(Terrestrial) Planetary Atmospheres I
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Atmosphere:
◦ Layer of gas that
surrounds a world

Thin for terrestrial
planets
◦ 2/3 of air within 10
km of Earth’s surface
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So what do atmospheres do?
◦ Pressure allows liquid phase of water
◦ Absorb and scatter light
 “Radiation shield”
 Ozone in Earth’s atmosphere absorbs UV radiation
◦ Wind and weather
◦ Can trap heat and warm the planet

Molecules move fast and collide
◦ 500 m/s on Earth
◦ They therefore push on surfaces

Aside: Why don’t they travel across a room
that fast?
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Consider how
gravity acts on a
bunch of molecules
in motion
◦ They “pile up” toward
the surface
◦ The atmosphere
below supports the
atmosphere above
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Planets are able to hold onto their
atmospheres longer if:
◦ They are large (stronger gravity)
◦ The temperature of the atmosphere is low
 Molecules don’t try as hard to escape
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Distance from the Sun
Albedo: Reflectivity of surface and
atmosphere
Greenhouse Effect: Trapping a planet’s
emitted radiation

Sunlight rejected by
planet
◦ Low Albedo:
 Darker: absorbs more
 Soil, trees, etc.
◦ High Albedo:
 Lighter: reflects more
 Cloud, ice caps, etc.

If the sunlight is
reflected, it can’t
warm the planet
Surface
Typical Albedo
Fresh Asphalt
0.04
Worn Asphalt
0.12
Coinifer Forest
0.09 to 0.15
Bare Soil
0.17
Green Grass
0.25
Desert Sand
0.40
Concrete
0.55
Fresh Snow
0.80 – 0.90
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Different materials
respond differently
to different
frequencies of light!
Clouds reflect
visible light. They
do not reflect UV.
Does albedo warm or cool a planet?
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Visible light from Sun absorbed by the ground
Ground returns absorbed radiation as a
continuous spectrum.
◦ Peaks in the infrared

Greenhouse gases absorb these infrared photons
◦ Water Vapor
◦ CO2 (Carbon dioxide)
◦ CH4 (Methane)

Keeps the lower
atmosphere and
ground warm
◦ Energy from the
photons can be
“exchanged” for kinetic
energy through
collisions

Cloudy nights can be
warmer than clear
nights!
Does the greenhouse effect warm
or cool a planet?

Be thankful for it…
◦ The infrared radiation emitted by Earth would
escape straight back into space if not for the
greenhouse effect.
◦ Earth would be at 3 oF if not for the greenhouse
effect.
◦ We wouldn’t have liquid water.
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Venus has a high albedo and reflects 75% of
incoming light. Why is it so hot (800 oF)?
Do Mercury and the Moon have a greenhouse
effect? Why or why not?
Earth’s atmosphere is mostly diatomic
nitrogen and oxygen (poor infrared
absorbers). How would the temperature
change if they were good infrared absorbers?
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Planetary climates are modeled as follows:
◦ Calculate Effective Temperature
 Assumes planet absorbs all radiation, emits freely
◦ Calculate Albedo Temperature
 Assumes that planet can reflect incident radiation
◦ Calculate Atmospheric Temperature
 Assumes atmosphere can inhibit radiation emission by
the planet
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Variation of
temperature with
height
Due to how
atmospheric gas
interacts with
sunlight
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X-rays:
◦ Can remove electrons from atoms
(Ionizes them)
◦ Can dissociate (break apart) molecules

Ultraviolet:
◦ Can dissociate (break apart) molecules
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Visible light:
◦ Usually transmitted, sometimes scattered
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Infrared light:
◦ Absorbed by molecules
◦ Causes rotation and vibration in molecules
X-rays
(Page 304)
Ultraviolet
Visible
Infrared
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The atmosphere
scatters visible light
◦ Think in terms of
light rays
◦ If no scattering,
would see stars with
Sun in view

Blue light scattered
more than red
◦ Red sunsets
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Troposphere gets
the infrared light
emitted by Earth
◦ Temperature drops
farther from surface
◦ Has convection and
storms
 Dense air
 Surface heat
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Infrared not significant
here
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UV light absorbed by
ozone here
◦ UV light from Sun
◦ The top layer absorbs more
than the bottom
◦ Gets hotter with height to a
point
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No convection, no weather
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Most gases absorb Xrays
They get absorbed by
the first dense gas they
encounter
◦ Exosphere not dense
enough
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This is the
thermosphere
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Gets hotter higher up
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Very low density gas
◦ Faster molecules
escape
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Boundary between
atmosphere and
space
Gas very hot, but you
wouldn’t feel it (low
density)