chapter 7 earth and the terrestrial worlds. mercury craters smooth plains cliffs

Chapter 7Earth and the Terrestrial Worlds

Mercury

craterssmooth plains

cliffs

volcanoesfew craters

Radar view of a twin-peaked volcano

some cratersvolcanoesriverbeds?

craterssmooth plains

volcanoescraters

mountainsriverbeds

Why have the planets turned out so differently, even though they formed at the same time from the same materials?

9.1 Earth as a Planet

Our goals for learning:

• Why is Earth geologically active?

• What processes shape Earth’s surface?

• How does Earth’s atmosphere affect the planet?

Why is Earth geologically active?

Earth’s Interior

• Core: Highest density; nickel and iron

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

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

Terrestrial Planet Interiors

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

Why do water and oil separate?

A. Water molecules repel oil molecules electrically.

B. Water is denser than oil, so oil floats on water.

C. Oil is more slippery than water, so it slides to the surface of the water.

D. Oil molecules are bigger than the spaces between water molecules.

Why do water and oil separate?

A. Water molecules repel oil molecules electrically.

B. Water is denser than oil, so oil floats on water.

C. Oil is more slippery than water, so it slides to the surface of the water.

D. Oil molecules are bigger than the spaces between water molecules.

Differentiation

• Gravity pulls high-density material to center

• Lower-density material rises to surface

• Material ends up separated by density

Thought Question

What is necessary for differentiation to occur in a planet?

A. It must have metal and rock in it.B. It must be a mix of materials of different density.

C. Material inside must be able to flow.

D. All of the above.

E. b and c.

Thought Question

What is necessary for differentiation to occur in a planet?

A. It must have metal and rock in it.B. It must be a mix of materials of different density.

C. Material inside must be able to flow.

D. All of the above.

E. b and c.

Lithosphere

• A planet’s outer layer of cool, rigid rock is called the lithosphere.

• It “floats” on the warmer, softer rock that lies beneath.

Thought Question

Do rocks s-t-r-e-t-c-h?

A. No—rock is rigid and cannot deform without breaking.

B. Yes—but only if it is molten rock.

C. Yes—rock under strain may slowly deform.

Thought Question

Do rocks s-t-r-e-t-c-h?

A. No—rock is rigid and cannot deform without breaking.

B. Yes—but only if it is molten rock.

C. Yes—rock under strain may slowly deform.

Strength of Rock

• Rock stretches when pulled slowly but breaks when pulled rapidly.

• The gravity of a large world pulls slowly on its rocky content, shaping the world into a sphere.

Heat Drives Geological Activity

Convection: hot rock rises, cool rock falls.

One convection cycle takes 100 million years on Earth.

Sources of Internal Heat

1. Gravitational potential energy of accreting planetesimals

2. Differentiation

3. Radioactivity

Heating of Interior over Time

• Accretion and differentiation when planets were young

• Radioactive decay is most important heat source today

Cooling of Interior• Convection

transports heat as hot material rises and cool material falls

• Conduction transfers heat from hot material to cool material

• Radiation sends energy into space

Thought Question

What cools off faster?

A. A grande-size cup of Starbucks coffee

B. A teaspoon of cappuccino in the same cup

Thought Question

A. A grande-size cup of Starbucks coffee

B. A teaspoon of cappuccino in the same cup

Thought Question

A. A big terrestrial planet

B. A tiny terrestrial planet

Thought Question

A. A big terrestrial planet

B. A tiny terrestrial planet

Role of Size

• Smaller worlds cool off faster and harden earlier.• Moon and Mercury are now geologically “dead.”

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:

surface area to volume ratio = 4r2

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

Planetary Magnetic Fields

Moving charged particles create magnetic fields.

A planet’s interior can create magnetic fields if its core is electrically conducting, convecting, and rotating.

Earth’s MagnetosphereEarth’s magnetic fields protects us from

charged particles from the Sun.The charged particles can create aurorae

(“Northern lights”).

Thought Question

If the planet core is cold, do you expect it to have magnetic fields?A. Yes, refrigerator magnets are cold, and

they have magnetic fields.B. No, planetary magnetic fields are

generated by moving charges around, and if the core is cold, nothing is moving.

Thought Question

If the planet core is cold, do you expect it to have magnetic fields?A. Yes, refrigerator magnets are cold, and they

have magnetic fields.B. No, planetary magnetic fields are generated

by moving charges around, and if the core is cold, nothing is moving.

Special Topic:How do we know what’s inside a planet?

• P waves push matter back and forth.

• S waves shake matter side to side.

Special Topic:How do we know what’s inside a planet?

• P waves go through Earth’s core, but S waves do not.

• We conclude that Earth’s core must have a liquid outer layer.

End of Lecture

10 Mar 2008

What processes shape Earth’s surface…

and might do the same on the other terrestrial planets?

Geological Processes

• 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

Impact Cratering

The Production of a Crater

Impact Cratering• Most cratering happened

soon after the solar system formed.

• Craters are about 10 times wider than objects that made them.

• Small craters greatly outnumber large ones.

Impact Craters

Meteor Crater (Arizona)Meteor Crater (Arizona) Tycho (Moon)Tycho (Moon)

Impact Craters

Meteor Crater (Arizona)Meteor Crater (Arizona)• ~50,000 years old~50,000 years old

Tycho (Moon)Tycho (Moon)

Impact Craters

Meteor Crater (Arizona)Meteor Crater (Arizona)• ~50,000 years old~50,000 years old

Tycho (Moon)Tycho (Moon)• ~108 million years old~108 million years old

Impact Craters

Meteor Crater (Arizona)Meteor Crater (Arizona)• ~50,000 years old~50,000 years old• ~4,000 feet across~4,000 feet across

Tycho (Moon)Tycho (Moon)• ~108 million years old~108 million years old

Impact Craters

Meteor Crater (Arizona)Meteor Crater (Arizona)• ~50,000 years old~50,000 years old• ~4,000 feet across~4,000 feet across

Tycho (Moon)Tycho (Moon)• ~108 million years old~108 million years old• ~50 miles across~50 miles across

Impact Craters

Meteor Crater (Arizona)Meteor Crater (Arizona)• ~50,000 years old~50,000 years old• ~4,000 feet across~4,000 feet across• ~550 feet deep~550 feet deep

Tycho (Moon)Tycho (Moon)• ~108 million years old~108 million years old• ~50 miles across~50 miles across

Impact Craters

Meteor Crater (Arizona)Meteor Crater (Arizona)• ~50,000 years old~50,000 years old• ~4,000 feet across~4,000 feet across• ~550 feet deep~550 feet deep

Tycho (Moon)Tycho (Moon)• ~108 million years old~108 million years old• ~50 miles across~50 miles across• ~3 miles deep~3 miles deep

Volcanism• Volcanism happens when

molten rock (magma) finds a path through lithosphere to the surface.

Volcanic Eruptions and Lava Flows

• To have volcanism, you need:

• To have volcanism, you need:– a molten interior

• To have volcanism, you need:– a molten interior– a thin lithosphere

• You don’t expect volcanism on small planets

• Molten rock is called lava after it reaches the surface.

• There are several different kinds of lava

Lava and Volcanoes

Runny lava makes flat Runny lava makes flat lava plains.lava plains.

Lava and Volcanoes

Slightly thicker lava Slightly thicker lava makes broad makes broad shield shield volcanoes.volcanoes.

Lava and Volcanoes

Slightly thicker lava Slightly thicker lava makes broad makes broad shield shield volcanoes.volcanoes.

Thickest lava makes Thickest lava makes steep steep stratovolcanoes.stratovolcanoes.

Outgassing

• Volcanism also releases gases from Earth’s interior into the atmosphere

Outgassing

• This is one way atmospheres are obtained

Outgassing

• This is one way atmospheres are obtained• There are other ways as well

Ways to add gas to an atmosphere

Ways to take gas out of an atmosphere

Tectonics

• Convection of the mantle creates stresses in the crust called tectonic forces.

Tectonics and Convection of the Mantle

Tectonics

• Compression forces make mountain ranges.

Tectonics

• Compression forces make mountain ranges.• A valley can form where the crust is pulled apart.

Tectonics

• Compression forces make mountain ranges.• A valley can form where the crust is pulled apart.• There’s a special kind of tectonics on Earth

Plate Tectonics on Earth

• Earth’s continents slide around on separate plates of crust.

• This is important for life here on Earth

• We’ll get to that later

Erosion• Erosion is a blanket term for weather-driven

processes that break down or transport rock.

Erosion• Erosion is a blanket term for weather-driven

processes that break down or transport rock.• Processes that cause erosion include

— Glaciers— Rivers— Wind

Erosion by Water

• The Colorado River continues to carve the Grand Canyon.

Erosion by Ice

• Glaciers carved the Yosemite Valley.

Erosion by Wind

• Wind wears away rock and builds up sand dunes.

Erosional Debris

• Erosion can create new features by depositing debris.

Erosional Debris

• Erosion can create new features by depositing debris.

• Extensive erosion happens on Earth because of our atmosphere

Effects of Atmosphere on Earth

• Erosion

• Earth’s atmosphere does other things:

• Erosion

• Radiation protection

• Erosion

• Greenhouse effect

• Erosion

• Greenhouse effect

• Makes the sky blue!

Radiation Protection

• All X-ray light is absorbed very high in the atmosphere.

Radiation Protection

• All X-ray light is absorbed very high in the atmosphere.

• Ultraviolet light is absorbed by ozone (O3).

Earth’s atmosphere absorbs light at most wavelengths.

The Greenhouse Effect

Greenhouse effect:

Certain molecules let sunlight through but trap escaping infrared photons.

(H2O, CO2, CH4)

The Green House Effect

The Greenhouse Effect

Which Molecules are Greenhouse Gases?

A Greenhouse Gas

• Any gas that absorbs infrared

• Greenhouse gas: molecules with two different types of elements (CO2, H2O, CH4)

• Not a greenhouse gas: molecules with one or two atoms of the same element (O2, N2)

Greenhouse Effect: Bad?

The Earth is much warmer because of the greenhouse effect than it would be without an atmosphere…but so is Venus.

Thought Question

Why is the sky blue?

A. The sky reflects light from the oceans.B. Oxygen atoms are blue.

C. Nitrogen atoms are blue.

D. Air molecules scatter blue light more than red light.

E. Air molecules absorb red light.

Thought Question

Why is the sky blue?

A. The sky reflects light from the oceans.B. Oxygen atoms are blue.

C. Nitrogen atoms are blue.

D. Air molecules scatter blue light more than red light.

E. Air molecules absorb red light.

Why the sky is blue

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

• Sunsets are red because less of the red light from the Sun is scattered.

End of Lecture

12 Mar 2008

What have we learned?

• Why is Earth geologically active?— Earth retains plenty of internal heat because

it is large for a terrestrial planet.

it is large for a terrestrial planet.— That heat drives geological activity, keeping

the core molten and driving geological activity.

it is large for a terrestrial planet.— That heat drives geological activity, keeping

the core molten and driving geological activity.

— The circulation of molten metal in the core generates Earth’s magnetic field.

• What geological processes shape Earth’s surface?

• What geological processes shape Earth’s surface?— Impact cratering, volcanism, tectonics, and

erosion

erosion• How does Earth’s atmosphere affect the planet?

— Erosion

— Erosion— Protection from radiation

— Erosion— Protection from radiation— Greenhouse effect

• Now let’s look at the other terrestrials

7.2 Mercury and the Moon: Geologically Dead

• Was there ever geological activity on the Moon or Mercury?

Was there ever geological activity on the Moon or Mercury?

• Some volcanic activity 3 billion years ago must have flooded lunar craters, creating lunar maria.

• The Moon is now geologically dead.

Cratering of Mercury

• Mercury has a mixture of heavily cratered and smooth regions like the Moon.

• And like the Moon, the smooth regions are likely ancient lava flows.

The Caloris basin is The Caloris basin is the largest impact the largest impact crater on Mercurycrater on Mercury

Region opposite the Region opposite the Caloris Basin is Caloris Basin is jumbled from jumbled from seismic energy of seismic energy of impactimpact

Tectonics on Mercury

• Long cliffs indicate that Mercury shrank early in its history

• Long cliffs indicate that Mercury shrank early in its history…as the core cooled

• Long cliffs indicate that Mercury shrank early in its history…as the core cooled…because the cooler core was smaller Why does

this make sense?

• Long cliffs indicate that Mercury shrank early in its history…as the core cooled…because the cooler core was smaller…and dragged the surface down as it shrank

• Was there ever geological activity on the Moon or Mercury?— Early cratering on Moon and Mercury is still

present, indicating that geological activity ceased long ago.

— Lunar maria resulted from early volcanism.

— Lunar maria resulted from early volcanism.— Tectonic features on Mercury indicate early

shrinkage.

7.3 Mars: A Victim of Planetary Freeze-drying

• What geological features tell us that water once flowed on Mars?

• Why did Mars change?

Mars versus Earth

• 50% Earth’s radius, 10% Earth’s mass

Mars versus Earth

• 50% Earth’s radius, 10% Earth’s mass• 1.5 AU from the Sun

Mars versus Earth

• 50% Earth’s radius, 10% Earth’s mass• 1.5 AU from the Sun• Axis tilt about the same as Earth

Mars versus Earth

• 50% Earth’s radius, 10% Earth’s mass• 1.5 AU from the Sun• Axis tilt about the same as Earth• Similar rotation period

Mars versus Earth

• 50% Earth’s radius, 10% Earth’s mass• 1.5 AU from the Sun• Axis tilt about the same as Earth• Similar rotation period• Thin CO2 atmosphere: little greenhouse

Mars versus Earth

• 50% Earth’s radius, 10% Earth’s mass• 1.5 AU from the Sun• Axis tilt about the same as Earth• Similar rotation period• Thin CO2 atmosphere: little greenhouse

• Main difference: Mars is SMALLER

Seasons on Mars

• Seasons on Mars are more extreme in the southern hemisphere because of its elliptical orbit.

Seasons on Mars

• Strong pole-to-pole winds occur when CO2 freezes out of the atmosphere at the winter pole (pressure ↓) and sublimates at the summer pole (pressure ↑)

Seasons on Mars

• Strong pole-to-pole winds occur when CO2 freezes out of the atmosphere at the winter pole (pressure ↓) and sublimates at the summer pole (pressure ↑)

• The winds are caused by the pressure differences

Storms on Mars

• Those seasonal winds on Mars can drive huge dust storms.

The surface of Mars has signs of geological activityand

of past liquid water

The surface of Mars appears to have ancient riverbeds.

The condition of craters indicates surface history.

Eroded crater

Close-up of eroded crater

Volcanoes…as recent as 180 million years ago…

Past tectonic activity…

Low-lying regions may once have had oceans.

OpportunitySpirit

Today, most water lies frozen underground (blue regions)

Some scientists believe accumulated snowpack melts carve gullies even today.

• 2004 Opportunity Rover provided strong evidence for abundant liquid water on Mars in the distant past.

• How could Mars have been warmer and wetter in the past?

Why did Mars change?

Climate Change on Mars• Mars has not had

widespread surface water for 3 billion years.

• The greenhouse effect probably kept the surface warmer before that.

• Which means it must have had thicker atmosphere

• So what happened?

Climate Change on Mars

• Magnetic field may have preserved early Martian atmosphere

• Magnetic field may have preserved early Martian atmosphere…How do magnetic fields do that?

• Magnetic field may have preserved early Martian atmosphere

• Solar wind may have stripped atmosphere after the interior cooled and the field decreased

• What geological features tell us water once flowed on Mars?— Dry riverbeds, eroded craters, and rock-strewn

floodplains all show that water once flowed on Mars.

— Mars today has ice, underground water ice, and perhaps pockets of underground liquid water.

• Why did Mars change?— Mars’ atmosphere must have once been much

thicker for its greenhouse effect to allow liquid water on the surface.

— Somehow Mars lost most of its atmosphere, perhaps because of a declining magnetic field.

7.4 Venus: A Hothouse World

• Is Venus geologically active?

• Why is Venus so hot?

Is Venus geologically active?

Cratering on Venus

• Impact craters, but fewer than Moon, Mercury, Mars

Volcanoes on Venus

• Many volcanoes, including both shield volcanoes and stratovolcanoes

Tectonics on Venus

• Fractured and contorted surface indicates tectonic stresses

Erosion on Venus

• Photos of rocks taken by lander show little erosion

Does Venus have plate tectonics?

• Most of Earth’s major geological features can be attributed to plate tectonics, which gradually remakes Earth’s surface.

• Venus does not appear to have plate tectonics, but from crater counts its entire surface seems to have been “repaved” ~750 million years ago.

• No one really understands the geology of Venus

Why is Venus so hot?

The greenhouse effect on Venus keeps its surface temperature at 470°C (880°F).

But why is the greenhouse effect on Venus so much stronger than on Earth?

Atmosphere of Venus• Venus has a very

thick carbon dioxide atmosphere with a surface pressure 90 times that of Earth.

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 are in rocks and oceans.

Atmosphere of Venus• Reflective clouds

contain droplets of sulfuric acid.

• The upper atmosphere has fast winds that remain unexplained.

• Though Venus should have as much water as Earth, there is very little water there

Runaway Greenhouse Effect

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

• Once the water was in the atmosphere, solar UV split it

Greater heat, more evaporation

More evaporation, stronger greenhouse effect

End of Lecture

24 Mar 2008

• Is Venus geologically active?— Its surface shows evidence of major volcanism and

tectonics during the last billion years.

— There is no evidence for erosion or plate tectonics.

• Why is Venus so hot?— The runaway greenhouse effect made Venus too hot

for liquid oceans.

— All carbon dioxide remains in the atmosphere, leading to a huge greenhouse effect.

7.5 Earth as a Living Planet

• What unique features on Earth are important for human life?

• How is human activity changing our planet?

• What makes a planet habitable?

What unique features of Earth are important for life?

1. Surface liquid water

2. Atmospheric oxygen

3. Plate tectonics

4. Climate stability

What unique features of Earth are important to human life?

3. Plate tectonics

Earth’s distance from the Sun and moderate greenhouse effect make liquid water possible.

3. Plate tectonics

4. Climate stabilityPHOTOSYNTHESIS (plant life) is required to make high concentrations of O2, which produces the protective layer of O3.

3. Plate tectonics

Plate tectonics are an important step in the carbon dioxide cycle.

Continental Motion

• Motion of continents can be measured with GPS

Continental Motion• Idea of continental drift

was inspired by puzzle-like fit of continents

• The continents have moved apart because of the upwelling of magma from the upper mantle along mid-ocean ridges

• This also recycles the seafloor and provides the basis for the carbon dioxide cycle

Seafloor Recycling

• Seafloor is recycled through a process known as subduction

Seafloor Recycling

• Seafloor is recycled through a process known as subduction

• Subduction also causes volcanoes and earthquakes

Plate Tectonics

• The convection drives plate tectonics, acting like a conveyor belt to move the plates of the Earth’s crust around

Plate Motions

• Measurements of plate motions tell us past and future layout of continents

Carbon Dioxide Cycle

1. Atmospheric CO2 dissolves in rainwater.

2. Rain erodes minerals that flow into the ocean.

3. Minerals combine with carbon to make rocks on ocean floor.

Carbon Dioxide Cycle

4. Subduction carries carbonate rocks down into the mantle.

5. Rock melts in mantle and outgasses CO2 back into atmosphere through volcanoes.

3. Plate tectonics

4. Climate stability The CO2 cycle acts like a thermostat for Earth’s temperature.

These unique features are intertwined:

• Plate tectonics create climate stability

• Climate stability allows liquid water

• Liquid water is necessary for life

• Life is necessary for atmospheric oxygen

How is human activity changing our planet?

Dangers of Human Activity

• Human-made CFCs in the atmosphere destroy ozone, reducing protection from UV radiation.

• Human activity is driving many other species to extinction.

• Human use of fossil fuels produces greenhouse gases that can cause global warming.

Global Warming

• Earth’s average temperature has increased by 0.5°C in the past 50 years.

• The concentration of CO2 is rising rapidly.

• An unchecked rise in greenhouse gases will eventually lead to global warming.

CO2 Concentration

• Global temperatures have tracked CO2 concentration for the last 500,000 years.

• Antarctic air bubbles indicate the current CO2 concentration is at its highest level in at least 500,000 years.

CO2 Concentration

• Most of the CO2 increase above previous peaks has happened in last 50 years

Modeling of Climate Change• Complex models of global

warming suggest that the recent temperature increase is indeed consistent with human production of greenhouse gases.

• And the increase is continuing, causing “calving” of vast ice sheets in Antarctica.

• If we aren’t careful, it’s quite possible that while Earth will still be habitable, it will be a very different place to inhabit.

• Which brings up the question: What is a “habitable planet”?

What makes a planet habitable?

• Located at an optimal distance from the Sun for liquid water to exist

What makes a planet habitable?

• Large enough for geological activity to release and retain water and atmosphere

Planetary Destiny

Earth is habitable because it is large enough to remain geologically active, and it is at the right distance from the Sun so oceans could form.

• What unique features of Earth are important for life?— Surface liquid water — Atmospheric oxygen— Plate tectonics— Climate stability

• How is human activity changing our planet?— Human activity is releasing carbon dioxide

into Earth’s atmosphere, increasing the greenhouse effect and producing global warming.

• What makes a planet habitable?— Earth’s distance from the Sun allows for

liquid water on Earth’s surface.— Earth’s size allows it to retain an atmosphere

and enough internal heat to drive geological activity.

chapter 7 earth and the terrestrial worlds. mercury craters smooth plains cliffs

density slide

radioactivity slide

water molecules

oil molecules

oil floats

oil separate

rigid rock

twin peaked volcano

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