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Planet Earth
APOD
Our Home
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Earth, Our Home
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Earth
a highly evolved planet
= altered dramatically since formation,
due to flow of energy from interior to
surface
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Earth, Our Home
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Planet Earth Facts
diameter (equator)
radius (equator)
diameter (poles)
mass
12,756 km
6,378 km
12,714 km
6 x 1024 kg
How do we know the mass of the Earth?
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Measuring the Mass
• Kepler’s Third Law (as revised by Newton
• something orbiting the Earth (moon or
satellite (must be small mass)
• period of the orbiting body
• average distance of the orbiting body
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Planet Earth Facts
average density = 5500 kg/m3
5.5 times the density of water
density: how well-packed matter is into
some volume of space
density = mass
volume
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Starting with what we know:
rocks:
iron:
density = 2000 - 4000 kg/m3
density -= 7800 kg/m3
rocks near the surface density = 2400 kg/m3
center must be ~ 12,000 kg/m3
core must be very dense, i.e. iron
compressed by weight of many
layers
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Model of the Interior
• core
• mantle
• crust
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CORE:
radius of 3500 km
nickel & iron alloy
solid inner core, molten outer core
temperature > 6000 K
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MANTLE:
2400 km thick
rock - iron, magnesium, silicon,
oxygen (olivine)
- silicates (compounds of
silicon and oxygen)
temperature 3800 K (base)
1300 K (top)
steady pressure causes mantle to flow
(like a plastic)
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CRUST: 8 km thick under the oceans
70 km thick under the continents
solid surface layer - rocks solidified from
molten lava = igneous rocks
basalt:
granite:
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silicates of aluminum, magnesium,
iron - ocean basins and
subcontinents
silicates of aluminum, sodium,
potassium - continents
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granite
lower density than basalt
continents float on the basalt
crust
lower density than mantle
crusts float on the mantle
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differentiated: layers of least dense
material on top, most dense on bottom
How did the Earth
get this way?
APOD
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Model for Early Earth
well-mixed at time of formation
then heated, dense material
sank to the core
Heat:
• stored energy from formation
• radioactive decay - energy is transferred
to the rocks and heats them
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Age: 4.6 billion years old
billion = 1000 million
• radioactive dating - decay of radioactive
isotopes
oldest rocks 4 billion years old
Greenland, Canada
allow 1/2 billion yeas for crust to
melt, then cool
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• coincides with age of meteorites
- 4.55 billion years old
• coincides with age of lunar rocks
- 4.6 billion years old
Solar System formed along with the Sun
4.6 billion years ago.
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Magnetic Field
Earth is like a giant bar magnet
- field from south magnetic
pole to north magnetic
pole
- axis of this magnet is 12o
off from the rotation axis
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Earth, Our Home
magnetic axis
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Magnetic Field
Variations:
changes intensity and direction
(polarity reversal)
- 9 times in 3.5 million years
Dynamo model:
• metallic core, partly liquid = good electrical
conductor
• acts like a large electromagnet and a giant
generator
• Earth’s rotation keeps this in motion
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Earth, Our Home
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Planetary magnetic field should:
fluid interior
rapid rotation
But this model does not always work out well!
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Atmosphere
• oxygen for breathing
• shields us from cancer-causing UV radiation
• warmth for the surface
• distributes heat evenly around the Earth
APOD
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Atmosphere
78 % Nitrogen (N2)
21 % Oxygen (O2)
1 % Argon
0.03 % CO2
traces
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Atmosphere & Light
• some of the light is absorbed - blocked
out by the atmosphere
atmospheric extinction
•sunlight is scattered by air molecules
- blue light scatters easily
=> sky is blue
- along the line of sight looks redder
=> sunset looks red
atmospheric reddening
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Greenhouse Effect
1/2 of the unreflected sunlight is absorbed
by the ground
- ground heats up
- air near the ground becomes warmer
This, in itself, would not give us the temperature
we currently enjoy; it would be -18o C
Somewhere there’s additional heat!
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Greenhouse Effect
Earth emits IR radiation - does not escape
to space.
1/2 is absorbed by the atmospheric
H2O and CO2
atmosphere heats up, heat radiates back to
ground
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Greenhouse Effect
Earth’s surface is warmed by:
direct sunlight
IR radiation from the atmosphere
Water and carbon dioxide are the chief
greenhouse gases.
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Ozone Layer
UV is blocked by the atmosphere at about
30 km above the surface
UV is absorbed => 3 Oxygen atoms to bind
into O3 (ozone)
human activity => sharp drop in ozone
1950 - 1980 decreased by 1/2
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Ozone Layer
CFC’s contribute to the depletion of the
ozone layer
aerosol propellants
solvents
refrigerants
volcanic eruptions
One chlorine atom destroys 100,000 O3 molecules
and there are about 1 million TONS of CFC’s
emitted per year. These remain for 100 years.
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Atmospheric Circulation
convection: warm air rises, cool air sinks
• sunlight heats ground and air
• heated air expands, becomes less dense
• expanded air rises
• denser, cooler air sinks to fill in behind
Local convection cells
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Earth, Our Home
cumulus clouds
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Atmospheric Circulation
Global convection cells
• warm air over equator is less dense
than cooler air over the poles
• warm air rises to the poles
• cool air from poles sinks down to fill in
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Atmospheric Circulation
And, if that wasn’t enough!
Earth rotates under all of this
- forms spiral air flows
cyclones !
APOD
Faster rotating planet, stronger cyclonic effects
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Atmospheric Circulation
Local up and down convection cells
Global north-south convection cells
Rapid rotation of the planet
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The Crust
surface of the Earth is not smooth and flat
why? why isn’t it eroded flat?
somehow it’s being replenished
heat flow: from Earth’s center drives the
evolution of our planet
heat: thermal energy transferred from one
place to another
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Earth:
interior
crust
atmosphere
oceans
all of these are at
different temperatures
heat in the interior eventually flows
to the surface
heat on the surface eventually flows
to space
1013 J/sec is lost
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Continental Drift
present continents were at one time one
large land mass
evidence:
• reversals of Earth’s polarity are preserved
in rock
• same patterns on both sides of Mid-Atlantic
Ridge
• record of past changes recorded in the
seafloor
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rift: separation of crustal masses
new material oozes out of the rift
• seafloor expands 3 cm/year - this would
be enough to push apart over a few
million years
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Volcanism & Plate Tectonics
volcanism:
molten material comes up
through the mantle and crust
ocean floor is being renewed with new material
continents:
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large plates floating on basalt
mantle
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Volcanism & Plate Tectonics
mountains:
sea floor trenches:
built from the collision of
two plates
plates get folded over
and driven back through
the mantle
surface of the Earth is constantly recycled!
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Volcanism & Plate Tectonics
earthquakes and volcanoes predominate
where plates are being created or destroyed
ocean basins are the youngest surfaces
center of continents are the oldest
rifts:
stretching, thinning, splitting of
Earth’s crust
HEAT FLOW drives plate tectonics
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Evolution of Oceans
volcanoes - bring up trapped gases
CO2, H2O - outgassing
• initially, no oceans - too hot
• Earth cooled, water vapor outgassed
• condensed as it cooled
evidence: measure current rate of outgassing
- can account for all of the oceans
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Evolution of the Atmosphere
model:
first atmosphere
hydrogen and helium
low mass particles
escaped into space
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Evolution of Atmosphere
second atmosphere:
CO2 (carbon dioxide)
SO2 ( sulfur dioxide)
H (hydrogen)
all
N (nitrogen)
outgassed
H2O (water)
from
CH4 (methane)
volcanoes
NH3 (ammonia)
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Evolution of Atmosphere
• large amounts of CO2 ended up in oceans
and rocks
• 3 billion years ago: mostly methane and
hydrogen
very little free oxygen, no ozone
• UV radiation interacted with methane,
ammonia and water
• atoms separated
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Evolution of Atmosphere
• hydrogen escaped
• oxygen formed the ozone layer
• nitrogen from the ammonia formed
our atmosphere
biological (plant life!) activity produced
the high abundance of oxygen and now
maintains it
600 million years ago - attained present level
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Evolution of Surface
Temperature
temperature:
depends on how much
energy from Sun
depends on how effectively
the greenhouse effect is
increase in CO2, 5% in last 30 years
could mean 2o C rise by 2020
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Earth, Evolved Planet
• most evolved planet
• active world, powered by internal heat
planetary formation:
• collection of debris in space
• gravity draws together
• gravitational potential energy
kinetic energy (heat)
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Earth, Evolved Planet
• collide, stick together (accretion)
• mass gains energy, temperature increases
• later arriving
debris makes
impact craters
Copyright by Calvin J. Hamilton
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Mass Extinction
NASA
Chicxulub impact crater
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Formation Process
4.6 billion years ago
accretion from smaller bodies
bombardment by large objects
planet of uniform materials
atmosphere - H and inert gases
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Formation Process
4.5 billion years ago
radioactive heating melted interior
Earth differentiated
original atmosphere was lost
second atmosphere outgassed by volcanoes
surface cooled, rain began to fall
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Formation Process
3.7 billion years ago
continents appeared
plate tectonics began
mountains grew
atmosphere evolved
2.2 billion years ago
crust cooled
plate activity began
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Formation Process
600 million years ago
formation and break up of continents
Earth looked much as it does now
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Earth, Our Home
changed greatly since formation
evolution driven by internal heat
stored and from radioactive decay
heat differentiated the interior
heat drove tectonics and volcanism
crust shaped by impacts
wind and water has eroded the crust
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