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Earth and the Other Terrestrial Worlds

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What are terrestrial planets like on the inside?

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Earth’s Interior

• Core: highest density; nickel and iron

• Mantle: moderate density; silicon, oxygen, etc.

• Crust: lowest density; granite, basalt, etc.

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Terrestrial Planet Interiors

• Applying what we have learned about Earth’s interior to other planets tells us what their interiors are probably like.

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Role of Size

• Smaller worlds cool off faster and harden earlier.• The Moon and (maybe) Mercury are now geologically

“dead.”

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Surface Area–to–Volume Ratio• Heat content depends on volume.• Loss of heat through radiation depends on surface

area.• Time to cool depends on surface area divided by

volume:

3

Surface area–to–volume ratio = 4r2

4 r3 3

r

• Larger objects have a smaller ratio and cool more slowly.

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What processes shape planetary surfaces?

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Processes That Shape Surfaces• Impact cratering

– Impacts by asteroids or comets• Volcanism

– Eruption of molten rock onto surface• Tectonics

– Disruption of a planet’s surface by internal stresses

• Erosion– Surface changes made by wind, water, or ice

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Role of Planetary Size

• Smaller worlds cool off faster and harden earlier.• Larger worlds remain warm inside, promoting

volcanism and tectonics.• Larger worlds also have more erosion because their

gravity retains an atmosphere.

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Role of Distance from Sun

• Planets close to the Sun are too hot for rain, snow, ice and so have less erosion.

• Hot planets have more difficulty retaining an atmosphere.• Planets far from the Sun are too cold for rain, limiting

erosion.• Planets with liquid water have the most erosion.

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Role of Rotation

• Planets with slower rotation have less weather, less erosion, and a weak magnetic field.

• Planets with faster rotation have more weather, more erosion, and a stronger magnetic field.

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What are the major geological features of Mars?

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Volcanism on Mars

• Mars has many large shield volcanoes.

• Olympus Mons is largest volcano in solar system.

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What geological evidence tells us that water once flowed on Mars?

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Dry Riverbeds?

• Close-up photos of Mars show what appear to be dried-up riverbeds.

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Erosion of Craters

• Details of some craters suggest they were once filled with water.

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Crater Walls• Gullies on crater

walls suggest occasional liquid water flows have happened less than a million years ago.

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What are the major geological features of Venus?

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Radar Mapping

• Its thick atmosphere forces us to explore Venus’s surface through radar mapping.

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Cratering on Venus

• Venus has impact craters, but fewer than the Moon, Mercury, or Mars.

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Volcanoes on Venus

• It has many volcanoes, including both shield volcanoes and stratovolcanoes.

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Erosion on Venus

• Photos of rocks taken by landers show little erosion.

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What is an atmosphere?

An atmosphere is a layer of gas that surrounds a world.

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Earth’s Atmosphere

• About 10 kilometers thick

• Consists mostly of molecular nitrogen (N2) and oxygen (O2).

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Atmospheric Pressure

Gas pressure Gas pressure depends on both depends on both density and density and temperature.temperature.

Adding air Adding air molecules molecules increases the increases the pressure in a pressure in a balloon.balloon.

Heating the air Heating the air also increases also increases the pressure.the pressure.

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Where does an atmosphere end?

• There is no clear upper boundary.

• Most of Earth’s gas is less than 10 kilometers from surface, but a small fraction extends to more than 100 kilometers.

• Altitudes less more than 60 kilometers are considered “space.”

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Effects of Atmospheres• They create pressure that determines whether

liquid water can exist on surface.• They absorb and scatter light.• They create wind, weather, and climate.• They interact with the solar wind to create a

magnetosphere.• They can make planetary surfaces warmer

through the greenhouse effect.

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How does the greenhouse effect warm a planet?

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Greenhouse Effect• Visible light passes

through the atmosphere and warms a planet’s surface.

• The atmosphere absorbs infrared light from the surface, trapping heat.

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“No Greenhouse” Temperatures

• Venus would be 510°C colder without greenhouse effect.

• Earth would be 31°C colder (below freezing on average).

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Why the Sky Is Blue

• Atmosphere scatters blue light from Sun, making it appear to come from different directions.

• Sunsets are red because red light scatters less.

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Earth’s Magnetosphere

• Magnetic field of Earth’s atmosphere protects us from charged particles streaming from Sun (the solar wind).

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Aurora

• Charged particles from solar wind energize the upper atmosphere near magnetic poles, causing an aurora.

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Sources of Gas

Outgassing Outgassing from volcanoesfrom volcanoes

Evaporation of Evaporation of surface liquid; surface liquid; sublimation of sublimation of surface icesurface ice

Impacts of Impacts of particles and particles and photonsphotons

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Losses of Gas

Condensation Condensation onto surfaceonto surface

Chemical Chemical reactions with reactions with surfacesurface

Large impacts Large impacts blasting gas blasting gas into spaceinto space

Thermal escape of atomsThermal escape of atoms Sweeping by solar windSweeping by solar wind

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What is Mars like today?

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Seasons on Mars

• The ellipticity of Mars’s orbit makes seasons more extreme in the southern hemisphere.

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Polar Ice Caps of Mars

• Carbon dioxide ice of polar cap sublimates as summer approaches and condenses at opposite pole.

Late winterLate winter Mid-springMid-spring Early summerEarly summer

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Polar Ice Caps of Mars

• Residual ice of the polar cap remaining during summer is primarily water ice.

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Dust Storms on Mars

• Seasonal winds can drive dust storms on Mars.• Dust in the atmosphere absorbs blue light,

sometimes making the sky look brownish-pink.

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What is Venus like today?

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Greenhouse Effect on Venus

• Thick carbon dioxide atmosphere produces an extremely strong greenhouse effect.

• Earth escapes this fate because most of its carbon and water is in rocks and oceans.

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Runaway Greenhouse Effect

• A runaway greenhouse effect would account for why Venus has so little water.