Terrestrial Atmospheres Solar 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 CO 2 ) block IR radiation Temperature raises to establish equilibrium GREENHOUSE EFFECT

Greenhouse Effect

Venus & Mars Mars is dry & cool Venus is an inferno Both have CO 2 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 H 2 O Water removes CO 2 from atmosphere (forming limestone) Life removes further CO 2 from Temperature differences Earth raised by ~35 K Venus raised by ~400K

Earth Mostly nitrogen and oxygen Other planets DO NOT have O 2 in atmosphere Byproduct of plant metabolism Presence of O 2 allows UV radiation to produce ozone (O 3 ) Ozone blocks harmful UV radiation

Earth

Layers of Earth’s Atmosphere Troposphere (surface to 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 CO 2, 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