Chapter 7 Earth and the Terrestrial Worlds

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Chapter 7Earth and the Terrestrial Worlds
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Mercurycraterssmooth plainscliffs
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Venusvolcanoesfew craters
Radar view of a twin-peaked volcano
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Marssome cratersvolcanoesriverbeds?
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Mooncraterssmooth plains
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Earthvolcanoescratersmountainsriverbeds
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Why have the planets turned out so differently,
even though they formed at the same time from the
same materials?
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9.1 Earth as a Planet

• Our goals for learning
• Why is Earth geologically active?
• What processes shape Earths surface?
• How does Earths atmosphere affect the planet?

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Why is Earth geologically active?
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Earths 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 Earths
  interior to other planets tells us what their
  interiors are probably like.

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Why do water and oil separate?

• Water molecules repel oil molecules electrically.
• Water is denser than oil, so oil floats on water.
• Oil is more slippery than water, so it slides to
  the surface of the water.
• Oil molecules are bigger than the spaces between
  water molecules.

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Why do water and oil separate?

• Water molecules repel oil molecules electrically.
• Water is denser than oil, so oil floats on water.
• Oil is more slippery than water, so it slides to
  the surface of the water.
• Oil molecules are bigger than the spaces between
  water molecules.

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Differentiation

• Gravity pulls high-density material to center
• Lower-density material rises to surface
• Material ends up separated by density

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Thought Question

• What is necessary for differentiation to occur
  in a planet?
• It must have metal and rock in it.
• It must be a mix of materials of different
  density.
• Material inside must be able to flow.
• All of the above.
• b and c.

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Thought Question

• What is necessary for differentiation to occur
  in a planet?
• It must have metal and rock in it.
• It must be a mix of materials of different
  density.
• Material inside must be able to flow.
• All of the above.
• b and c.

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Lithosphere

• A planets outer layer of cool, rigid rock is
  called the lithosphere.
• It floats on the warmer, softer rock that lies
  beneath.

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Thought Question

• Do rocks s-t-r-e-t-c-h?
• Norock is rigid and cannot deform without
  breaking.
• Yesbut only if it is molten rock.
• Yesrock under strain may slowly deform.

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Thought Question

• Do rocks s-t-r-e-t-c-h?
• Norock is rigid and cannot deform without
  breaking.
• Yesbut only if it is molten rock.
• Yesrock under strain may slowly deform.

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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.

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Heat Drives Geological Activity

• Convection hot rock rises, cool rock falls.
• One convection cycle takes 100 million years on
  Earth.

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Sources of Internal Heat

• Gravitational potential energy of accreting
  planetesimals
• Differentiation
• Radioactivity

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Heating of Interior over Time

• Accretion and differentiation when planets were
  young
• Radioactive decay is most important heat source
  today

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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

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Thought Question

• What cools off faster?
• A grande-size cup of Starbucks coffee
• A teaspoon of cappuccino in the same cup

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Thought Question

• What cools off faster?
• A grande-size cup of Starbucks coffee
• A teaspoon of cappuccino in the same cup

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Thought Question

• What cools off faster?
• A big terrestrial planet
• A tiny terrestrial planet

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Thought Question

• What cools off faster?
• A big terrestrial planet
• A tiny terrestrial planet

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Role of Size

• Smaller worlds cool off faster and harden
  earlier.
• Moon and 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

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

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Planetary Magnetic Fields

• Moving charged particles create magnetic fields.
• A planets interior can create magnetic fields if
  its core is electrically conducting, convecting,
  and rotating.

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Earths Magnetosphere

• Earths magnetic fields protects us from charged
  particles from the Sun.
• The charged particles can create aurorae
  (Northern lights).

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Thought Question

• If the planet core is cold, do you expect it to
  have magnetic fields?
• Yes, refrigerator magnets are cold, and they have
  magnetic fields.
• No, planetary magnetic fields are generated by
  moving charges around, and if the core is cold,
  nothing is moving.

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Thought Question

• If the planet core is cold, do you expect it to
  have magnetic fields?
• Yes, refrigerator magnets are cold, and they have
  magnetic fields.
• No, planetary magnetic fields are generated by
  moving charges around, and if the core is cold,
  nothing is moving.

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Special TopicHow do we know whats inside a
planet?

• P waves push matter back and forth.
• S waves shake matter side to side.

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Special TopicHow do we know whats inside a
planet?

• P waves go through Earths core, but S waves do
  not.
• We conclude that Earths core must have a liquid
  outer layer.

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What processes shape Earths surface?
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Geological Processes

• Impact cratering
• Impacts by asteroids or comets
• Volcanism
• Eruption of molten rock onto surface
• Tectonics
• Disruption of a planets surface by internal
  stresses
• Erosion
• Surface changes made by wind, water, or ice

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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.

The Production of a Crater
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Impact Craters
Meteor Crater (Arizona)
Tycho (Moon)
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Volcanism

• Volcanism happens when molten rock (magma) finds
  a path through lithosphere to the surface.
• Molten rock is called lava after it reaches the
  surface.

Volcanic Eruptions and Lava Flows
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Lava and Volcanoes
Runny lava makes flat lava plains.
Slightly thicker lava makes broad shield
volcanoes.
Thickest lava makes steep stratovolcanoes.
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Outgassing

• Volcanism also releases gases from Earths
  interior into the atmosphere.

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Tectonics

• Convection of the mantle creates stresses in the
  crust called tectonic forces.
• Compression forces make mountain ranges.
• A valley can form where the crust is pulled
  apart.

Tectonics and Convection of the Mantle
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Plate Tectonics on Earth

• Earths continents slide around on separate
  plates of crust.

Plate Tectonics on Earth
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Erosion

• Erosion is a blanket term for weather-driven
  processes that break down or transport rock.
• Processes that cause erosion include
• Glaciers
• Rivers
• Wind

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Erosion by Water

• The Colorado River continues to carve the Grand
  Canyon.

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Erosion by Ice

• Glaciers carved the Yosemite Valley.

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Erosion by Wind

• Wind wears away rock and builds up sand dunes.

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Erosional Debris

• Erosion can create new features by depositing
  debris.

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How does Earths atmosphere affect the planet ?
Which Molecules are Greenhouse Gases?
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Effects of Atmosphere on Earth

• Erosion
• Radiation protection
• Greenhouse effect
• Makes the sky blue!

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Radiation Protection

• All X-ray light is absorbed very high in the
  atmosphere.
• Ultraviolet light is absorbed by ozone (O3).

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The Greenhouse Effect
Which Molecules are Greenhouse Gases?
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Earths atmosphere absorbs light at most
wavelengths.
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Greenhouse effect Certain molecules let
sunlight through but trap escaping infrared
photons. (H2O, CO2, CH4)
The Green House Effect
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Thought Question

• Why is the sky blue?
• The sky reflects light from the oceans.
• Oxygen atoms are blue.
• Nitrogen atoms are blue.
• Air molecules scatter blue light more than red
  light.
• Air molecules absorb red light.

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Thought Question

• Why is the sky blue?
• The sky reflects light from the oceans.
• Oxygen atoms are blue.
• Nitrogen atoms are blue.
• Air molecules scatter blue light more than red
  light.
• Air molecules absorb red light.

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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)

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Greenhouse Effect Bad?

• The Earth is much warmer because of the
  greenhouse effect than it would be without an
  atmospherebut so is Venus.

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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.

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What have we learned?

• Why is Earth geologically active?
• Earth retains plenty of internal heat because 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 Earths magnetic field.

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What have we learned?

• What geological processes shape Earths surface?
• Impact cratering, volcanism, tectonics, and
  erosion
• How does Earths atmosphere affect the planet?
• Erosion
• Protection from radiation
• Greenhouse effect

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7.2 Mercury and the Moon Geologically Dead

• Our goals for learning
• Was there ever geological activity on the Moon or
  Mercury?

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

• Some volcanic activity 3 billion years ago must
  have flooded lunar craters, creating lunar maria.
• The Moon is now geologically dead.

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Cratering of Mercury

• Mercury has a mixture of heavily cratered and
  smooth regions like the Moon.
• The smooth regions are likely ancient lava flows.

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Cratering of Mercury
Region opposite the Caloris Basin is jumbled from
seismic energy of impact
The Caloris basin is the largest impact crater on
Mercury
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Tectonics on Mercury

• Long cliffs indicate that Mercury shrank early in
  its history.

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What have we learned?

• Was there ever geological activity on the Moon or
  Mercury?
• Early cratering on Moon and Mercury is still
  present, indicating that activity ceased long
  ago.
• Lunar maria resulted from early volcanism.
• Tectonic features on Mercury indicate early
  shrinkage.

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7.3 Mars A Victim of Planetary Freeze-drying

• Our goals for learning
• What geological features tell us that water once
  flowed on Mars?
• Why did Mars change?

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Mars versus Earth

• 50 Earths radius, 10 Earths 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

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Seasons on Mars

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

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Storms on Mars

• Seasonal winds on Mars can drive huge dust storms.

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What geological features tell us water once
flowed on Mars?
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The surface of Mars appears to have ancient
riverbeds.
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Eroded crater
The condition of craters indicates surface
history.
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Close-up of eroded crater
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Volcanoesas recent as 180 million years ago
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Past tectonic activity
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Low-lying regions may once have had oceans.
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Low-lying regions may once have had oceans.
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Opportunity
Spirit
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• 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?

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Today, most water lies frozen underground (blue
regions) Some scientists believe accumulated
snowpack melts carve gullies even today.
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Why did Mars change?
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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.
• Somehow Mars lost most of its atmosphere.

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Climate Change on Mars

• Magnetic field may have preserved early Martian
  atmosphere.
• Solar wind may have stripped atmosphere after
  field decreased because of interior cooling.

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What have we learned?

• 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.

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7.4 Venus A Hothouse World

• Our goals for learning
• Is Venus geologically active?
• Why is Venus so hot?

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Is Venus geologically active?
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Cratering on Venus

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

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Volcanoes on Venus

• Many volcanoes, including both shield volcanoes
  and stratovolcanoes

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Tectonics on Venus

• Fractured and contorted surface indicates
  tectonic stresses

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Erosion on Venus

• Photos of rocks taken by lander show little
  erosion

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Does Venus have plate tectonics?

• Most of Earths major geological features can be
  attributed to plate tectonics, which gradually
  remakes Earths surface.
• Venus does not appear to have plate tectonics,
  but its entire surface seems to have been
  repaved 750 million years ago.

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Why is Venus so hot?
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Why is Venus so hot?

• The greenhouse effect on Venus keeps its surface
  temperature at 470C.
• But why is the greenhouse effect on Venus so much
  stronger than on Earth?

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Atmosphere of Venus

• Venus has a very thick carbon dioxide atmosphere
  with a surface pressure 90 times that of Earth.

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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 are in rocks and oceans.

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Atmosphere of Venus

• Reflective clouds contain droplets of sulfuric
  acid.
• The upper atmosphere has fast winds that remain
  unexplained.

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Runaway Greenhouse Effect
More evaporation, stronger greenhouse effect
Greater heat, more evaporation

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

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Thought Question

• What is the main reason why Venus is hotter than
  Earth?
• Venus is closer to the Sun than Earth.
• Venus is more reflective than Earth.
• Venus is less reflective than Earth.
• The greenhouse effect is much stronger on Venus
  than on Earth.
• Human activity has led to declining temperatures
  on Earth.

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Thought Question

• What is the main reason why Venus is hotter than
  Earth?
• Venus is closer to the Sun than Earth.
• Venus is more reflective than Earth.
• Venus is less reflective than Earth.
• The greenhouse effect is much stronger on Venus
  than on Earth.
• Human activity has led to declining temperatures
  on Earth.

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What have we learned?

• 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.

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7.5 Earth as a Living Planet

• Our goals for learning
• What unique features on Earth are important for
  human life?
• How is human activity changing our planet?
• What makes a planet habitable?

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What unique features of Earth are important for
life?

• Surface liquid water
• Atmospheric oxygen
• Plate tectonics
• Climate stability

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What unique features of Earth are important to
human life?
Earths distance from the Sun and moderate
greenhouse effect make liquid water possible.

• Surface liquid water
• Atmospheric oxygen
• Plate tectonics
• Climate stability

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What unique features of Earth are important to
human life?

• Surface liquid water
• Atmospheric oxygen
• Plate tectonics
• Climate stability

PHOTOSYNTHESIS (plant life) is required to make
high concentrations of O2, which produces the
protective layer of O3.
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What unique features of Earth are important to
human life?

• Surface liquid water
• Atmospheric oxygen
• Plate tectonics
• Climate stability

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

• Motion of continents can be measured with GPS

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Continental Motion

• Idea of continental drift was inspired by
  puzzle-like fit of continents
• Mantle material erupts where seafloor spreads

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Seafloor Recycling

• Seafloor is recycled through a process known as
  subduction

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Plate Motions

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

Plate Tectonics on Earth
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Carbon Dioxide Cycle

• Atmospheric CO2 dissolves in rainwater.
• Rain erodes minerals that flow into the ocean.
• Minerals combine with carbon to make rocks on
  ocean floor.

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Carbon Dioxide Cycle

• Subduction carries carbonate rocks down into the
  mantle.
• Rock melts in mantle and outgases CO2 back into
  atmosphere through volcanoes.

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Long-Term Climate Change

• Changes in Earths axis tilt might lead to ice
  ages.
• Widespread ice tends to lower global temperatures
  by increasing Earths reflectivity.
• CO2 from outgassing will build up if oceans are
  frozen, ultimately raising global temperatures
  again.

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What unique features of Earth are important to
human life?

• Surface liquid water
• Atmospheric oxygen
• Plate tectonics
• Climate stability

The CO2 cycle acts like a thermostat for Earths
temperature.
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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 many other connections between these can you
think of?
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How is human activity changing our planet?
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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.

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Global Warming

• Earths average temperature has increased by
  0.5C in the past 50 years.
• The concentration of CO2 is rising rapidly.
• An unchecked rise in greenhouse gases will
  eventually lead to global warming.

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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.

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CO2 Concentration

• Most of the CO2 increase has happened in last 50
  years!

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Modeling of Climate Change

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

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What makes a planet habitable?

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

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What makes a planet habitable?

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

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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.

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What have we learned?

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

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What have we learned?

• How is human activity changing our planet?
• Human activity is releasing carbon dioxide into
  Earths atmosphere, increasing the greenhouse
  effect and producing global warming.
• What makes a planet habitable?
• Earths distance from the Sun allows for liquid
  water on Earths surface.
• Earths size allows it to retain an atmosphere
  and enough internal heat to drive geological
  activity.