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Chapter 20
Planet Earth
Guidepost
Astronomy has been described as the science of
everything above the clouds. Planetary astronomers,
however, must also think about what lies below the
clouds because Earth is the basis for comparison with
all other Earthlike planets. We know Earth well, and we
can apply what we know about Earth to other worlds.
There is another reason for studying Earth in an
astronomy course. Astronomy is really about us.
Astronomy is exciting and fascinating because it helps
us understand what we are and where we are in the
universe. Thus, we cannot omit Earth from our
discussion—it is where we are.
The next two chapters will discuss the Earthlike planets,
but that will not end our thoughts of Earth. The moons of
Guidepost (continued)
the giant outer worlds will seem Earthlike in strange
ways, and our discussion of the smaller bodies of our
solar system will alert us to the dangers Earth faces.
Throughout the rest of this book, we will remain painfully
aware of the fragile beauty of our planet.
Outline
I. The Early History of Earth
A. Four Stages of Planetary Development
B. Earth as a Planet
II. The Solid Earth
A. Earth's Interior
B. The Magnetic Field
C. Earth's Active Crust
III. The Atmosphere
A. Origin of the Atmosphere
B. Human Effects on Earth's Atmosphere
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
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
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.
Seismic Waves
(SLIDESHOW MODE ONLY)
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
• 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
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)
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
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
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.
The Atmosphere
Earth had a primeval atmosphere from remaining
gasses captured during formation of Earth
Atmospheric
composition
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
Planetary Atmospheres
(SLIDESHOW MODE ONLY)
Human Effects on Earth’s Atmosphere
1) The Greenhouse Effect
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.
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
• 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!
Human Effects on the Atmosphere (2)
2) Destruction of the
Ozone Layer
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!
New Terms
comparative planetology
seismic waves
seismograph
pressure (P) waves
shear (S) waves
mantle
plastic
bow shock
magnetosphere
Van Allen belts
plate tectonics
midocean rise
subduction zone
basalt
folded mountain range
rift valley
primeval atmosphere
secondary atmosphere
ozone layer
albedo
greenhouse effect
global warming
Discussion Questions
1. If we visited a planet in another solar system and
discovered oxygen in its atmosphere, what might we
guess about its surface?
2. If liquid water is rare on the surface of planets, then
most terrestrial planets must have CO2-rich
atmospheres. Why?
Quiz Questions
1. What do we call the study of planets through contrast and
comparison?
a. Geology.
b. Planetary Science.
c. Contrastive Planetology.
d. Comparative Planetology.
e. None of the above.
Quiz Questions
2. When Earth formed it melted and differentiated. What was
the source of heat that melted Earth?
a. The infall of matter that formed Earth.
b. The decay of radioactive elements.
c. Sunlight striking Earth's surface.
d. Both a and b above.
e. All of the above.
Quiz Questions
3. With a small telescope we see that the Moon has regions
that are relatively smooth, and others that are saturated with
impact craters. What does this tell us about the Moon's
development?
a. The Moon never went through the differentiation stage.
b. The Moon never experienced surface flooding by lava.
c. The Moon doesn't have much slow surface evolution.
d. Both a and b above.
e. All of the above.
Quiz Questions
4. Earth's interior can be divided up into four zones: the inner
core, the outer core, the mantle, and the crust. Which of these
zones has the lowest density?
a. The inner core.
b. The outer core.
c. The mantle.
d. The crust.
e. All four zones have the same density.
Quiz Questions
5. What evidence do we have that Earth differentiated?
a. The curved paths of seismic waves indicate that the interior
density of Earth is greater than can be explained by
compression alone.
b. Deep wells have brought up iron fragments of Earth's core.
c. Diamond miners have to endure high temperatures in deep
mines.
d. The Earth-Moon system has a lot of angular momentum.
e. Aristotle's theory of the four elements requires this to be the
case.
Quiz Questions
6. How do P waves and S waves differ?
a. P waves oscillate parallel to their direction of travel, whereas
S waves oscillate perpendicular to their direction of travel.
b. P waves can pass through liquids, whereas S waves cannot.
c. S waves can pass through liquids, whereas P waves cannot.
d. Both a and b above.
e. Both a and c above.
Quiz Questions
7. How does temperature and pressure vary with depth deep
inside Earth?
a. Temperature and pressure both increase with depth.
b. Temperature and pressure both decrease with depth.
c. Temperature increases with depth, and pressure decreases
with depth.
d. Temperature decreases with depth, and pressure increases
with depth.
e. Temperature and pressure do not vary with depth inside the
Earth.
Quiz Questions
8. What distinguishes Earth's inner core from its outer core?
a. The inner core is rock and the outer core is iron-nickel.
b. The inner core is iron-nickel and the outer core is rock.
c. The inner core is liquid and the outer core is solid.
d. The inner core is solid and the outer core is liquid.
e. Both b and d above.
Quiz Questions
9. What creates Earth's strong dipole magnetic field?
a. The attractive force between massive particles.
b. The conduction of solar wind particles through solid Earth.
c. The conduction of solar wind particles around Earth's
ionosphere.
d. The Sun's magnetic field induces an opposing magnetic field
in Earth.
e. Convection in Earth's outer liquid iron-nickel core, combined
with Earth's rotation.
Quiz Questions
10. What erases the impact craters on Earth and is responsible
for most of the landforms that we see?
a. Plate tectonics.
b. Water and ice erosion.
c. Erosion by the solar wind.
d. Both a and b above.
e. Both b and c above.
Quiz Questions
11. What evidence do we have that Earth's outer core is a
metallic liquid?
a. Volcanic activity at Earth's surface.
b. S waves are blocked, resulting in an S-wave shadow zone.
c. Earth has a strong magnetic field.
d. Both a and b above.
e. Both b and c above.
Quiz Questions
12. What drives the moving plates of Earth's crust?
a. Convection in Earth's outer liquid iron core.
b. Convection in Earth's rocky mantle.
c. The Moon's tidal force.
d. Both a and b above.
e. Both b and c above.
Quiz Questions
13. Where are deep ocean trenches located relative to the
moving plates of Earth's crust?
a. Along the midocean rises.
b. At subduction zones.
c. Along strike-slip boundaries.
d. Both a and b above.
e. All of the above.
Quiz Questions
14. Where on Earth are two plates being pushed apart,
creating new crust?
a. Along the center of the Atlantic Ocean.
b. Along the center of the Red Sea.
c. Along the Indian plate / Asian plate boundary.
d. Both a and b above.
e. All of the above.
Quiz Questions
15. What type of tectonic zone is responsible for the Andes
Mountains that are located all along the western side of South
America?
a. A midocean rise.
b. A subduction zone.
c. A strike-slip zone.
d. The convergence of two continental plates.
e. None of the above.
Quiz Questions
16. What is the likely source of Earth's early atmosphere that
consisted of carbon dioxide and water vapor?
a. It was outgassed from Earth's interior during volcanic activity.
b. It arrived in the form of icy planetesimals that impacted
Earth.
c. These two gases are the products of radioactive decay of
long-lived radioactive elements in Earth's crust.
d. Both a and b above.
e. None of the above.
Quiz Questions
17. What happened to the majority of the carbon dioxide that
was formerly in Earth's atmosphere?
a. Most of it remains in the atmosphere today.
b. Most of it resides in living plants and animals.
c. Most was ionized and eroded away by the intense solar wind
of the youthful Sun.
d. Most was dissociated by ultraviolet photons, and the carbon
and oxygen escaped into space.
e. Most of it dissolved into the oceans and now is in the form of
limestone rocks of Earth's crust.
Quiz Questions
18. What adverse affect is human activity having on today's
atmosphere?
a. By burning coal, oil, and natural gas deposits we are
increasing the carbon dioxide content of the atmosphere.
b. By cutting down woodlands, we are increasing the amount of
carbon dioxide in the atmosphere.
c. We are releasing chlorofluorocarbons, a gas that breaks
down ozone molecules, into the atmosphere.
d. Both a and b above.
e. All of the above.
Quiz Questions
19. Why would a decrease in the density of the ozone layer
cause public health problems?
a. Humans need ozone, which is oxygen, for respiration.
b. Ozone is used in refrigerators to reduce bacteria levels in
food.
c. The ozone layer blocks ultraviolet light from the Sun, and
ultraviolet light causes skin cancer.
d. Both a and b above.
e. All of the above.
Quiz Questions
20. The 2000 US presidential election depended on the results
in the state of Florida, a state entirely less than 80 feet above
sea level. In the end the environmental candidate, Al Gore, lost
the election. Thus carbon dioxide continues to be added to the
atmosphere at a high rate. Why should Floridians be
concerned with what might called, "Gore's Revenge"?
a. Carbon dioxide is a greenhouse gas.
b. The greenhouse effect occurs when atmospheric gases
allow visible sunlight to penetrate and hinder the escape of
infrared radiation released from a planet's surface.
c. The greenhouse effect raises the temperature near the
surface of a planet.
d. If Earth's polar ice caps melt, sea level will rise a few
hundred feet.
e. All of the above.
Answers
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