Download Light: The Cosmic Messenger

Earth and the Other Terrestrial Worlds
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What are terrestrial planets like
on the inside?
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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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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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Role of Size
• Smaller worlds cool off faster and harden earlier.
• The Moon and (maybe) Mercury are now geologically
“dead.”
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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:
2
p
r
4
=3
Surface area–to–volume ratio =
4 pr 3 r
3
• Larger objects have a smaller ratio and cool more
slowly.
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What processes shape planetary
surfaces?
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Processes That Shape 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
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Role of Planetary Size
• Smaller worlds cool off faster and harden 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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Role of Distance from Sun
• Planets close to the Sun are too hot for rain, snow, ice
and so have less erosion.
• Hot planets have more difficulty retaining an atmosphere.
• Planets far from the Sun are too cold for rain, limiting
erosion.
• Planets with liquid water have the most erosion.
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Role of Rotation
• Planets with slower rotation have less weather, less
erosion, and a weak magnetic field.
• Planets with faster rotation have more weather, more
erosion, and a stronger magnetic field.
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What are the major geological
features of Mars?
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Volcanism on Mars
• Mars has many
large shield
volcanoes.
• Olympus Mons is
largest volcano in
solar system.
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What geological evidence tells us
that water once flowed on Mars?
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Dry Riverbeds?
• Close-up photos of Mars show what appear
to be dried-up riverbeds.
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Erosion of Craters
• Details of some
craters suggest
they were once
filled with water.
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Crater Walls
• Gullies on crater
walls suggest
occasional liquid
water flows have
happened less
than a million
years ago.
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What are the major geological
features of Venus?
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Radar Mapping
• Its thick atmosphere forces us to explore Venus’s
surface through radar mapping.
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Cratering on Venus
• Venus has impact
craters, but fewer
than the Moon,
Mercury, or Mars.
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Volcanoes on Venus
• It has many
volcanoes, including
both shield volcanoes
and stratovolcanoes.
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Erosion on Venus
• Photos of rocks
taken by landers
show little
erosion.
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What is an atmosphere?
An atmosphere is a layer of gas that surrounds a world.
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Earth’s Atmosphere
• About 10 kilometers
thick
• Consists mostly of
molecular nitrogen
(N2) and oxygen
(O2).
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Atmospheric Pressure
Gas pressure
depends on both
density and
temperature.
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Adding air
molecules
increases the
pressure in a
balloon.
Heating the air
also increases
the pressure.
Where does an atmosphere end?
• There is no clear
upper boundary.
• Most of Earth’s gas is
less than 10 kilometers
from surface, but a
small fraction extends
to more than 100
kilometers.
• Altitudes less more
than 60 kilometers are
considered “space.”
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Effects of Atmospheres
• They create pressure that determines whether
liquid water can exist on surface.
• They absorb and scatter light.
• They create wind, weather, and climate.
• They interact with the solar wind to create a
magnetosphere.
• They can make planetary surfaces warmer
through the greenhouse effect.
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How does the greenhouse effect
warm a planet?
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Greenhouse Effect
• Visible light passes
through the
atmosphere and
warms a planet’s
surface.
• The atmosphere
absorbs infrared light
from the surface,
trapping heat.
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“No Greenhouse” Temperatures
• Venus would be 510°C colder without greenhouse
effect.
• Earth would be 31°C colder (below freezing on
average).
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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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Earth’s Magnetosphere
• Magnetic field of Earth’s atmosphere protects us from
charged particles streaming from Sun (the solar wind).
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Aurora
• Charged particles from solar wind energize the upper
atmosphere near magnetic poles, causing an aurora.
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Sources of Gas
Outgassing
from volcanoes
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Evaporation of
surface liquid;
sublimation of
surface ice
Impacts of
particles and
photons
Losses of Gas
Thermal escape of atoms
Condensation
onto surface
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Sweeping by solar wind
Chemical
reactions with
surface
Large impacts
blasting gas
into space
What is Mars like today?
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Seasons on Mars
• The ellipticity of Mars’s orbit makes seasons more
extreme in the southern hemisphere.
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Polar Ice Caps of Mars
Late winter
Mid-spring
Early summer
• Carbon dioxide ice of polar cap sublimates as
summer approaches and condenses at opposite pole.
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Polar Ice Caps of Mars
• Residual ice of the
polar cap remaining
during summer is
primarily water ice.
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Dust Storms on Mars
• Seasonal winds can drive dust storms on Mars.
• Dust in the atmosphere absorbs blue light,
sometimes making the sky look brownish-pink.
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What is Venus like today?
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Greenhouse Effect on Venus
• Thick carbon
dioxide atmosphere
produces an
extremely strong
greenhouse effect.
• Earth escapes this
fate because most of
its carbon and water
is in rocks and
oceans.
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Runaway Greenhouse Effect
• A runaway greenhouse effect would account for why
Venus has so little water.
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