object…
– What are terrestrial planets like on the
inside?
– What is geological activity and what
causes it?
– Why do some planetary interiors create
magnetic fields?
Common Processes
Sharing a Family History of:
• Terrestrial planets all have solid surfaces
– Dominated by rocky materials
 Rocks are naturally occurring aggregates of minerals
 Minerals are naturally occurring homogeneous solids with a
definite chemical composition and ordered atomic arrangement
 All metals
t l andd ices
i
are minerals
i
l
• Solid surfaces can break, stretch, compress, or melt.
• Stable for long periods of time (millions to billions
of years) → Preserves ancient geologic history!
• High melting points → Silicate volcanism
Different surface expressions
Don’t get distracted by the surface differences!
Most of that is due to atmospheres (or not)
They’re almost identical on the inside!
•
•
•
•
Impact Cratering
Volcanism
Faulting
Resurfacing
And a common genetic history…
• Applying what we have learned about Earth’s
interior to other planets tells us what their interiors
are probably like
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A word about…
Planetary Interiors
Gravitational
Interactions tell
us a great deal
about how a body
is put together.
Surface Crust
Plastic Mantle
Solid/Liquid Core
Q: How do we know?
A: Seismic Waves
• Vibrations that
travel through
Earth’s interior
tell us what
Earth is like on
the inside
• Ditto for
moonquakes and
marsquakes
Special Topic:
How do we know what’s inside a planet?
Interiors of the Big 4
• Mercury is densest and
largest core relative to
planet size
• Venus has relatively
y
thick crust
• Earth has relatively
thin crust
• Mars has extremely
thick crust and very
large core.
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
Scientific Visualization
• 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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Earth’s Interior
• Core: Highest
density; nickel
and iron
• Mantle: Moderate
density; silicon,
oxygen, etc.
• Crust: Lowest
density; granite,
basalt, etc.
Lithosphere
• A planet’s outer
layer of cool, rigid
rock is called the
lithosphere
• It “floats” on the
warmer, softer
rock that lies
beneath
High Density Interiors
• Hot accretion
• Differentiation
when planets
were young
• Radioactive
decay is most
important heat
source today
Differentiation
• Gravity attracts high
mass (high-density)
material to center
• Lower-density
material is displaced
p
to surface
– Atmospheric gases
– Silicate rocks
– Nickel/Iron core
• Material ends up
segregated by
density
Strength of Rock
• Rock stretches when
pulled slowly but
breaks when pulled
rapidly
• The gravity of a large
world pulls slowly on
its rocky content,
shaping the world into a
sphere
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
3
Role of Size
Surface Area to Volume Ratio
• Heat content depends on volume
• Loss of heat through radiation depends on surface
area
• Time to cool depends on surface area divided by
volume
surface area to volume ratio =
• Smaller worlds cool off faster and harden earlier
• Moon and Mercury are now geologically “dead”
• Fully cooled planets dubbed “rocks”
Magnetic Fields
4r 2 3

4 3 r
r
3
• Larger objects have smaller ratio and cool more
slowly
Sources of Magnetic Fields
• Motions of
charged particles
creates magnetic
i
fields
Sources of Magnetic Fields
So What?
• A world can have
a magnetic field if
charged particles
are moving inside
• 3 requirements:
– Molten interior
– Convection
– Moderately rapid
rotation
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9.2 Shaping Planetary Surfaces
Why the Different Surfaces?
• Learning Goals:
– What processes shape planetary surfaces?
– Why do the terrestrial planets have different
geological histories?
– How does a planet’s surface reveal its
geological age?
Surface Expressions
• History of interior processes
– Still molten → Still active volcanism
– Lava resurfacing
• Atmospheres
– Outgassing
– Retention of volatiles
– Erosion
• Cratering History
• Tectonics
Impact Craters
Impact Cratering
• Most cratering
happened soon after
solar system formed
– Early
a y heavy
eavy
bombardment
• Craters are about 10
times wider than object
that made them
• Small craters greatly
outnumber large ones
Volcanism
• Volcanism happens
when molten material
(magma) finds a path
thro gh lithosphere to
through
the surface
• Molten material is called
lava after it reaches the
surface
• Can be any composition
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Volcanic Edifices
Outgassing
• Volcanism releases gases from
planetary interior into
atmosphere
t
h
Runny lava makes flat
lava plains
Slightly thicker lava
makes broad shield
volcanoes
Thickest lava makes
steep stratovolcanoes
Tectonics
Plate Tectonics on Earth
• Convection of the mantle creates stresses in the crust
called tectonic forces
• Compression forces make mountain ranges
• Extension forces (pulling apart) create valleys
Erosion
Fluvial Erosion
• Generic term for processes that break down or
transport rock
• Erosion Processes include
–
–
–
–
–
Rain & Rivers
Wind
Wave action
Micrometeorite gardening
Viscous Relaxation (including glaciers)
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Glacial Erosion
Aeolian Erosion
Micrometeorite Erosion
Clues to Erosion
Gardening
• Erosion followed by
deposition shows up in
bedded rock deposits
Only effective with thin atmospheres
Different Surfaces indicate
different histories
Size Matters
• Smaller worlds cool off faster and crystallize earlier
• Larger worlds remain warm inside, promoting
volcanism and tectonics
• Larger worlds also have more erosion because their
gravity retains an atmosphere
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Distance from Sun
• Planets close to Sun are too hot for rain, snow, ice
and so have less erosion
– More difficult for hot planet to retain atmosphere
• Planets with liquid water have most erosion
• Planets far from Sun are too cold for liquids,
limiting erosion
How old are the surfaces?
We have a Rosetta Stone…
History of Cratering
Role of Rotation
• Planets with slower rotation have less
weather and less erosion
• Planets with faster rotation have more
weather and more erosion
Cratering of Moon
• Some areas of
Moon are more
heavily cratered
than others
• Younger
regions were
flooded by lava
after Early
Heavy
Bombardment
Calibrating the Cratering Curve
• Most cratering
happened in first
billion years
• More Craters =
Older Surface
• Heavilty-cratered
surfaces have not
changed much in
last 3-4 billion years
• Distinction: Surface
age versus rock age?
8
Cratering Curves
• Calibrated curves from the Moon are the
basis of all inner solar system surface ages
– Why not for the outer solar system?
 Different
flux rates
 Viscous relaxation much
more common
• Age of surface is not the same thing as age
of the rock
9.3 Geology of the Moon and Mercury
• Learning Goals
– What geological processes shaped our
Moon?
– What geological processes shaped
Mercury?
– What happens when a big impact wipes out the
existing surface?
Both are Geologically Dead
Lunar Geology
• Geologically “dead”
because major
geological processes
have virtually stopped:
– No plate tectonics
– No atmospheric erosion
– No flowing surface
water
– Interiors frozen solid?
Lunar Interior
far
near
far
• Giant impact theory says Moon is mostly formed from Earth’s
mantle material
• Initially molten
• Strong gravity from Earth pulled fluid mass to near side
• Dominance of maria on near side
• Oldest units are lunar highlands (similar to
Earth mantle)
• Intermediate age are lunar maria made by
floods of runny lava from late major impacts
• Youngest are smaller bright impacts (e.g.
Tycho)
Lunar Maria
• Smooth, dark
lunar maria are
less heavily
cratered
t d than
th
lunar highlands
– younger
• Maria were made
by flood of runny
lava
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Formation of Lunar Maria
Early surface
covered with
craters
Large impact
crater
weakens
crust
Heat buildup allows
lava to well
up to surface
Tectonic Features
• Formed after
lava floods
• Wrinkles arise
from cooling
and contraction
of lava flood
Cooled lava
is smoother
and darker
than
surroundings
Geology of Mercury
Cratering on Mercury
• A mixture of heavily cratered and smooth regions
like the Moon
• Smooth regions are likely ancient lava flows
Caloris Basin
Caloris basin is
largest impact crater
on Mercury
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Tectonics on Mercury
Almost a planet killer
The Caloris impact blew off the antipodes: Chaotic Terrain
• Long cliffs indicate that Mercury shrank early in its
history
The Myth of Mars
9.4 Geology of Mars
• Learning Goals:
– The Myth of Mars
– Major
j geological
l i l
features of Mars?
– Geological evidence of
water on Mars?
• Percival Lowell misinterpreted surface
features seen in telescopic images of Mars
The Myth of Mars
• Original
observation of
“caneli” in Italian
(channels)
• Known similarity
of seasons and
length of day
The Myth of Mars
original
g
recent high resolution image
deliberate fakery
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Mars Environment
Major Geological Features
• Extremely thin (6 mbar) atmosphere
• Very cold – Highs of 40°F at Noon on Equator
Cratering & the Crustal Dichotomy
Volcanism on Mars
• Many large shield
volcanoes
• Olympus Mons is
largest volcano in
solar system
• Amount of cratering differs greatly across surface
• Many early craters have been erased
Tectonics on Mars
Evidence of Water
• Orbital
images show
many
examples of
what appear
to be dried-up
riverbeds
• System of valleys known as Valles Marineris
thought to originate from tectonics
12
Shorelines?
Evidence of Water
• Much evidence
of a planetwide
permafrost layer
• Enough to cover
all of Mars to
depth of 20m
Where did all the water go?
Erosion of Craters
• Details of some
craters suggest
they were once
filled with
water
• Stratified
sedimentary
deposits
Martian Rocks
• Exploration of impact craters has revealed that
Mars’ deeper layers were affected by water
The Record in the Rocks
• Mars rovers have found rocks that appear to have
formed in water
Hydrogen Content
• Map of hydrogen content (blue) shows that lowlying areas contain more water ice
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Present Day Water Flow
Term Exam #3
• 35 objective questions at 2 points each
• Covering Chapters 7, 8, 9, and 10
• 3 essay/short answer questions @ 10 points each.
each
– Select 3 from 5
• Open note and open book, but…
– Never memorize anything you can look
up…except where to look it up.
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