The Earth-Moon System
The Earth’s Atmosphere
• Unique among terrestrial
planets in its composition
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78% Nitrogen (N2)
21% Oxygen (O2)
0.9% Argon (Ar)
0.1% other gases including
0.03% Carbon Dioxide (CO2)
• The atmospheres of the other
terrestrial planets are mostly
CO2.
The Earth and Moon as seen from Mars!
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Formation of Earth’s Atmosphere
• There are two theories about the origin
of Earth’s atmosphere:
– Through volcanic eruptions or from impacts
gases were released from rocks on Earth.
– Impacts from comets brought gases to Earth.
• Scientists are testing volcanic emissions
and studying comets for answers to
these questions.
• Either way the original atmosphere was
likely very different from what we have
today.
• Changes came from the breakdown of
molecules by sunlight and Earth’s weak
Mt. St. Helens volcano
gravity allowing certain gases to escape.
• Plants provided most of the Earth’s
oxygen.
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Probing the Interior of the Earth
• Seismic waves are of two
types: S and P
– P waves compress material
and travel easily through
liquid or solid
– S waves move material
perpendicular to the wave
direction of travel and only
propagate through solids
Interior Structure
• Observations show P waves but no S waves at
detecting stations on the opposite side of the Earth
from the origin of an Earthquake
 the Earth has a liquid core!
Earth’s Interior
• The thin crust of Earth rides on
an elastic layer of rock called
the mantle.
• Below the mantle lies the liquid
outer core composed of iron
and nickel
• At the center is the solid inner
core also composed of iron and
nickel
• Motions within the mantle
cause the crust to be dragged
along. The crust is broken up
into “plates” that shift around
causing earthquakes, volcanoes
and forming mountain ranges
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Plate Tectonics
• Rifting
– Hot, molten material rises from deep in the Earth’s interior
in great, slow plumes that work their way to the surface
– Near the surface, these plumes spread and drag the surface
layers from below
– The crust stretches, spreads, and breaks the surface in a
phenomenon called rifting
Subduction
• Subduction
– Where cool
material sinks, it
may drag crustal
pieces together
buckling them
upward into
mountains
– If one piece of
crust slips under
the other, the
process is called
subduction
Earth’s Magnetic Field
The strength and orientation of the
Earth’s magnetic field changes very
slowly over time. In the past the
magnetic poles have even reversed.
• The motion of molten
iron beneath the Earth’s
crust combined with the
spin of the Earth is
thought to be the cause of
Earth’s magnetic field
• Like a magnet Earth has a
magnetic north pole and a
magnetic south pole
• These magnetic poles are
offset from the
geographic poles by about
11 degrees. A compass
therefore does not point
to “True North”.
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Earth’s
Magnetosphere
• Earth’s magnetic field
extends into space. The
region of space enclosed
by Earth’s magnetic
field is called the
magnetosphere.
• It protects Earth from
dangerous high-energy
particles streaming from
the Sun. This stream is
called the Solar Wind.
• Some high-energy
particles are trapped in
Earth’s field and form
the Van Allen Belts
• The Solar Wind warps
Earth’s field into a teardrop shape.
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The Aurora
• Near the Earth’s North and
South magnetic poles glowing
arcs and streamers in the sky
are often seen at night. This is
the Aurora.
• They are caused by energetic
electrons and protons moving
along the magnetic field and
striking air molecules causing
them to glow.
• When the density or speed of
the solar wind increases the
Earth’s magnetosphere can get
compressed.
• During this time of
compression and expansion
the location and intensity of
Aurora can change.
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The Moon
Footprint of Apollo 11 astronaut
in soft lunar regolith
• The Moon is about 1/4 the
size of Earth.
• The surface is covered in
impact craters. Crater sizes
range from a few inches to
several hundred miles
across.
• The Moon has some dark,
smoother areas and other
rough brighter areas.
• Over billions of years
impacts have pulverized
the surface into a fine
powder called regolith.
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Near Side and Far Side
• The Moon is locked into a
1:1 spin orbit resonance
with the Earth. The Moon
spins 1 time on its axis for
every 1 orbit around the
Earth.
• Because of this the Moon
always has one side facing
the Earth. This is the
“Near Side”.
• The side not facing the
Earth is called the “Far
Side”.
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Near and Far Sides of the Moon
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Robert Gendler, 1999
Apollo 16, 1972
Types of Lunar Surfaces
• There are two major types
of lunar surfaces:
– Maria- dark, relatively
smooth areas
– Highlands - light colored,
very rough cratered areas
• Highlands are made of
less dense rock and are
older.
• Maria are made of more
dense rock and are
younger
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Formation of Maria and Highlands
• As the Moon formed, denser
material sank towards the core
while lighter material floated to the
surface.
• The Highlands were formed after
the Moon’s crust cooled.
• This surface was constantly being
impacted during the early years of
the solar system.
• Much later, but before the Moon’s
interior had cooled, very large
impacts broke through the crust.
Denser rock flowed into the cracks
and filled the large craters that
formed. This dark dense material
formed the Maria.
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The Moon’s Interior
• The Maria are located on
the near side of the Moon.
• The crust of the moon is
thinner on the near side of
the Moon than on the far
side.
• The Moon’s core is
thought to be shifted
slightly in the direction
towards the Earth. This
shift may have caused the
crust on the far side of the
Moon to cool more and
become thicker than the
crust on the near side
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Formation of the Moon
• Several theories have been
proposed to explain the
origin of the Moon.
• The one that seems to
explain the observed
properties of the Moon the
best is that a very large
object struck the Earth and
the debris from this impact
was blasted off of Earth
and slowly coalesced into
the Moon.
• This theory explains the
composition of the Moon
and its orbital inclination.
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