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Terrestrial AtmospheresSolar System Astronomy
Chapter 8
Primary & Secondary Primary atmospheres
Formed with planet Form by sweeping up gas in accretion disk
Secondary atmospheres Acquired later Form by volcanism or comets striking
Comets are mostly water ice
Atmospheric Escape/Loss Temperature measures gas speed
High T reaches escape velocity Low mass gives low escape velocity
Hot & small planets lose atmospheres
Mercury & Moon Lost primary atmospheres Little secondary atmosphere
Less internal heat/volcanism Secondary atmospheres also escape
Earth, Venus, Mars Hot during formation
Lost primary atmosphere Significant volcanism
Produced secondary Earth & Venus are large
High escape velocity But they evolved very differently…
Greenhouse Effect Important on Earth & Venus
Incoming sunlight heats planet Outgoing IR cools planet
Some gasses (especially CO2) block IR radiation Temperature raises to establish equilibrium
GREENHOUSE EFFECT
Greenhouse Effect
Venus & Mars Mars is dry & cool Venus is an inferno Both have CO2 atmospheres
Venus’ is 2500 times as great as Mars’ Nearly 100× Earth’s Venus is larger
More volcanoes More atmosphere More greenhouse effect Higher escape velocity
Venus & Earth Venus was hotter than Earth early
No water on surface
Earth retained H2O Water removes CO2 from atmosphere
(forming limestone) Life removes further CO2 from
Temperature differences Earth raised by ~35 K Venus raised by ~400K
Earth Mostly nitrogen and oxygen
Other planets DO NOT have O2 in atmosphere Byproduct of plant metabolism
Presence of O2 allows UV radiation to produce ozone (O3)
Ozone blocks harmful UV radiation
Earth
Earth
Layers of Earth’s Atmosphere Troposphere (surface to 10-15 km)
Temperature & pressure decline with altitude Temperature stops declining at tropopause
Stratosphere (5-15 km) Temperature rises with altitude
Ozone absorbs light, heating stratosphere
Layers of Earth’s Atmosphere Mesosphere (50-90 km)
Temperature declines with altitude No ozone
Ionosphere (>90 km) UV radiation and solar wind ionizes atoms
Layers of Earth’s Atmosphere
Earth’s magnetic field Earth’s magnetic field, the
magnetosphere, extends out into space Blocks much of the solar wind Particles from the solar wind collide with the
atmosphere This creates the northern and southern lights
(auroras)
Earth’s magnetic field
Convection Transport of energy by rising/falling
hot/cool gas Important for transporting heat Incoming sunlight heats the ground Hot air at the surface rises Rising air expands and cools Denser, cooler air sinks.
Convection Winds
Parts of the Earth are heated differently Vertical circulation of air (convection)
distributes surface heating Global winds carry heat from hot to cool
regions On Earth, Venus, and Mars, the circulation
depends on heating pattern and rotation period
Venus Hot, dense atmosphere, completely cloud-
covered Surface pressure 92× that on Earth Mainly CO2, strong greenhouse effect
Surface temperature about 740 K Thick atmosphere means nearly uniform
temperatures over the entire planet
Mars Cold, thin atmosphere.
No oxygen, no ozone Thin atmosphere = extreme temperature
variations Equator: up to 293 K (20 C) Pole: down to -150 C
Consequently large winds, which can make big dust storms
Moon & Mercury Almost totally airless
Combination of temperatures and low escape velocity means any atmosphere is lost
No erosion from wind, so old, cratered surfaces are retained
Mars
Mars
Mars