Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Eighth Edition
CHAPTER 6
Earth and Moon
WHAT DO YOU THINK?
1.
2.
3.
4.
5.
6.
How old is the Earth? How do we know??
Can Earth’s ozone layer, which is now being depleted,
be replenished?
What is the greenhouse effect? Is Global Warming
real?
What causes the ocean tides?
Do we see all parts of the Moon’s surface at some time
throughout the lunar cycle?
How did the moon form? How do we know?
When did the planets form?


We cannot find the age of a planet, but
we can find the ages of the rocks that
make it up.
We can determine the age of a rock
through careful analysis of the
proportions of various atoms and
isotopes within it.
Radioactive Decay
Some
isotopes
decay into
other nuclei.
 A half-life is
the time for
half the
nuclei in a
substance to
decay.

Thought Question
Suppose you find a rock originally made of
potassium-40, half of which decays into argon-40
every 1.25 billion years. You open the rock and
find 15 atoms of argon-40 for every atom of
potassium-40. How long ago did the rock form?
A.
B.
C.
D.
1.25 billion years ago
2.5 billion years ago
3.75 billion years ago
5 billion years ago
Thought Question
Suppose you find a rock originally made of
potassium-40, half of which decays into argon-40
every 1.25 billion years. You open the rock and
find 15 atoms of argon-40 for every atom of
potassium-40. How long ago did the rock form?
A.
B.
C.
D.
1.25 billion years ago
2.5 billion years ago
3.75 billion years ago
5 billion years ago
Dating the Solar System
Age dating of
meteorites that are
unchanged since they
condensed and
accreted tells us that
the solar system is
about 4.6 billion years
old.
Dating the Solar System
•
•
•
•
Radiometric dating tells us that the oldest
moon rocks are 4.4 billion years old.
The oldest meteorites are 4.55 billion
years old.
Planets probably formed 4.5 billion years
ago.
But Earth’s crust rocks date to 3 or 4
billion years only… why?
Dating the Earth…

“Professor, didn’t YOU say every
measurement in science is uncertain???
Couldn’t YOU be wrong?”
“The Earth is the center of the universe.”
 “The Earth is flat.”
 “The continents are fixed upon Earth.”
 “Chernobyl and Three-Mile-Island are safe”
 “Intelligence is determined by race.”

Dating the Earth…

Good science…
Seeks natural laws to explain similar
phenomena
 Adapts to new data
 Looks for patterns and corroboration
 Requires independent lines of evidence
 Depends upon experiment, not personal
values

The Origin of the Moon: An
Example of the Process of Science
Where did the
moon come
from?
 How do we
know?

How do we explain the
existence of our Moon?
•Terrestrial planets don’t have moons….
•Our Moon is VERY large compared to its “partner”
•Composition is both like Earth…
•Rocky materials, similar to Earth’s Crust
•…and NOT like Earth:
•No water, no atmosphere, not a large metal core
How do we explain the
existence of our Moon?
•FOUR (4) Theories
• Formed at the same time, alongside Earth
• Formed from Earth’s materials
• Captured as it passed by
• Result of Giant Impact
How do we explain the
existence of our Moon?
•Formed at the same time, alongside Earth
•The “Sister” Theory…
•But then
•Should have the same composition…
•Should have water & “volatiles”
•Should have similar density
•It doesn’t, so rule this one out…
How do we explain the
existence of our Moon?
•Formed from Earth’s materials
•As earth formed, it “spun out”
•“Fission Theory”
•But then…. again
•Should have water & “volatiles”
•Should have similar density
•It doesn’t, so rule this one out…
How do we explain the
existence of our Moon?
•Captured as it passed by
•“Capture Theory”
•But then….
•How can this be done?
•It is very unlikely, so rule this one out…
Captured Moons
•
The unusual moons of some planets
may be captured planetesimals.
How do we explain the
existence of our Moon?
•Result of Giant Impact
•Mars-sized impactor strikes Earth a glancing blow
• Impactor destroyed, creates a debris ring
•Gravity takes over….the Moon is born.
•Lacks “volatiles” and water & Iron
•Computer models show this to be possible!
Giant Impact
Giant impact stripped matter from Earth’s crust
Stripped matter began to orbit
Then accreted into Moon
Earth as a Planet
Our goals for learning:
Why is Earth geologically active?
 What processes shape Earth’s surface?
 How does Earth’s atmosphere affect the
planet?

Why is Earth geologically
active?
Earth’s Interior
Core: Highest
density; nickel
and iron
 Mantle:
Moderate
density; silicon,
oxygen, etc.
 Crust: Lowest
density; granite,
basalt, etc.

Terrestrial Planet Interiors

Applying what we have learned about Earth’s
interior to other planets tells us what their
interiors are probably like.
Why do water and oil separate?
A.
B.
C.
D.
Water molecules repel oil molecules
electrically.
Water is denser than oil, so oil floats
on water.
Oil is more slippery than water, so it
slides to the surface of the water.
Oil molecules are bigger than the
spaces between water molecules.
Why do water and oil separate?
A.
B.
C.
D.
Water molecules repel oil molecules
electrically.
Water is denser than oil, so oil
floats on water.
Oil is more slippery than water, so it
slides to the surface of the water.
Oil molecules are bigger than the
spaces between water molecules.
Differentiation
Gravity pulls
high-density
material to
center
 Lower-density
material rises to
surface
 Material ends up
separated by
density

Thought Question
What is necessary for differentiation to occur in a
planet?
A.
B.
C.
D.
E.
It must have metal and rock in it.
It must be a mix of materials of different density.
Material inside must be able to flow.
All of the above.
b and c.
Thought Question
What is necessary for differentiation to occur in a
planet?
A.
B.
C.
D.
E.
It must have metal and rock in it.
It must be a mix of materials of different density.
Material inside must be able to flow.
All of the above.
b and c.
Lithosphere
A planet’s outer
layer of cool,
rigid rock is
called the
lithosphere.
 It “floats” on the
warmer, softer
rock that lies
beneath.

Thought Question
Do rocks s-t-r-e-t-c-h?
A. No—rock is rigid and cannot deform
without breaking.
B. Yes—but only if it is molten rock.
C. Yes—rock under strain may slowly
deform.
Thought Question
Do rocks s-t-r-e-t-c-h?
A. No—rock is rigid and cannot deform
without breaking.
B. Yes—but only if it is molten rock.
C. Yes—rock under strain may slowly
deform.
Strength of Rock
Rock stretches when
pulled slowly but
breaks when pulled
rapidly.
 The gravity of a large
world pulls slowly on
its rocky content,
shaping the world into
a sphere.

Heat Drives Geological Activity
Convection: hot rock
rises, cool rock falls.
One convection cycle
takes 100 million
years on Earth.
Sources of Internal Heat
1.
2.
3.
Gravitational
potential energy
of accreting
planetesimals
Differentiation
Radioactivity
Heating of Interior over Time

Accretion and
differentiation
when planets
were young

Radioactive
decay is most
important heat
source today
Cooling of Interior



Convection
transports heat
as hot material
rises and cool
material falls
Conduction
transfers heat
from hot
material to cool
material
Radiation
sends energy
into space
Thought Question
What cools off faster?
A. A grande-size cup of Starbucks coffee
B. A teaspoon of cappuccino in the same
cup
Thought Question
What cools off faster?
A. A grande-size cup of Starbucks coffee
B. A teaspoon of cappuccino in the same
cup
Thought Question
What cools off faster?
A. A big terrestrial planet
B. A tiny terrestrial planet
Thought Question
What cools off faster?
A. A big terrestrial planet
B. A tiny terrestrial planet
Role of Size


Smaller worlds cool off faster and harden
earlier.
Moon and Mercury are now geologically “dead.”
Surface Area to Volume Ratio



Heat content depends on volume.
Loss of heat through radiation depends on
surface area.
Time to cool depends on surface area divided
by volume:
surface area to volume ratio


4r 2 3
=

4 3 r
r
3
Larger objects have a smaller ratio and cool
more slowly.
Planetary Magnetic Fields
Moving charged particles create magnetic
fields.
A planet’s interior can create magnetic fields if
its core is electrically conducting, convecting,
and rotating.
Earth’s Magnetosphere
Earth’s magnetic fields protects us from
charged particles from the Sun.
The charged particles can create aurorae
(“Northern lights”).
Thought Question
If the planet core is cold, do you expect it to
have magnetic fields?
A. Yes, refrigerator magnets are cold, and
they have magnetic fields.
B. No, planetary magnetic fields are
generated by moving charges around,
and if the core is cold, nothing is moving.
Thought Question
If the planet core is cold, do you expect it to
have magnetic fields?
A. Yes, refrigerator magnets are cold, and
they have magnetic fields.
B. No, planetary magnetic fields are
generated by moving charges around,
and if the core is cold, nothing is
moving.
Special Topic:
How do we know what’s inside a
planet?

P waves
push matter
back and
forth.

S waves
shake matter
side to side.
Special Topic:
How do we know what’s inside a
planet?

P waves go
through Earth’s
core, but S
waves do not.

We conclude that
Earth’s core must
have a liquid
outer layer.
What processes shape Earth’s
surface?
Geological Processes

Impact cratering
—

Volcanism
—

Eruption of molten rock onto surface
Tectonics
—

Impacts by asteroids or comets
Disruption of a planet’s surface by internal
stresses
Erosion
—
Surface changes made by wind, water, or
ice
Impact Cratering



The Production of a Crater
Most cratering
happened soon after
the solar system
formed.
Craters are about 10
times wider than
objects that made them.
Small craters greatly
outnumber large ones.
Impact Craters
Meteor Crater (Arizona)
Tycho (Moon)
Volcanism
Volcanism happens
when molten rock
(magma) finds a
path through
lithosphere to the
surface.
 Molten rock is called
lava after it reaches
the surface.

Volcanic Eruptions and Lava Flows
Lava and Volcanoes
Runny lava makes flat
lava plains.
Slightly thicker lava
makes broad shield
volcanoes.
Thickest lava makes
steep stratovolcanoes.
Outgassing

Volcanism also releases gases from Earth’s
interior into the atmosphere.
Tectonics



Convection of the mantle creates stresses in the crust
called tectonic forces.
Compression forces make mountain ranges.
A valley can form where the crust is pulled apart.
Tectonics and Convection of the Mantle
Plate Tectonics on Earth

Plate Tectonics on Earth
Earth’s
continents slide
around on
separate plates
of crust.
Erosion
Erosion is a blanket term for weatherdriven processes that break down or
transport rock.
 Processes that cause erosion include
— Glaciers
— Rivers
— Wind

Erosion by Water

The Colorado
River
continues to
carve the
Grand
Canyon.
Erosion by Ice

Glaciers
carved the
Yosemite
Valley.
Erosion by Wind

Wind wears
away rock and
builds up sand
dunes.
Erosional Debris

Erosion can
create new
features by
depositing
debris.
How does Earth’s atmosphere
affect the planet ?
Which Molecules are Greenhouse Gases?
Effects of Atmosphere on Earth
1.
2.
3.
4.
Erosion
Radiation protection
Greenhouse effect
Makes the sky blue!
Radiation Protection
All X-ray light is
absorbed very high in
the atmosphere.
 Ultraviolet light is
absorbed by ozone
(O3).

The Greenhouse Effect
Which Molecules are Greenhouse Gases?
Earth’s atmosphere absorbs light at most wavelengths.
Greenhouse
effect:
Certain
molecules let
sunlight through
but trap escaping
infrared photons.
(H2O, CO2, CH4)
The Green House Effect
Thought Question
Why is the sky blue?
A.
B.
C.
D.
E.
The sky reflects light from the oceans.
Oxygen atoms are blue.
Nitrogen atoms are blue.
Air molecules scatter blue light more than red light.
Air molecules absorb red light.
Thought Question
Why is the sky blue?
A.
B.
C.
D.
E.
The sky reflects light from the oceans.
Oxygen atoms are blue.
Nitrogen atoms are blue.
Air molecules scatter blue light more than red
light.
Air molecules absorb red light.
A Greenhouse Gas

Any gas that absorbs infrared

Greenhouse gas: molecules with two different
types of elements (CO2, H2O, CH4)

Not a greenhouse gas: molecules with one or
two atoms of the same element (O2, N2)
Greenhouse Effect: Bad?
The Earth is much warmer because of the
greenhouse effect than it would be without
an atmosphere…but so is Venus.
Why the sky is blue

Atmosphere scatters
blue light from the
Sun, making it
appear to come from
different directions.

Sunsets are red
because less of the
red light from the Sun
is scattered.
Origin of Earth’s Water

Water may have
come to Earth by
way of icy
planetesimals
and comets from
the outer solar
system.
Summary of Key Ideas
Earth: A Dynamic, Vital World





Earth’s atmosphere is about four-fifths nitrogen and one-fifth
oxygen. This abundance of oxygen is due to the biological
processes of life-forms on the planet.
Earth’s atmosphere is divided into layers named the
troposphere, stratosphere, mesosphere, and ionosphere.
Ozone molecules in the stratosphere absorb ultraviolet light
rays.
The outermost layer, or crust, of Earth offers clues to the
history of our planet.
Earth’s surface is divided into huge plates that move over the
upper mantle. Movement of these plates, a process called
plate tectonics, is caused by convection in the mantle. Also,
upwelling of molten material along cracks in the ocean floor
produces seafloor spreading. Plate tectonics is responsible for
most of the major features of Earth’s surface, including
mountain ranges, volcanoes, and the shapes of the continents
and oceans.
Earth: A Dynamic, Vital World



Study of seismic waves (vibrations produced by
earthquakes) shows that Earth has a small, solid inner
core surrounded by a liquid outer core. The outer core is
surrounded by the dense mantle, which in turn is
surrounded by the thin, low-density crust. Earth’s inner
and outer cores are composed primarily of iron. The
mantle is composed of iron-rich minerals.
Earth’s magnetic field produces a magnetosphere that
surrounds the planet and blocks the solar wind.
Some charged particles from the solar wind are trapped
in two huge, doughnut-shaped rings called the Van Allen
radiation belts. A deluge of particles from a coronal mass
ejection by the Sun can initiate an auroral display.
The Moon and Tides




The Moon has light-colored, heavily cratered highlands
and dark-colored, smooth-surfaced maria.
Many lunar rock samples are solidified lava formed
largely of minerals also found in Earth rocks.
Anorthositic rock in the lunar highlands was formed
between 4.0 and 4.3 billion years ago, whereas the mare
basalts solidified between 3.1 and 3.8 billion years ago.
The Moon’s surface has undergone very little geologic
change over the past 3 billion years.
Impacts have been the only significant “weathering”
agent on the Moon; the Moon’s regolith (pulverized rock
layer) was formed by meteoritic action. Lunar rocks
brought back to Earth contain no water and are depleted
of volatile elements.
The Moon and Tides




Frozen water may have been discovered at the Moon’s
poles.
The collision-ejection theory of the Moon’s origin,
accepted by most astronomers, holds that the young
Earth was struck by a huge asteroid, and debris from this
collision coalesced to form the Moon.
The Moon was molten in its early stages, and the
anorthositic crust solidified from low-density magma that
floated to the lunar surface. The mare basins were
created later by the impact of planetesimals and were
then filled with lava from the lunar interior.
Gravitational interactions between Earth and the Moon
produce tides in the oceans of Earth and set the Moon in
synchronous rotation. The Moon is moving away from
Earth, and, consequently, Earth’s rotation rate is
decreasing.
Key Terms
anorthosite
capture theory
cocreation theory
collision-ejection theory
continental drift
convection
core
coronal mass ejection
crust
dynamo theory
ejecta blanket
fission theory
highlands
impact breccias
ionosphere (thermosphere)
mantle
mare (plural maria)
mare basalt
mascons
mesosphere
neap tide
northern lights (aurora
borealis)
ozone layer
planetary differentiation
plate tectonics
regolith
rille
seafloor spreading
seismic waves
seismograph
solar wind
southern lights (aurora
australis)
spring tide
stratosphere
synchronous rotation
troposphere
Van Allen radiation belts
WHAT DID YOU THINK?


Can Earth’s ozone layer, which is now being depleted,
be naturally replenished?
Yes. Ozone is created continuously from normal oxygen
molecules by their interaction with the Sun’s ultraviolet
radiation.
WHAT DID YOU THINK?


Who was the first person to walk on the Moon, and when
did this event occur?
Neil Armstrong was the first person to set foot on the
Moon. He and Buzz Aldrin flew on the Apollo 11
spacecraft piloted by Michael Collins. Armstrong and
Aldrin set down the Eagle Lander on the Moon on July
20, 1969.
WHAT DID YOU THINK?


Do we see all parts of the Moon’s surface at some time
throughout the lunar cycle of phases?
No. Because the Moon’s rotation around Earth is
synchronous, we always see the same side. The far side
of the Moon has been seen only from spacecraft that
pass or orbit it.
WHAT DID YOU THINK?


Does the Moon rotate and, if so, how fast?
The Moon rotates at the same rate that it revolves
around Earth. If the Moon did not rotate, then, as it
revolved, we would see its entire surface from Earth,
which we do not.
WHAT DID YOU THINK?


What causes the ocean tides?
The tides are created by orbital and gravitational forces,
primarily from the Moon and, to a lesser extent, from the
Sun.
WHAT DID YOU THINK?


When does the spring tide occur?
Spring tides occur twice monthly, during each full and
new Moon.