Transcript which would release a little bit more CO 2

Lecture 24
Terrestrial planets: atmospheres
Atmospheres are created by three processes:
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
Outgassing
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Evaporation/sublimation
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Impacts
All planets probably had minimal (primary.
H,He) atmospheres at some point after they
formed

How do you keep an atmosphere?

Gravity attracts gas towards the planet.
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Thermal motions of the gas can cause it to escape.
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Gravity determines the escape velocity
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Temperature determines the thermal velocity.
Large planets hold onto atmospheres easier than
small planets
Cooler planets hold onto atmospheres easier than
hot ones.
Original primary atmospheres (H,He) were swept
away from the terrestrial planets early in their life.
Jovian planets still have these primary atmospheres
Present day terrestrial atmospheres are secondary
atmospheres, formed primarily by outgassing
(mostly carbon dioxide - CO2)
or, in the case of the Earth, a tertiary atmosphere of
oxygen and nitrogen (secondary atmosphere
modified by life and presence of liquid water)

Which moons have atmospheres?
Jupiter's Io (tenuous sulfur dioxide)
 Jupiter's Europa (tenuous oxygen)
 Saturn's largest moon Titan (dense nitrogen &
methane)
 Neptune's largest moon Triton (tenuous nitrogen &
methane).
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Moon and Mercury


“black sky”
The little atmosphere that exists
consists of particles of the solar wind
that are temporarily trapped.
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Mars
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Very little atmosphere today (CO2)
 Mars
had standing and running water on its surface
in the past.

Therefore, it must have had a more substantial
atmosphere in the past
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Does it have water today? Yes - frozen in polar ice
caps and possibly beneath its soil
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Earth
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A moderate atmosphere today
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Mostly nitrogen (N2), with some oxygen (O2),
carbon dioxide (CO2), etc.
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Enough to enable liquid water to exist
(temperature and pressure adequate)
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Together the air & water produce erosion
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Venus
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Densest atmosphere of all Terrestrials
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Mostly CO2
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Temperature at surface hot enough to melt lead
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Pressure at the surface ~ 90 times that on Earth
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Perpetual cloud cover, sulfuric acid rain
How do we account for these differences?
Composition of outgassed gasses
The dominant gasses arising from outgassing
were carbon dioxide and water vapor, with minor
amounts of nitrogen, sulfer, argon, …
Each terrestrial planet’s outgassed atmosphere
was roughly the same.
How do we account for these differences?
Composition of outgassed gasses
(Carbon dioxide and water vapor)
Temperature appropriate for liquid water?
Distance from the sun, amount of greenhouse
gasses in atmosphere.
Oceans absorb carbon dioxide -> carbonate rocks
How do we account for these differences?
Composition of outgassed gasses
(Carbon dioxide and water vapor)
Temperature appropriate for liquid water?
(Carbon dioxide dissolves in oceans)
Interaction of light and atmospheres
Dissociation of water and ammonia molecules
by UV light, and warming by the greenhouse effect
How do we account for these differences?
Composition of outgassed gasses
(Carbon dioxide and water vapor)
Temperature appropriate for liquid water?
(Carbon dioxide dissolves in oceans)
Interaction of light and atmospheres
(dissociation and the greenhouse effect)
 Photosynthetic life - generation of oxygen
Interactions of light with atmospheric gasses
•
X rays
• ionize atoms & molecules
• dissociate molecules
• absorbed by almost all gases
•
Ultraviolet (UV)
• dissociate some molecules
• absorbed well by O3 & H2O
•
Visible (V)
• passes right through gases
• some photons are scattered
•
Infrared (IR)
• absorbed by greenhouse gases
Exosphere
hottest layer, v. rarified
Thermosphere
absorbs X-rays, ionized,
ionosphere, reflects
some radio, aurora
Mesosphere
•
weakly absorbs UV
Stratosphere
•
strongly absorbs UV,
ozone (O3), stratified (no
convection)
Troposphere
•
absorbs IR (greenhouse);
convective; weather
The greenhouse effect
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Planets heat up by absorbing the Sun’s visible light
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Planets cool off by radiating infrared out to space
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Greenhouse gasses trap infrared radiation in
troposphere (lowest level of atmosphere), heating lower
atmosphere.

greenhouse gasses (e.g., H2O, CO2, CH4 - methane)
transmit visible light but absorb infrared light
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Greenhouse effect raises
temperature of lower
atmosphere
Greenhouse effect is
critical to the existence of
life on Earth – it raises
temperatures to
“habitable” level, permits
liquid water
Terrestrial planets: atmospheres
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Question: Why does Venus have so much more
atmosphere than Earth?
The answer is found in what Venus’
atmosphere is made of: CO2 (carbon monoxide)
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What happens if there is a lot of CO2 in a
planet’s atmosphere?
Due to the large amount of CO2 in its
atmosphere, the surface temperature on
Venus is over 700 K, instead of the 230 K
that it should be at this distance from the
Sun.


Does this also explain why Venus has so
much atmosphere???
YES!
Evolution of
Atmospheres:
Earth vs. Venus
because water can exist in liquid form
 On Earth there are oceans
Original CO2 has dissolved into oceans,
rocks (carbonates) which keeps
levels of CO2 just balanced in
atmosphere
 keeps planet WARM but not HOT
if planet were hotter, CO2, H2O would
be boiled out of oceans and baked
out of rocks
 more CO2, H2O enter Atmosphere
Liquid water may have existed early in
Evolution of
Atmospheres: Venus’ history – but most vaporized into
Earth vs. Venus atmosphere: T was hotter on Venus
H2O vapor is a greenhouse gas - trapped
energy making planet hotter; eventually T
so high that water boils
‘runaway’ because more H20 goes into the
Atmosphere as it evaporates; no water left
on planet to dissolve CO2 – out of balance!
eventually stabilized when H20 broken down
by UV sunlight and no further CO2 to bake
out of the Venus surface
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Earth has about the same amount of CO2 as Venus
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Much of the Earth’s CO2 is ‘frozen’ into the rocks
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However, if we could raise the temperature of our
atmosphere a little bit, it would release a little bit more CO2
into the air
This would trap a little bit more heat, raising the temperature
a little bit more…
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This would release a little bit more CO2…

…which would trap a little bit more heat…

…which would raise the temperature a little bit
more…
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…which would release a little bit more CO2…

…which would trap a little bit more heat…


…which would raise the temperature a little bit
more…
…which would release a little bit more CO2…
You get the idea!
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This is called the runaway greenhouse effect
It happened on Venus because Venus is closer
to the Sun
So - Earth has less atmosphere because most
of our CO2 is still frozen in rocks
Global warming concern
There has been a large Increase in atmospheric CO2
Human activity is responsible for this increase.
CO2 is a strong greenhouse gas
There is a strong correlation between the increase in
carbon dioxide in the atmosphere and the global
temperature.
Essentially all evidence indicates that human activity is
a significant contributor to this trend (50%, 100% ???).
Can it be halted? reversed?
red) Ozone is broken apart
by ultraviolet radiation.
What Determines a Planet’s Surface Temperature?
In the absence of the Greenhouse Effect:

the planet's distance from the Sun

the planet’s overall reflectivity
•
the higher the albedo (reflectivity), the less light
absorbed  planet cooler
What Determines a Planet’s Surface Temperature?

Greenhouse Effect.
cannot change incoming Sunlight, so it cannot
change the total energy returned to space
 it increases the energy (heat) in lower atmosphere,
keeping the surface warmer
 It works like a blanket
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Greenhouse Effect on the Planets
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Greenhouse Effect warms Venus, Earth, & Mars
 on
Venus: it is very strong
 on Earth: it is moderate
 on Mars: it is weak
 avg. temp. on Venus & Earth would be freezing
without it
Global warming concern
There has been a large Increase in atmospheric CO2
Human activity is responsible for this increase.
CO2 is a strong greenhouse gas
There is a strong correlation between the increase in
carbon dioxide in the atmosphere and the global
temperature.
Essentially all evidence indicates that human activity is
a significant contributor to this trend (50%, 100% ???).
Can it be halted? reversed?