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Astronomy 311: Lecture 4 - Terrestrial Planetary Atmospheres
• An atmosphere is a layer of gas that surrounds a planet.
• On Earth, atmosphere is about 10km thick: O2 , N2 , H2 O, CO2.
• Atmospheric pressure decreases as you go up. Principle of hydrostatic equilibrium applies.
• Ideal gas law: P = ρT × constant.
• Average pressure at sea level is 1.03kg/m2 which is 1 bar of pressure.
• Atmospheres create pressure that can determine whether water is in liquid
form on the surface.
• Atmospheres can absorb and scatter light: daytime skies are blue in worlds
with atmospheres.
• Atmospheres create winds and weather.
• Atmospheres can contain gases which make planetary surfaces warmer than
they would be otherwise using the greenhouse effect.
• Greenhouse effect
– Visible light from Sun absorbed by the ground.
– Ground reradiates this radiation in the form of I=infra-red (IR) light
because this is the appropriate wavelength for a body of that temperature (Wien’s law).
– Then some gases in lower atmosphere ”trap” this IR radiation for a
while and are heated by it.
– These greenhouse gases are water vapor (H2 O, CO2 , and CH4 ).
– These gases aborb an IR photon coming from the ground and then
reradiate it in a random direction. Further collisions occur.
– Escape of this IR radiation is slowed down but ensuing molecular motion heats the surrounding air.
– The more greenhouse gases that are present the greater the degree of
surface warming.
– These gases abosrb IT because their molecular structure makes them
prone to begin rotating or vibrating when struck by a IR photon.
– Energy balance is not altered.
• Without the greenhouse effect, planet’s temperature much cooler.
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• Planet’s temperature without greenhouse affected by distance from Sun
(inverse square law) and the planet’s reflectivity.
• Black absorbs more than white.
• Rflectivity depends on surface composition and color.
• For example, the temperature of a planet’s surface is roughly
T = 280K × 4
s
(1 − ref )
.
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• X rays ionize most gases and dissociate molecules.
• UV rays dissociate molecules.
• Visible light: blue light is scattered more than red light.
• IR photons abosrbed by molecules causing them to rotate and vibrate.
• Troposphere, Stratosphere, Thermosphere and Exosphere.
• Sky is blue because blue light is scattered more than red light.
• Sunsets are red because you are watching the beam rather than the scattered light.
• Greenhouse effect warms the troposhere and because more is absorbed
closer to the ground, temperature drops with altitude in troposhere.
• Without greenhouse effect not such a difference in temp. between mountain
tops and sea level.
• Convection in troposphere: storms, weather etc.
• At top of troposphere IR photons no longer collide and go into space.
• Now only the effect of sunlight on atmosphere.
• Now primary source of atmospheric heating is UV photons from Sun.
• This is stronger at higher altitudes so temp increases with altitude in lower
stratosphere.
• Convection cannot occur in stratosphere because of this temp. inversion.
• No weather and no rain: pollution stays here.
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• Planet can only have a stratosphere if its atmosphere contains molecules
that are good at absorbing UV photons: Ozone, O3 plays this role on Earth.
• Earth is the only terrestrial world with a stratosphere.
• Nearly all gases are good X-ray absorbers so X rays are absorbed by the
first gases they encounter in atmosphere.
• Desnity in exosphere too low so not much absorption there.
• Most X rays absorbed in thermosphere. Virtually no Xrays penetrate below
thermosphere.
• Part of thermosphere is ionized: ionosphere. This layer reflects radio waves
and hence important for communications.
• Exosphere is low-density boundary between atmosphere and space. High
temperature but low density.
• Mercury and Moon have contain tiny atmpsheres: very small, no structure,
essentially an exosphere.
• Mars, Earth and Venus have a warm troposhere at the base caused by the
greenhouse effect and a warm thermosphere at the top caused by X-ray
absorption.
• Only Earth has a stratosphere.
• Venus, Earth and Mars have all experienced and are experiencing the greenhouse effect in order of severity, starting with the most severe.
• Solar wind: outpouring of subatomic charged particles from Sun.
• Earth’s strong magnetic field creates a magentosphere that deflects these
charged particles around the planet.
• Some enter through the poles and reside in the Van-Allen belts encircling
Earth.
• The interaction of these charged particles produces aurora eg. Northern
lights.
• Climate Change
– Solar brightening due to the Sun brightening somewhat during its main
sequence lifetime.
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– Tilt of the earth’s axis can change due to gravitational tugs from other
planets: Earth’s axis changes from 22 to 25 degrees. Greater tilt means
more severe seasons. These tilt changes are called Milankovitch cycles.
Mars experiences more extreme changes in axis tilt than the Earth.
– Changes in reflectivity of a planet. One possible cause for changes in
Earth’s reflectivity is human activity, deforestation, smog etc. Smog is
a good reflector and so one theory is that increased smog has actually
reduced increasing temperatures due to increased greenhous gas emission becuase the smog reflects a lot of sunlight back. Hence efforts to
clean up smog pollution may actually accelerate greenhouse warming.
– Changes in greenhouse gas abundance.
• Origin of Atmospheres
– During their births, planets captures some H,He but the planetesimals
were too small to retain these primitive atmospheres during the solar
T-Tauri phase.
– It is thought that volcanic outgassing has been the primary source for
terrestrial atmospheres: terrestrials made up of metal and rock but
impacts of ice-rich planetessimals from beyond the frost line brought
in water and gas that became trapped in the interiors. This was further
”solidified” when the crust formed and the mantle started to cool from
the outside in. Volcanic outgassing releases H2 O, CO2, N2 , H2 S, SO2 .
It is thought that this occurred either during the molten mantle stage
or as a consequence of a catastrophic collision for the Earth.
– Recent work though suggests that argon can stay trapped in the mantle
even at very high temperatures, with mantle convection and volcanism.
– This suggests its is difficult for the Earth to outgass all the argon
trapped in the mantle. Argon is also a byproduct of the radioactive
deavy of potassium.
– But the earth’s atmosphere contains 1of the crust is the suggestion.
Thus argon trapped in the mantle billions of years ago may still be
there.
– Mars and Venus have similar mantle materials so the situation may
be similar for them as well.
– Can also form by evaporation of liquid of sublimation of surface ices
into gases.
– After outgassing, some gases condense to become liquids or ices eg.
Earth’s oceans and polar caps.
– There is a continual exchange between surface liquids/ices and the
atmosphere.
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– Outgassing is the primary origin for atmospheres on Mars, Venus and
Earth.
– Planets can loose atmospheric gas by thermal escape, condensation,
solar wind sweeping up particles, chemical reactions and large impacts
blasting gases into space.
– For thermal escape, the average velocity of a particle, mass m is related
to the temperature by
Vthermal =
q
2kT /m,
so of this equals or exceeds
q the escape velocity of the planet, mass
and radius M, R, Vesc = 2GM/R, the particle will escape. Here k is
Boltzmann’s constant.
– Mars is cold and dry with a very low atmospheric pressure - water is
unstable - it can evaporate easily. Because of its orbit, its distance
from the Sun varies much more than the Earth and has an effect on
its seasons: seasons are more extreme in the southern than northern
hemispheres.
– Martian atmosphere is mainly CO2 , no ozone, so there is considerable
UV radiation on the surface.
– Mars has cooled more than Earth, hence its magnetic field is much
less, it lost its magnetosphere and so the solar wind was able to ”blow
away” the Martian atmosphere.
– Venus has a thick atmosphere with a strong greenhouse effect - thats
why it is so hot. Rotates slowly, thus a weak Coriolis effect and weak
winds: too hot for rain to fall. Little axis tilt implies no seasons.
– It may have got so hot due to a runaway greenhouse effect.
– Earth’s planetary temperature just right for outgassed water vapour
to condense and form oceans. Oceans dissolve CO2 and moderate the
greenhouse effect. The Earth’s atmosphere stays stable due to the
CO2 cycle: too cool, less perciptation, rainwater dissolves less CO2
from the atmosphere, so more CO2 in the atmosphere which leads to
a strengthened greenhouse effect. If its too warm, more precipitation,
more CO2 from atmosphere dissolves in rainwater and into oceans and
less CO2 in the atmosphere etc.
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