Chapter 7 Earth and the Terrestrial Worlds

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Chapter 7 Earth and the Terrestrial Worlds

• Understanding the similarities and differences
  between the planets of the solar system, in
  particular, the four terrestrial planets, can
  tell us how Earth becomes the way it is today.
• The similarities and differences of the
  terrestrial worlds.
• The small terrestrial worlds Mercury and our
  Moon.
• The large terrestrial worlds Mars, Venus, and
  Earth.
• What makes the environment on Earth suitable for
  life?
• Mars?
• Whats the future of Earth?

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Similarities and Differences of the Terrestrial
Worlds

• From a distance, they appear very similar
• rocky and small (we really cant see the surface
  of Venus directly) !
• No or few moons
• No rings

• Examined close-up, They are very different
• Mercury and Earths Moon are airless and barren
• Mars has a very thin atmosphere
• Earth has oxygen, water, and life!
• Venus has a thick atmosphere and very hot!

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• Internal Structure
• Surface Features
• Atmosphere
• What makes the Earth hospitable to life?
• Venus
• Mars
• Global Warming?

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Internal Structure of the Terrestrial Planets

• The internal structure of the terrestrial planets
  are similar. They all have
• Core High density metal
• Mantle Medium density rocky materials, such as
  silica (SiO2), hot, semi-solid
• Crust lowest density rocks, such as granite and
  basalt (black lava rock)
• The layering of different density materials
  occurs due to differentiation heavy materials
  sink to the bottom while lighter material rise to
  the top
• Lithosphere The coolest and most rigid layer of
  rock near a planets surface.
• Molten lava of Earth exists at a very
  narrow region beneath the lithosphere

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Inside the Earth More Details
Lava comes from a thin layer under the
lithosphere
Illustration by J. C. Butler http//www.uh.edu/jb
utler/physical/chapter19.html
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Heating of the Terrestrial Planets

• The interiors of the terrestrial planets are
  heated by
• Gravitational potential energy of the accreting
  planetesimals are converted into thermal energy.
• Radioactive heating
• Radioactive Heating
• Radioactive materials (e.g., uranium, potassium,
  thorium) decay by emitting subatomic particles
  (alpha particlenuclei of helium, beta particle
  electrons or positron, neutron, proton, etc.) and
  often gamma-ray, which collide with surrounding
  atoms, heating them up.
• Potassium-40 ? Argon-40
• Uranium-234 ? . ? Lead-206

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Earths Magnetic Field

• Another important characteristics of the Earth
  is its magnetic fields, which shield us from the
  bombardment of the high-energy charged particles,
  mostly from the Sun.
• The rapid rotating liquid outer metal core of
  Earth generate magnetic field.
• The charged particles from the Sun must move
  along the magnetic field lines, and are directed
  to the north and south polar regions. The
  interactions between the charged particles and
  the molecules of the atmosphere cause the glow of
  atmosphere near the north and south poles ?
  aurora borealis and aurora australis
• Without magnetic field, solar wind can strip much
  of the Earths atmosphere

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Building a Magnet

• We can generate magnetic field by circulating
  electric charges (running a electric current) in
  a spiral path.
• We do not have a complete theory of how the
  magnetic field of the Earth is formed yetbut
  generally, it is believed to form by the rotation
  of the Earth carries the electrically conducting
  molten metals in the core around, generating
  Earths magnetic field.

Lines indicate points with equal magnetic field
strength
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Reaching Inside the Earth

• We can study the interior structure of the Earth
  by studying how seismic waves travel through
  Earth
• Seismic waves propagate through Earth in two
  modes
• P wave Primary (Pressure, or Pushing) wave
• P wave can travel through any material.
• S wave Secondary (Shear, or side-to-side) wave.
• S wave cannot travel through liquid.

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• Internal Structure
• Surface Features
• Atmosphere
• What makes the Earth hospitable to life?
• Global Warming?

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Surface Features

• Processes shaping the surface of the planets
• Impact cratering the blasting of bowl-shaped
  impact craters by asteroids or comets striking a
  planets surface.
• Volcanism the eruption of molten rock, or lava,
  from a planets interior onto its surface.
• Tectonics the disruption of a planets surface
  by internal stresses.
• Erosion the wearing down or building up of
  geological features by wind, water, ice, and
  other phenomena of planetary weather.

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Impact Crater in Arizona
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Volcanism

• Volcanism occurs when underground molten rock
  finds a path through the lithosphere to the
  surface
• In addition to shaping the surface of the planet,
  it explains the existence of our atmosphere and
  oceanwater and gases trapped in the interior of
  Earth are released into the atmosphere through
  volcanic activities
• The best resultsthe formation of our island
  paradiseHawaii!

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Plate Tectonic

• Tectonics is particularly important on Earth,
  because the underlying mantle convection
  fractured Earths lithosphere into more than a
  dozen pieces, or plates. These plates move over,
  under, and around each other, leading to a
  special brand of tectonics that we call plate
  tectonics.

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Erosion

• Erosion is a blanket term for a variety of
  processes that break down or transport rock
  through the action of ice, liquid, or gas.
• The shaping of valleys by glaciers (ice),
• the carving of canyons by rivers (liquid),
• and the shifting of sand dunes by wind (gas)
• are all examples of erosion.
• Mauna Kea and Mauna Loa are very tall, but as we
  move toward the west, the volcanoes gets lower
  and lower

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• Internal Structure
• Surface Features
• Atmosphere
• What makes the Earth hospitable to life?
• Global Warming?

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The Atmosphere of the Terrestrial Worlds

• According to the Nebular Theory, the terrestrial
  planets were formed by metallic and rocky
  planetesimals. So,
• Where did the gas come from?
• The gases came from comets and asteroids impact
  during the period of heavy bombardment.
• The gases are trapped in the interior of the
  planets, later released through volcanic
  out-gassing.

• But, why are their atmosphere so different?
• How come Earth has so much H2O?
• How come Earth dont have much CO2?
• How come Earth has so much O2?

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Why Mercury and the Moon dont have an Atmosphere?

• Mercury and the Moon dont have an atmosphere
  because they are too small. Their weak
  gravitation field is not enough to keep the gas.
• Given the same temperature
• Light gases escape easily,
• Heavier gases are trapped by gravity

Given the same temperature, the thermal velocity
of lighter gases are higher compared with
velocity with heavier gases. Therefore, the
lighter gases have better chance of acquiring a
speed greater than the escape velocity of the
Earth and escape(remember that the escape
velocity of the Earth is about 14 km/sec, and
independent of the mass of the escaping object).
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Mercury and Earths Moon

• Similarities between Mercury and the Moon
• Size
• No Atmosphere
• Dense impact craters on the surface ? No
  geological activities to alter the surface
  features after the period of heavy bombardment
  they are geologically dead long time ago
• Large day/night temperature difference

Surface of Mercury looks very similar to the Moon

• The similarities between these two worlds can be
  explained by their small sizes
• Small size ? low surface gravity ? low escape
  velocity ? gas cannot be trapped by gravity on
  the surface.
• No atmosphere ? large day/night temperature
  difference
• Small size ? small initial heat content ? they
  cool off fast ? low level of geological activities

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Venus, Mars and Earth

• Venus and Earth are very similar in size, and
  Mars is a little smaller. Their surface features
  are somewhat similar also they both have few
  impact craters, and they all have volcanoes and
  evidence of tectonic activities.
• Although there is a very large difference in the
  amount of gases on Venus and Mars, their chemical
  compositions are very similar high percentage
  of CO2
• Among the three large terrestrial worlds, the
  surface environment of Earth is very unique.

• The surface of the Earth is characterized by
• Abundant surface liquid water
• Abundant atmospheric oxygen
• Plate tectonics
• Climate stability
• These are features that are very important to
  support life on Earth.

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Surface Temperature of the Terrestrial Worlds
The No Greenhouse Temperature of a planet
depends on its distance from the Sun, and its
albedo (or the reflectivity of a surface or a
body) However, the presence of an atmospheres
can drastically change the surface temperature of
a planet.
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The No Greenhouse Temperature

• Without an atmosphere, the surface materials of
  the planets absorb some of the visible light from
  the Sun.
• The temperature of the surface material
  increases, depending on the amount of energy it
  absorbs (the albedo).
• The planet surfaces re-radiates the absorbed
  energy in the form of thermal radiation.
• The re-radiated energy is equal to the absorbed
  energy.
• The amount of energy the planet surface radiates
  depends on its temperature. The equilibrium
  temperature is around a few hundred degree
  Kelvin
• Therefore, the surface of the planets emits in
  the infrared wavelength range

Objects with temperature of a few hundred degree
Kelvin emit thermal radiation in the IR
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How does the Atmosphere Affects the Environment
on the Surface?

• X-rays are absorbed by the atoms and molecules of
  the atmosphere at high altitude
• UV is absorbed by the ozone (O3) in the
  stratosphere
• Visible light reaches the ground and warms the
  surface.
• IR radiation is absorbed by the water vapor
• Radio waves are not affected by the atmosphere

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The Effects of the Atmosphere on Planet Surface
Temperature

• Depending on the composition of the atmosphere,
  the effect can be very different
• Venus 96 CO2, 3.5 N2
• ? T 740 K.
• Earth 77 N2, 21 O2, 1 Argon (dry air),
• Variable H2O ( 10...order-of-magnitude)
• Small amount of CO2 (0.03)
• ? 288 K.
• Why?
• Greenhouse Effect!