Download The Earth and the Moon

The Earth and the Moon
• We will discuss the
general characteristics
of the Earth as a point
of useful comparison
and contrast with other
Solar System bodies.
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Interior Structure of the Earth
• Core (largely Fe-Ni)
– inner solid core
– outer liquid core
• Mantle (rocky
material)
• Crust (granite, basalts)
• Atmosphere (mostly
N2 and O2)
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The Interior of the Earth
• Observations of the interior structure
depend on propagation of seismic waves.
• Seismic waves come in two varieties:
– S-waves: “Shear waves”
• Matter is displaced perpendicular to direction of
propagation.
– P-waves: “Pressure waves”
• Compressional waves, like sound waves.
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P-Waves
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S-Waves
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Seismic Waves
• P-waves travel faster
than S-waves
– multiple stations can
thus pinpoint the
epicenter
• S-waves limited to
103° from epicenter.
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Seismic Waves
• Higher densities near
center cause seismic
waves to speed up and
refract
• No direct P-waves are
observed between
103° and 142°
Shadow
zone 103º to
142º
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The Surface of the Earth
• Lithosphere is broken into about a dozen
plates that move quasi-rigidly, floating on a
partially molten upper mantle.
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The Surface of the Earth
• Rift zone: where plates spread apart. Young
surface.
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The Surface of the Earth
• Subduction zone: Collision of plates,
results in mountain building.
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Rift Zone
• Mid-Atlantic ridge is
an example of a rift
zone.
• North American and
Eurasian plates are
pulling apart here.
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Rift Zone
• The Mid-Atlantic
Ridge runs right
through Iceland, and
we can actually see the
two plates pulling
apart.
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Icelandic Shield Volcano
• “Skjaldbreidur”,
or “Broken
Shield”, the
archetype of
shield volcanoes.
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Hawaii: Drifting over a Hot Spot
• The Hawaiian Island
chain is formed by a
plate drifting over a
permanent hot spot in
the Earth’s mantle.
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Mauna Loa
• Mauna Loa on Hawaii is the world’s largest
shield volcano.
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Continental Drift
Triassic
Permian
225 Myrs ago
200 Myrs ago
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Continental Drift
Cretaceous
Jurassic
135 Myrs ago
65 Myrs ago
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Continental Drift
Cretaceous
Present
65 Myrs ago
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Continental Drift
• The northward motion
of the Pacific plate
relative to the North
American plate
produces the San
Andreas fault in
California.
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Fault line
19
The Earth’s Atmosphere
• Primeval atmosphere
– Accumulated during formation.
– Consists of hydrogen (H2), helium (He),
ammonia (NH3), and methane (CH4).
• Secondary atmosphere
– Outgassing during differentiation process.
– Carbon dioxide (CO2) and water (H2O).
• Keep this in mind when we discuss Venus!
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The Changing Atmosphere
• Liquid water dissolves CO2 out of
atmosphere.
• CO2 reacts with other dissolved substances
– Forms SiO2 (sand), CaCO3 (limestone), and
other solid carbonates.
• Methane and ammonia are dissociated by
solar ultraviolet radiation.
– There is no protective ozone (O3) layer.
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The Appearance of Life
• About 3 billion years ago, photosynthesis
starts introduction of free oxygen (O2) into
the atmosphere.
• Oxygen is highly reactive, and must be
constantly replenished.
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Composition of Present
Atmosphere
Percentage
75.5%
23.1%
1.3%
0.05%
trace
variable
Constituent
N2
O2
Ar
CO2
Ne, He, CH4, Kr
H2O vapor
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Atmospheric Structure
Equation of Hydrostatic Equilibrium
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Structure of the
Atmosphere
• Troposphere:
– Lowest, densest
level. 80-85% of
the atmospheric
mass is in this
layer, within 10 km
of ground
– Absorbs re-radiated
IR emission from
the ground (colder
at higher altitude).
– Convective
motions.
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Structure of the
Atmosphere
• Stratosphere:
– Convective
motions replaced
by laminar flow.
– Absorption of solar
UV by ozone, so
warmer at higher
altitude.
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Structure of the
Atmosphere
• Mesosphere:
– Above ozone layer,
ineffective heating
because the density
is so low.
– Most important
coolant is CO2.
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Structure of the
Atmosphere
• Ionosphere:
– Partially ionized
(by solar extreme
UV radiation) gas.
– High temperature
because cooling
ineffective.
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Atmospheric Circulation
• Hadley circulation
– warmest air at subsolar
point rises
– good model for a slow
rotator, but not for
Earth
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Atmospheric Circulation
• Modified Hadley
circulation
– atmosphere coupled to
the surface by friction;
coriolis forces break up
the Hadley cells.
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Effect of the Earth’s Atmosphere
on Astronomy
• Atmospheric effects on light:
– scatters
– absorbs
– refracts
All wavelength-dependent
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Scattering of Radiation
• Depends on characteristic size L relative to
wavelength λ
• Case 1: L « λ (scattering by small particles,
such as aerosols [small airborne particles])
– Iscatter ∝ λ–4 (Rayleigh scattering)
– Blue light more easily scattered
– Accounts for blue skies, red sunset
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Scattering of Radiation
• Case 2: L ≈ λ (scattering or red/infrared
photons by dust particles ~1 μm)
– Iscatter ∝ λ–1
– Wavelength dependence of scattering much
weaker in infrared than optical
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Scattering of Radiation
• Case 3: L » λ (scattering of optical light by
water droplets)
– Iscatter ∝ λ0
– Wavelength independence is why clouds are
white (τ ≈ 1) or gray (τ » 1).
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The Earth’s Magnetosphere
• Magnetic field generated by convective
motion in molten core.
– Deflection of solar wind around Earth.
– Trapping of particles in “van Allen Belts”
– Interaction between solar particles produces
aurorae.
– Magnetic field reverses polarity every million
years or so (geological evidence).
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The Earth’s Magnetosphere
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Interaction of the Solar Wind and
the Earth’s Magnetosphere
Magnetopause or Stagnation Radius
van Allen Belts
Particle drift
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Aurora Borealis from the Space
Shuttle
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The Northern Lights or Aurora
Borealis
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Northern Lights are
concentrated around the
North Magnetic Pole
Magnetic Pole
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The Moon
• The Moon is in
synchronous rotation
because of tidal
friction.
• On account of
librations, only 59% of
the Moon’s surface
can be seen from
Earth.
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Lunar Librations
• A time lapse movie showing changing phases of the moon
along with librations
– Also notice the Moon’s changing angular size
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The Moon
Maria
• Moon’s surface has lighter
(higher elevations) and
darker (lower elevations).
• Galileo noted that the
darker areas are relatively
smooth. He called them
“seas”.
– Mare (singular)
– Maria (plural)
• Maria concentrated on
lunar near side.
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The Moon
• Maria concentrated on lunar near side
(maria shown in blue/violet).
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Clementine Mosaic of the Moon
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Edge of Mare
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Lunar Highlands
• Lighter color.
• Saturated with craters.
• An older surface, not altered since heavy
cratering era of planetary formation.
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Age of Lunar Surface
• Apollo samples (some
400 kg of rock
returned by six lunar
missions) show:
– Age of highlands: ~ 4
billion years
The rock shown here is
4.4 billion years old!
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Age of Lunar Surface
• Apollo samples (some
400 kg of rock
returned by six lunar
missions) show:
– Age of highlands: ~4
billion years
– Age of lowlands: ~3.5
billion years
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Cratering History of the Moon
• Era of heavy cratering.
– ~3.5 billion years ago
(formation of
highlands)
– Unlike Earth, erosion
is inefficient at
obliterating craters.
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More recent large
impacts and subsequent
flooding formed maria.
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Formation of Maria
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Crater Formation
1 Impact (at speeds up
to 72 km s-1 for headon impact)
Solar escape speed at 1 AU
= 42 km s-1
+ Earth orbital speed
= 30 km s-1
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Crater Formation
2 Deep penetration and
vaporization
• Example: 1 km
radius rock:
4π 3
M=
R ρ
3
4π
3 3
=
10 (3000 )
3
= 1.3 × 1013 kg
( )
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Crater Formation
2 Deep penetration and
vaporization
• Impact energy:
1
E = mV 2
2
(
= 0.5 ×1.3 ×10 × 7.3 × 10
13
)
4 2
= 3 ×10 22 joules
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How Much Energy Is This?
• 1 Megaton TNT = 4.2 ×1015 joules
• 1.3×1022 joules = 107 Megatons
• The largest man-made nuclear weapons are
about 20 Megatons.
– Would level all of Columbus inside the I-270
outerbelt.
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Crater Formation
3 Circular crater and
ejecta
– The 1 km rock
produces a crater of
diameter 100 km with
5 km high walls.
– This is the size of some
of the more prominent
lunar craters such as
Copernicus.
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Copernicus
Rays
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Crater Formation
4 Ejecta form walls and
a surrounding blanket.
– “Rebound” produces a
central peak.
– Surface underneath is
fractured.
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Crater Formation
• A “glancing blow” can
produce a chain of
secondary craters.
Secondary
craters
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Lunar Surface
• No evidence for
volcanoes (in contrast
to Earth, Venus, and
Mars).
• Rilles, domes, and
wrinkled ridges are
evidence of past lava
flows.
Hadley Rille
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Wrinkled Ridges
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Domes
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Lunar Regolith
• Entire lunar
surface
covered with
dust, most of
it lunar crust
that has been
pulverized
by impacts.
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Differences Between Lunar and
Terrestrial Rocks
1 All lunar rocks are igneous.
2 Lunar rocks do not have a trace of water.
– Earth rocks contain up to 3% water.
3 Iron in lunar rocks is not oxidized.
4 Lunar rocks are depleted in elements with
low boiling points.
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Lunar Interior
• Moon is
differentiated,
yet geologically
dead.
– Smaller than
Earth, so shorter
cooling time.
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Origin of Moon
• Probable origin:
impact of Marssized protoplanet
with differentiated
Earth. This accounts
for:
– composition
differences (absence
of volatile elements
in lunar rocks).
– small iron core of
the Moon.
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