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