Download Lecture 13

Comparing the Terrestrial Worlds
Reading: Chapter 13, Celestial profile cards (Appendix C)
Comparing the Terrestrial Worlds
Terrestrial worlds have similar origins but have evolved
very differently.
Why are there no Martians, Venusians, Mercurians or
Lunatics ?
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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.
•  A planet’s outer layer
of cool, rigid rock is
called the lithosphere.
It “floats” on the
warmer, softer rock that
lies beneath
Seismic Waves
How do we know what’s inside a
planet?
•  Vibrations
that travel
through
Earth’s
interior tell us
what Earth is
like on the
inside
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Seismic Waves
How do we know what’s inside a
planet?
•  P waves push matter
back and forth
•  S waves shake matter
side to side
•  P waves go through
Earth’s core but S
waves do not
•  We conclude that
Earth’s core must
have a liquid outer
layer
Reason for layers:
Differentiation
•  Gravity pulls
high-density
material to center
•  Lower-density
material rises to
surface
•  Material ends up
separated by
density
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Thought Question
What is necessary for differentiation to occur in a
planet?
a) It must have metal and rock in it
b) It must be a mix of materials of different density
c) Material inside must be able to flow
d) All of the above
e) b and c
Heating of Interior
•  Accretion and
differentiation
when planets
were young
•  Radioactive
decay is most
important heat
source today
•  Interior heat
drives geological
activity
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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
Terrestrial Planet Interiors
•  Applying what we have learned about Earth’s
interior to other planets tells us what their interiors
are probably like
•  The terrestrial worlds are made up of rock and
metal.
•  They are all differentiated:
–  Rocky, low-density crusts
–  High-density metal cores
–  Mantles composed of dense rock between
the cores and crusts
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Role of Size in Heating and Cooling
•  Smaller worlds cool off faster and harden earlier
•  Mercury and the Moon are now geologically “dead” since
their interiors have cooled down.
Sources of Magnetic Fields
•  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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Processes that Shape Planetary Surfaces
•  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
Impact Cratering
•  Most cratering happened
soon after solar system
formed
•  All planets were equally
impacted
•  Craters are about 10 times
wider than object that
made them
•  Small craters greatly
outnumber large ones
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Impact Craters
Meteor Crater (Arizona)
Standard crater: Mars
Tycho (Moon)
History of Cratering
•  Most cratering
happened in first
billion years
•  A surface with
many craters has not
changed much in 3
billion years
•  Erosion can erase
craters. Thus fewer
craters indicate
geological activity
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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
•  Volcanism also releases gases
from interior into atmosphere:
Outgassing
Volcano on Io
Iceland
Tectonics
•  Convection of the mantle creates stresses in the crust
called tectonic forces
•  Compression forces make mountain ranges
•  Valley can form where crust is pulled apart
•  Earth’s continents slide around on separate plates of
crust: plate tectonics
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Erosion
•  Weather-driven processes that break
down or transport rock
•  Processes that cause erosion include
glaciers, rivers, wind
•  Erosion not only wears down
features, it also builds them: sand
dunes, river deltas sedimentary
rock
Sand blown by wind
canyons carved by rivers
valleys shaped by glaciers
Comparison of Planetary Surfaces
•  Mercury & the Moon
•  heavily cratered {scars from the heavy bombardment}
•  some volcanic plains
•  Venus
•  volcanoes and bizarre bulges
•  Mars
•  volcanoes and canyons
•  apparently dry riverbeds {evidence for running
water?}
•  Earth
•  all of the above plus liquid water and life
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Geology of Earth
•  most active geology
•  volcanoes & tectonics
•  ongoing plate tectonics
•  moderate atmosphere
•  N2 O2 H2O
•  H2O exists in liquid state
•  rampant erosion
•  few craters
•  life
Continental Motion
•  Earth’s crust is divided into
moving sections called
plates
•  Idea of continental drift was
inspired by puzzle-like fit
of continental plates
•  Mantle material erupts
where seafloor spreads
•  Motion of continents can be
measured with GPS
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Surface Features
Earthquakes, volcanism, and mountain
building are linked to motions of the crust and
the location of plate boundaries.
Surface Features
•  Himalayas are forming
from a collision between
plates
•  Red Sea is forming
where plates are
pulling apart
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Rifts, Faults, Earthquakes
•  San Andreas
fault in
California is a
plate boundary
•  Motion of plates
causes
earthquakes
Hot Spots
•  Hawaiian islands have formed where plate is
moving over volcanic hot spot
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Earth’s Atmosphere
•  Consists mostly of
molecular nitrogen
(N2) and oxygen
(O2)
•  Most of Earth’s gas
is < 10 km from
surface, but a small
fraction extends to
>100 km
Thought Question
Why is the sky blue?
a) The sky reflects light from the oceans.
b) Oxygen atoms are blue.
c) Nitrogen atoms are blue.
d) Air molecules scatter blue light more than red light.
e) Air molecules absorb red light.
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Thought Question
Why is the sky blue?
a) The sky reflects light from the oceans.
b) Oxygen atoms are blue.
c) Nitrogen atoms are blue.
d) Air molecules scatter blue light more than red light.
e) Air molecules absorb red light.
Why the sky is blue
•  Atmosphere scatters
blue light from Sun,
making it appear to
come from different
directions
•  Sunsets are red because
red light scatters less
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Magnetospheres
•  The Sun ejects a stream of charged particles, called the
solar wind.
•  it is mostly electrons, protons, and Helium nuclei
•  Earth’s magnetic field attracts and diverts these
charged particles to its magnetic poles. This protective
“bubble” is called the magnetosphere
•  the particles spiral along magnetic field lines and
emit light
•  this causes the aurora (aka northern & southern
lights)
•  Other terrestrial worlds have no strong magnetic fields
Earth’s Magnetosphere
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Greenhouse Effect
•  Visible Sunlight passes
through a planet’s
atmosphere.
•  Some of this light is
absorbed by the planet’s
surface.
•  Planet re-emits this energy
(heat) as infrared (IR) light.
•  IR light is “trapped” by the
atmosphere. its return to
space is slowed
•  The overall surface
temperature is thus higher
than if there were no
atmosphere at all.
Greenhouse Gases
•  Key to Greenhouse Effect…
gases which absorb IR light
effectively:
•  water [H2O]
•  carbon dioxide [CO2]
•  methane [CH4]
•  These are molecules which
rotate and vibrate easily.
•  they re-emit IR light in a
random direction
•  The more greenhouse gases
which are present, the
greater the amount of
surface warming.
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Earth’s Water and CO2
•  Earth’s temperature
remained cool
enough for liquid
oceans to form
•  Oceans dissolve
atmospheric CO2,
enabling carbon to
be trapped in rocks
Nitrogen and Oxygen
•  Most of Earth’s
carbon and oxygen
is in rocks, leaving a
mostly nitrogen
atmosphere
•  Plants release some
oxygen from CO2
into atmosphere
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Ozone and the Stratosphere
•  Ultraviolet light can
break up O2
molecules, allowing
ozone (O3) to form
•  Without plants to
release O2, there
would be no ozone
in stratosphere to
absorb UV light
Carbon Dioxide Cycle
1. 
2. 
3. 
4. 
5. 
Atmospheric CO2 dissolves
in rainwater
Rain erodes minerals which
flow into ocean
Minerals combine with
carbon to make rocks on
ocean floor
Subduction carries
carbonate rocks down into
mantle
Rock melt in mantle and
outgas CO2 back into
atmosphere through
volcanoes
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Earth’s Thermostat
•  Cooling allows CO2 to build up in atmosphere
•  Heating causes rain to reduce CO2 in atmosphere
Dangers of Human Activity
•  Human-made
CFCs in
atmosphere destroy
ozone, reducing
protection from
UV radiation
•  Human use of
fossil fuels
produces
greenhouse gases
that can cause
global warming
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Global Warming
•  Earth’s average
temperature has
increased by 0.5°C in
past 50 years
•  Concentration of CO2 is
rising rapidly
•  An unchecked rise in
greenhouse gases is
causing global warming
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