Download Chap 20 Earth and Comparative Planetology

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 towards the center
at which an 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
Where plates move away
upward and downward 
formation of mountain ranges, from each other, molten lava
can rise up from below 
some with volcanic activity,
earthquakes
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.
Earth’s atmosphere
• Layer of gasses surrounding the
Earth held in place by gravity
• Shields the sun’s ultraviolet rays
• Retains heat (greenhouse effect
• Reduces temperature extremes
between day and night
Earth’s Atmosphere
consists of four major
layers
Top layer is the Exosphere which
includes the magnetosphere but not
counted among the major layers
Most of our weather happens in the
troposphere. 80% of total mass of
atmosphere. 8-16 km deep.
In stratosphere, about 25 km from the
surface of the Earth is the ozone layer
which protects the Earth from most of
the Sun’s harmful UV rays. Lower
portion is influenced by jet streams
Mesosphere has coldest temperatures
of about -90°C
Thermosphere has greatest
temperature range from -90° - 1200° C
The Ionosphere
• Layer that stretches between the lower
mesosphere and into the exosphere, running
contiguously with the thermosphere.
• 4 levels that reflect different types of radio
waves
– D: from 60-80 km reflects radio waves
– E: from 80-120 km reflects short wave
– F1: around 170km
– F2: roughly from 250-300km
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
1. In which layer of the atmosphere does
the weather occur?
a.
b.
c.
d.
Mesosphere
Troposphere
Thermosphere
Stratosphere
1. In which layer of the atmosphere does
the weather occur?
a.
b.
c.
d.
Mesosphere
Troposphere
Thermosphere
Stratosphere
2. A little less than 20% of the
atmosphere’s mass is in the __
a.
b.
c.
d.
Stratosphere
Troposphere
Mesosphere
Thermosphere
2. A little less than 20% of the
atmosphere’s mass is in the __
a.
b.
c.
d.
Stratosphere
Troposphere
Mesosphere
Thermosphere
3. The greatest atmospheric
temperature range is found in the
a.
b.
c.
d.
Stratosphere
Troposphere
Mesosphere
Thermosphere
3. The greatest atmospheric
temperature range is found in the
a.
b.
c.
d.
Stratosphere
Troposphere
Mesosphere
Thermosphere
4. The primary gases in the
atmosphere are
a.
b.
c.
d.
Nitrogen, oxygen, carbon dioxide
Nitrogen, oxygen, argon
Carbon dioxide, nitrogen, argon
Carbon dioxide, argon, water vapor
4. The primary gases in the
atmosphere are
a.
b.
c.
d.
Nitrogen, oxygen, carbon dioxide
Nitrogen, oxygen, argon
Carbon dioxide, nitrogen, argon
Carbon dioxide, argon, water vapor
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
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!
5. Primeval atmosphere was
probably altered most by
a. The sun’s solar waves
b. Dinosaurs
c. Outgassing and impacts from meteors and
comets
d. Space junk
5. Primeval atmosphere was
probably altered most by
a. The sun’s solar waves
b. Dinosaurs
c. Outgassing and impacts from meteors and
comets
d. Space junk
6. Global warming is caused by
a.
b.
c.
d.
Car exhaust and deforestation
Carbon dioxide emissions and deforestation
Politicians
Industrial waste and deforestation
6. Global warming is caused by
a.
b.
c.
d.
Car exhaust and deforestation
Carbon dioxide emissions and deforestation
Politicians
Industrial waste and deforestation
7. The greenhouse effect is caused
by
a. Sun’s heat reflected from the surface trapped
by carbon dioxide and methane
b. Heat caused by human activity like cars,
factories, etc.
c. The effect of a dome over the Earth’s
atmosphere trapping heat
d. Mars
7. The greenhouse effect is caused
by
a. Sun’s heat reflected from the surface trapped
by carbon dioxide and methane
b. Heat caused by human activity like cars,
factories, etc.
c. The effect of a dome over the Earth’s
atmosphere trapping heat
d. Mars