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

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.

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

Earth formed 4.566 billion years ago in the inner solar nebula.
Two sources of heat in Earth’s interior:
• Potential energy of infalling material
• Decay of radioactive material
By 4.4 billion years, Earth had a crust and oceans
Heavy bombardment ended about 3.8
Byr ago
Most traces of bombardment
(impact craters) now destroyed by erosion and volcanic activity

Direct exploration of Earth’s interior (e.g. drilling) is impossible.
Earth’s interior can be explored through seismology: earthquakes produce seismic waves .
P ressure waves:
Two types of seismic waves:
S hear waves:
Particles vibrate back and forth
Particles vibrate up and down

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.

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 = temperature at which an element melts (transition from solid to liquid)
Melting point increases with increasing pressure towards the center
=> Inner core becomes solid

• Earth’s core consists mostly of iron + nickel: conducts electricity!!
• Convective motions and rotation of the core generate a 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

As high-energy particles leak into the lower magnetosphere, they excite molecules near the Earth’s magnetic poles, causing the aurora

About 2/3 of Earth’s surface is covered by water.
Earth is the only planet which has liquid water on its surface!
Mountains are relatively rapidly eroded away by the forces of water.

Earth’s crust is composed of several tectonic plates, which are in constant motion with respect to each other

Plate tectonics
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Evidence for plate tectonics on the ocean floor : mid-ocean rise … and in geologically active regions all around the Pacific

Where plates move toward each other, you get mountain ranges, some with volcanic activity, earthquakes
Where plates move away from each other, molten lava can rise up from below  volcanic activity

Convection in the mantle is the driving force of plate tectonics!
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Upwelling is occurring at the Mid-Atlantic ridge and Red Sea
Subduction occurs along the
West Coast of North and
South America

Mediterranean
Sea
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Saudi Arabia
Red Sea
Egypt
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Hawaii
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Iceland
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mantle
CO
2 is released from volcanoes

Normally, CO
2 cycles through the atmosphere, crust, and ocean.
This cycle is a thermostat that regulates our climate.

The nebula was very hot at 1 AU.
Earth has almost no H or He because it was too small to trap gases from the nebula, and these gases never condense into rocks.
Planetesimals with carbon, nitrogen, and oxygen (as ice, minerals) came in from 3-5 AU
(asteroid belt)
1) Outgassing: Release of gasses bound in compounds in the Earth’s interior through volcanic activity
2) Later bombardment with icy meteoroids and comets

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:
 Photosynthesis produces O
2
 Microbes produce CH
4
, NH
3
, etc.
 Burning fossil fuels produces CO
2
 Agriculture produces CH
4
 Chlorofluorocarbons (CFCs) destroy ozone And many others

The Earth would be as cold as the moon,
FROZEN SOLID, without the greenhouse effect.
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• Human activity (CO
2 emissions + deforestation) is drastically increasing the concentration of greenhouse gases.
• Observational data show that the average temperature on Earth has increased by about 1 o F since start of industrial era : “Global warming”
• Not all parts of the world are getting warmer!!

2

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• Polar ice caps begin to melt:
 Sea levels rise
 Earth gets darker --> absorbs more sunlight
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Qori Kalis Glacier, Peru
1978 and 2002
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Grinnell Glacier, Glacier
National Park (USA)
1900 and 1998

• About half the world’s population gets their water from glaciers
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Siachen Glacier in India & Pakistan

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Ozone (= O
3
) absorbs UV radiation
Chlorofluorocarbons (CFCs)
(refrigerants) destroy ozone
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!

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

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 CO
2
-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
1.
d
2.
d
3.
c
4.
d
5.
a
6.
d
7.
a
8.
d
9.
e
10.
d
11.
e
12.
b
13.
b
14.
d
15.
b
16.
d
17.
e
18.
e
19.
c
20.
e