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Chapter 7
Earth: The Standard of
Comparative Planetology
How the Earth was Formed?
Guidepost (1)
The planets formed from the solar nebula 4.6 billion years ago, and now
you are ready to visit the planets and get to know them as individuals.
That will help you confirm your understanding of the origin of the planets
and will reveal new principles of planetary evolution.
You begin your study with Earth, and that means you must see your
home world in a new way, not as a location for exciting vacations,
international trade, and strategic political agreements, but as a planet. In
this chapter, you will answer four essential questions:
• How does Earth fit among the terrestrial planets?
• How was Earth changed since it formed?
• What is the inside of Earth like?
• How was Earth’s atmosphere formed and evolved?
Guidepost (2)
Thinking about Earth’s interior will help you answer an important
question about science:
• How can scientists know about things they cannot see?
Like a mountain climber establishing a base camp before
attempting the summit, you will, in this chapter, establish your basis
of comparison on Earth. In the following chapters you will visit
worlds that are un-Earthly but, in many ways, familiar.
Outline
I.
A Travel Guide to the Terrestrial Planets
A. Five Worlds
B. Core, Mantle, and Crust
C. Atmospheres
II. The Early History of Earth
A. Four Stages of Planetary Development
B. Earth as a Planet
III. The Solid Earth
A. Earth's Interior
B. The Magnetic Field
C. Earth's Active Crust
IV. Earth’s Atmosphere
A. Origin of the Atmosphere
B. Human Effects on Earth’s Atmosphere
I. A Travel Guide to the Terrestrial Planets
A Travel Guide to the Terrestrial Planets
Similar internal
structures:
• Liquid heavymetal core
• Liquid mantle of
lighter rocks
• Solid Crust
Earth, Venus, and Mars have atmospheres, dominated by
similar physical processes (e.g., greenhouse effect).
Chapter Summary (1)
How does Earth fit among the terrestrial planets?
•Earth is the standard of comparative planetology in the study of the
terrestrial planets because we know it best and because it contains all
of the phenomena found on the other terrestrial planets.
•The terrestrial planets include Earth, the moon, Mercury, Venus, and
Mars. Earth’s moon is included because it is a complex world and
makes a striking comparison with Earth.
•The terrestrial worlds differ mainly in size, but they all have low-density
crusts, mantles of dense rock, and metallic cores.
•Comparative planetology warns you to expect that cratered surfaces
are old, that heat flowing out of a planet drives geological activity, and
that the nature of a planet’s atmosphere depends on the size of the
planet and its temperature.
Scientific Argument…..
• Why do you expect the inner planets to be
high-density worlds?
• What made of those craters?
II. The Early History of Earth
The Early History of Earth
Earth formed 4.6 billion years
ago from the inner solar nebula.
Four main stages of
evolution:
Two sources of heat in Earth’s
interior:
• Potential energy of infalling
material
• Decay of radioactive material
Most traces of bombardment
(impact craters) now destroyed
by later geological activity
Chapter Summary (2)
How has Earth changed since it formed?
•Earth formed rapidly from the solar nebula and was hot enough to be
molten.
•Earth has passed through four stages as it has evolved: (1)
differentiation; (2) cratering; (3) flooding by lava and water; and (4) slow
surface evolution.
•Earth is peculiar in that it has large amounts of liquid water on its
surface, and that water drives strong erosion that alters the surface
geology.
•Earth is also peculiar in that it is the only known home for life.
Scientific Argument…..
• Why should you think Earth went through an
early stage of cratering?
III. The Solid Earth
Earth’s Interior
Direct exploration of Earth’s interior (e.g. drilling) is impossible.
Earth’s interior can be explored through seismology:
earthquakes produce seismic waves.
Two types of seismic waves:
Pressure waves:
Shear waves:
Particles
vibrate back
and forth
Particles
vibrate up
and down
Seismology (Animation – From Book)
Seismic waves do not
travel through Earth in
straight lines or at
constant speed.
They are bent by or
bounce off transitions
between different
materials or different
densities or
temperatures.
Such information can
be analyzed to infer
the structure of
Earth’s interior.
Earth’s Interior (2)
Basic structure:
Solid crust
Solid mantle
Liquid core
Solid inner core
Earth’s interior gets hotter towards the center.
Earth’s core is as hot as the sun’s surface; metals are liquid.
Melting point
increases with
increasing pressure
Melting point =
temperature at which an towards the center
element melts (transition
from solid to liquid)
=> Inner core
becomes solid
Earth’s Magnetic Field (Video)
• Earth’s core consists
mostly of iron +
nickel: high electrical
conductivity
• Convective motions
and rotation of the
core generate a
dipole magnetic field
The Role of Earth’s Magnetic Field (Video)
Earth’s magnetic field protects Earth from high-energy
particles coming from the sun (solar wind).
Surface of first
interaction of solar
wind with Earth’s
magnetic field =
Bow shock
Region where
Earth’s magnetic
field dominates =
magnetosphere
Some high-energy particles leak through the magnetic field and produce
a belt of high-energy particles around Earth: Van Allen belts
The Aurora (Polar Light) - Video
As high-energy particles leak into the lower
magnetosphere, they excite molecules near the
Earth’s magnetic poles, causing the aurora
The Active Earth
About 2/3 of Earth’s
surface is covered
by water.
Mountains are
relatively rapidly
eroded away by the
forces of water.
Tectonic Plates: Video
Earth’s crust is composed of several distinct tectonic plates, which
are in constant motion with respect to each other  Plate tectonics
Evidence for plate tectonics can
be found on the ocean floor
… and in geologically active
regions all around the Pacific
Plate Tectonics: Video
Tectonic plates move with respect to each other.
Where plates move toward
each other, plates can be
pushed upward and downward Where plates move away
 formation of mountain ranges, from each other, molten
some with volcanic activity,
lava can rise up from
earthquakes
below  volcanic activity
Active Zones Resulting from Plate Tectonics
Volcanic hot spots due to
molten lava rising up at plate
boundaries or through holes in
tectonic plates
Earth’s Tectonic History -
History of Geological Activity
Surface formations visible today have emerged
only very recently compared to the age of Earth.
Chapter Summary (3)
What is the inside of Earth like?
•Seismic waves generated by earthquakes can be detected by
seismographs all over the world and can reveal Earth’s internal
structure.
•Pressure (P) waves can travel through a liquid, but shear (S) waves
cannot. Observations show that S waves cannot pass through Earth’s
core, and that is evidence that the core is liquid.
•Earth’s magnetic field is generated by the dynamo effect in the liquid,
convecting, rotating, conducting core.
•The magnetic field shields Earth from the solar wind by producing a
bow shock and a magnetosphere around the planet. Radiation belts
called the Van Allen belts and auroras are also produced by the field.
Chapter Summary (4)
•
Earth is dominated by plate tectonics that breaks the crust into
moving sections. Plate tectonics is driven by heat flowing upward
from the interior.
•
Tectonic plates are made of low-density, brittle rock that floats on
the hotter plastic upper layers of the mantle.
•
Volcanism and earthquakes are common along the edge of the
plates.
•
The continents are drifting slowly on the plastic mantle, and their
arrangement changes with time. Where they collide, they can form
folded mountain ranges.
•
Most geological features on Earth, such as mountain ranges and the
Grand Canyon, have been formed recently. The first billion years of
Earth’s geology are almost entirely erased by plate tectonics and
erosion.
Scientific Argument….
• What evidence indicates that Earth has a liquid
metal core?
List two evidences (indirect).
IV. Earth’s Atmosphere
The Atmosphere - Video
Earth had a primeval atmosphere from remaining
gasses captured during formation of Earth
Atmospheric
composition
was severely
altered (
secondary
atmosphere)
through a
combination of
two processes:
1) Outgassing: Release of
gasses bound in compounds in
the Earth’s interior through
volcanic activity
2) Later bombardment with icy
meteoroids and comets
The Structure of Earth’s Atmosphere
Composition of Earth’s
atmosphere is further
influenced by:
• Chemical reactions
in the oceans,
• Energetic radiation
from space (in
particular, UV)
The ozone
layer is
essential for life
on Earth since
it protects the
atmosphere
from UV
radiation
• Presence of life on Earth
The temperature of the atmosphere depends critically on its
albedo = percentage of sun light that it reflects back into space
Depends on many factors, e.g., abundance of water vapor in
the atmosphere
Human Effects on Earth’s Atmosphere
1) The Greenhouse Effect - Video
Earth’s surface is heated by the
sun’s radiation.
Heat energy is re-radiated from
Earth’s surface as infrared radiation.
CO2, but also other gases in the
atmosphere, absorb infrared light
 Heat is trapped in the
atmosphere.
This is the Greenhouse Effect.
The Greenhouse Effect occurs naturally
and is essential to maintain a
comfortable temperature on Earth,
but human activity, in particular
CO2 emissions from cars and
industrial plants, is drastically
increasing the concentration of
greenhouse gases.
Do You Know
What is
Global
Warming?
Global Warming
• Human activity (CO2 emissions + deforestation) is
drastically increasing the concentration of
greenhouse gases.
• As a consequence, beyond any reasonable doubt,
the average temperature on Earth is increasing.
• This is called Global Warming - Video
• Leads to melting of glaciers and polar ice caps
( rising sea water levels) and global climate
changes, which could ultimately make Earth
unfit for human life!
What would happen if
Earth were to be
6 degrees WARMER?
Human Effects on the Atmosphere (2)
2) Destruction of the Ozone Layer - Video
Ozone (= O3) absorbs UV radiation,
(which has damaging effects on
human and animal tissue).
Chlorofluorocarbons (CFCs) (used,
e.g., in industrial processes,
refrigeration and air conditioning)
destroy the Ozone layer.
Destruction of the ozone layer as a
consequence of human activity is
proven (e.g., growing ozone hole
above the Antarctic);
Must be stopped and reversed by
reducing CFC use, especially in
developed countries!
Video:
Ozone 2004-2010 (NASA OMI daily image animation)
Several Common Misconceptions….
• The greenhouse effect is bad.
• Human output of CO2 is minor compared to natural sources
such as volcanoes.
• The observed warming of Earth is due to natural causes
rather than the greenhouse effect.
• All Earth will warm at the same rate if there is global warming.
• Ozone is bad because people hear it mentioned as a pollutant
of city air, produced by auto emissions.
• Global warming and ozone depletion are two names for the
same thing….
Scientific Argument….
• Why does Earth’s atmosphere contain little
carbon dioxide and lots of oxygen?
Chapter Summary (1)
How does Earth fit among the terrestrial planets?
•Earth is the standard of comparative planetology in the study of the
terrestrial planets because we know it best and because it contains all
of the phenomena found on the other terrestrial planets.
•The terrestrial planets include Earth, the moon, Mercury, Venus, and
Mars. Earth’s moon is included because it is a complex world and
makes a striking comparison with Earth.
•The terrestrial worlds differ mainly in size, but they all have low-density
crusts, mantles of dense rock, and metallic cores.
•Comparative planetology warns you to expect that cratered surfaces
are old, that heat flowing out of a planet drives geological activity, and
that the nature of a planet’s atmosphere depends on the size of the
planet and its temperature.
Chapter Summary (2)
How has Earth changed since it formed?
•Earth formed rapidly from the solar nebula and was hot enough to be
molten.
•Earth has passed through four stages as it has evolved: (1)
differentiation; (2) cratering; (3) flooding by lava and water; and (4) slow
surface evolution.
•Earth is peculiar in that it has large amounts of liquid water on its
surface, and that water drives strong erosion that alters the surface
geology.
•Earth is also peculiar in that it is the only known home for life.
Chapter Summary (3)
What is the inside of Earth like?
•Seismic waves generated by earthquakes can be detected by
seismographs all over the world and can reveal Earth’s internal
structure.
•Pressure (P) waves can travel through a liquid, but shear (S) waves
cannot. Observations show that S waves cannot pass through Earth’s
core, and that is evidence that the core is liquid.
•Earth’s magnetic field is generated by the dynamo effect in the liquid,
convecting, rotating, conducting core.
•The magnetic field shields Earth from the solar wind by producing a
bow shock and a magnetosphere around the planet. Radiation belts
called the Van Allen belts and auroras are also produced by the field.
Chapter Summary (4)
•
Earth is dominated by plate tectonics that breaks the crust into
moving sections. Plate tectonics is driven by heat flowing upward
from the interior.
•
Tectonic plates are made of low-density, brittle rock that floats on
the hotter plastic upper layers of the mantle.
•
Volcanism and earthquakes are common along the edge of the
plates.
•
The continents are drifting slowly on the plastic mantle, and their
arrangement changes with time. Where they collide, they can form
folded mountain ranges.
•
Most geological features on Earth, such as mountain ranges and the
Grand Canyon, have been formed recently. The first billion years of
Earth’s geology are almost entirely erased by plate tectonics and
erosion.
Chapter Summary (5)
How has Earth’s atmosphere formed and evolved?
•Because Earth formed hot, it never had a primeval atmosphere.
•Because Earth formed in a molten state, its first atmosphere was
probably mostly carbon dioxide, nitrogen, and water vapor. Most of the
carbon dioxide was dissolved in seawater, and plant life has added
oxygen.
•The greenhouse effect can warm a planet if gases such as carbon
dioxide in the atmosphere are transparent to light but opaque to
infrared.
•The natural greenhouse effect warms Earth and makes it comfortable
for life, but greenhouse gases added by industrial civilization are
responsible for global warming.
Chapter Summary (6)
•
The ozone layer high in Earth’s atmosphere protects the surface
from ultraviolet radiation, but certain chemicals called
chlorofluorocarbons released in industrial processes attack the
ozone layer and thin it. This is allowing more harmful ultraviolet
radiation to reach Earth’s surface.