Download Planet Earth - Wayne State University Physics and Astronomy

Planet Earth
1 February 2005
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Basic Facts
The Earth is a medium-sized planet with a
diameter of 13,000 km
It is one of the inner or terrestrial planets
It is composed primarily of heavy elements, such as
iron, silicon, and oxygen
It has much less light elements, such as hydrogen and
helium, than the outer planets
Earth's orbit around the Sun is nearly circular
The Earth is the only planet in our solar system
that is neither too hot nor too cold
It is warm enough to support liquid water on its
surface
It is “just right” to sustain life — at least life as we
know it
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Basic Properties
Some properties of the Earth
Semi-major axis
1.00 AU
Orbital period
1.00 year
Mass
5.98 x 1024 kg
Diameter
Escape velocity
Rotation period
12,756 km
11.2 km/s
23 h 56 m 4 s
Surface area
5.1 x 108 km2
Atmospheric
pressure
1.00 bar
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Earth's Interior (1)
The interior of the Earth is difficult to study even
with today's amazing technology
Its composition and structure must be determined
indirectly from observation made near or at the
surface only
Earth’s skin or crust is
a layer only a few
kilometers deep
The Earth is composed
largely of metals and
silicate rock
Most of this material
is in a solid state, but
some of it is hot
enough to be molten
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Earth's Interior (2)
The structure of the interior of the Earth has
been probed in great detail by measuring the
transmission of seismic waves through it
Seismic waves are waves that spread through
the interior of the Earth from earthquakes or
explosions
Seismic waves travel through Earth rather like
sound waves through a struck bell
In a bell, the sound frequencies depend on
what material the bell is made of and how it
was constructed
Similarly, the way seismic vibrations behave
depends on the composition and structure of
the planet
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Earth’s Internal Layers (1)
The Earth is divided into four main layers: crust,
mantle, core, and inner core
The crust is the top layer, the part we know best
The crust under the oceans, which covers 55% of the
surface, is typically about 6 km thick and is composed of
volcanic rocks called basalt
Basalts are produced by cooling
volcanic lava and made primarily
of silicon, oxygen, iron,
aluminum, and magnesium
The continental crust, which covers
45% of the surface, is 20 to 70 km
thick and is mainly composed of
another class of volcanic rocks
called granite
The crust makes up only about
0.3% of the Earth’s total mass
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Earth Internal Layers (2)
The mantle is the largest part of the solid Earth,
stretching from the base of the crust down to a
depth of 2,900 km
The mantle is more or less solid, but may deform
and flow slowly due to its high pressures and
temperatures
Below the mantle is Earth’s dense metallic core
In addition to iron, it contains nickel and sulfur, all
compressed to a very high density
The core is 7,000 km in diameter
Its outer part is liquid
The inner core is 2,400 km in
diameter and is probably
solid
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Rocks (1)
Basalt & granites are two examples of a class of
rocks called igneous rocks
They are rocks that have cooled from a molten
state
All volcanically produced rocks are igneous
There are two other kinds of rocks
Sedimentary rocks are made of fragments of
igneous rocks or the shells of living organisms
deposited by wind or water and cemented without
melting
Metamorphic rocks are produced when high
temperature or pressure alters igneous rocks
physically or chemically
These are commonly found on Earth, but not on
other planets
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Rocks (2)
A fourth group of rocks are called primitive
rocks
Their formation dates back to formation of the
planet
They have largely escaped chemical modification
by heating
Thus, they represent the original material out of
which the planetary system was made
No primitive rock is left on the Earth because it
was heated early in its history
Primitive rocks may be found in comets,
asteroids, or small planetary satellites
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Differentiation
The separation of the earth interior into
layers is an example of differentiation
Differentiation observed on Earth is
evidence that it was once warm enough
for the mantle rocks to melt
It allows the heavier metals to sink to the
center and form a very dense core
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Earth’s Magnetic Field
Much about Earth's interior can be learned
from the Earth's magnetic field
Earth behaves in some ways as if a giant bar
magnet were inside it
The magnet is roughly aligned with the rotational
axis of the planet
Earth’s magnetic field is generated by
moving material in Earth’s liquid metallic
core
The circulating liquid metal sets up an electric
current, which in turn produces a magnetic field
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Earth’s Magnetosphere (1)
The Earth's magnetic field extends into surrounding
space and traps small quantities of electric charges, such
as electrons, that roam about the solar system
Within this region, called the magnetosphere, Earth’s
field dominates over the weak interplanetary magnetic
field extending outward from the Sun
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Cross-sectional view of Earth’s magnetosphere as
revealed
by spacecraft missions
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Earth’s Magnetosphere (2)
It was discovered in 1958 by instruments on
the first U.S. Earth satellite, Explorer 1
This satellite recorded the ions (charged particles)
trapped in the inner part of the magnetosphere
This region has a fairly complex structure
It is composed of more than one layer or part
The layer discovered in 1958 is called Van
Allen Belts after the physicist who built the
instrumentation for Explorer 1 and correctly
interpreted its measurements
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Solar Wind
Charges trapped in the magnetosphere flow
outward from the Sun
Phenomenon is called solar wind
Flow of charged particles from the Sun is
large
Trapped by the Earth’s magnetosphere
Their flow produces a deformation of magnetic
field lines
Elongation far beyond Earth pointing away from the Sun
Magnetosphere typically extends to 60000 km - 10
earth radii - from the earth - towards the Sun
Away from the sun, sizeable magnetic fields are
measurable at a distance as large as the Moon's
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Geology
Study of processes that shape the crust
Although a fairly mature science, it is
not until very recently that geologists
were successful in understanding how
landforms are created
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Plate Tectonics (1)
A theory that explains how slow motions of
the earth mantle move large segments of the
crust
Resulting in slow drifting of the continents
Formation of mountains and other large scale
geological features
Earth's crust and mantle divided into ~12
major plates fit together like the pieces of a
puzzle
Plates observed to move slowly relative to
one another
In some places, such as the Atlantic ocean,
the plates are moving apart, elsewhere they
are forced together
Plate Tectonics (2)
Driving power of the plates motion is
provided by slow convection of the
mantle
Convection: a process by which heat
escapes from the interior of the mantle
and produces and upward of warmer
materials while cooler materials found
above slowly sink down
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Plate Motion
The plates’ motion brings them to
collide into one another and brings
about dramatic changes on the surface
of the Earth
Basically four types of interactions are
observed between the crustal plates:
They can pull apart
One plate can burrow under another
The can slide alongside each other
They can jam into each other
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Rift Zones
Plates pull apart from each other along rift zone
An important rift zone is found in the Mid-Atlantic
ridge
Few rift zones are also found on land
E.g. central African rift - these rifts shall eventually break
apart the African continent
Most of the rift zones are however found in the
oceans
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Subduction Zones
The point of contact where two plates come together is
called subduction zone
Continental masses cannot be subducted but the thinner
oceanic plates can be “easily” pushed down into the
upper mantle
Subduction zones often marked by an ocean trench
Subducted plates forced down into regions of high
temperature+pressure, eventually melts several hundred
kilometers below the surface
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Crust Regeneration
Calculations of the rate at which the sea floor
is spreading reveal the approximate age of
oceanic crust
60000 km of active rifts identified
Average separation of 4 cm per year
Correspond to an added area of 2 km2 per
year
Enough to renew the entire oceanic crust in about
100 million years
Less than 3% the age of the planet
Oceans are a fairly recent feature of the
planet
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Fault Zones
Crustal plates slide parallel to each
another along much of their lengths
Boundaries so formed lead to the
formation of cracks or faults
Along active fault zones, the motion of
one plate relative to the other may
amount to several centimeters per year
- basically the same as the spreading
along the rifts
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San Andreas Fault
On the boundary between
the Pacific and North
American plates
Runs from the Gulf of
California to the Pacific
Ocean northwest of San
Francisco
Pacific plate (west side)
moves north carrying
along Los Angeles, San
Diego, and parts of
Southern California
In a few million years, LA will be an island off
the coast of San Francisco
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Beware of Faults!
Plates slide roughly alongside each other
The creeping motions of the plates builds up
stresses in the crust
The stresses are eventually released in
sudden, violent slippages, a.k.a. earthquakes
Average motion of the plates is constant
The longer the interval between earthquakes
the greater the stress and the larger the energy
released when the surface finally moves
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More about
San Andreas
The San Andreas Fault,
near Parkfield, has
slipped every 22 years
during the past century
moving an average of
about 1 m each time
In contrast, the average interval
between major Earthquakes in the Los
Angeles region is about 140 years
the average motion is about 7 m
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Mountain Building
When two continental masses are brought
together by the motion of the crustal plates,
they are forced against each other under great
pressure
The surface buckles and folds forcing some of
the rock deep below the surface and others to
raise to large heights (sometimes many
kilometers!)
This is how mountain ranges are formed on
Earth
The Alps result from the interaction of the African
Plate with the European plate
We will see, however, that other mechanisms
lead to formation of mountains on other planets
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Volcanoes
Volcanoes mark the
location where molten
rock, called magma,
rises from the upper
mantle through the crust
Volcanoes are formed numerously along
oceanic rift zones where rising hot material
pushes plates away from one another
Volcanic activity is also observed in
subduction zones
In both cases, the volcanic activity brings to
the surface large amount of materials from
the upper mantle
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More about Volcanoes
Volcanic activity also found
near mantle "hot spots" areas
far from plate boundaries
but where heat rises from the interior of the
planet.
Best known hot spot lies under Hawaii
Supplies in magma three active volcanoes - two
of which are on land, and the third in the ocean.
It is estimated that the Hawaiian hot spot has
been active for at least 100 million years.
Shaping the Pacific plate, the hot spot has
generated a 3500-km long chain of volcanic
islands
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Earth’s Atmosphere
Provides the air we breathe
The air of the atmosphere exerts a constant
pressure (on the ground)
The atmosphere pressure at sea level is used to
define the pressure unit called bar
Humans have existed mostly at sea level and are
thus accustomed to such a pressure
The total mass of the atmosphere is ~ 5x1018 kg
Although this sounds like a lot, it constitutes only
one millionth of the total mass of the Earth
Yet it composition is quite vital to us humans and
other living creatures on the surface of this Earth
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Structure of Earth’s Atmosphere
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Troposphere
Altitude range:
Sea level - 9 miles
Densest area of the atmosphere
Most weather occurs and almost all aircraft fly in
this region
Temperatures drop as elevation increases
Warm air, heated on the surface, rises and is
replaced by descending currents of cooler air
The circulation generates clouds and other
manifestations of weather
As one rises through the troposphere, one finds the
temperature drops rapidly with increasing elevation
The temperature is near 50oC below freezing at
the top of the troposphere
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Stratosphere
Altitude range:
9 - 31 miles
Dry and less dense
The air in this layer moves horizontally and
does not move up and down within it
Temperatures here increase with elevation
Near the top of the stratosphere, one finds a
layer of ozone (O3)
Ozone is a good absorber of ultraviolet light
It thus protects the surface from the sun's
ultraviolet radiation and makes it possible for life
to exist on the planet
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Mesosphere
Altitude range:
31 - 62 miles (50 - 100 km)
Temperatures fall as low as -93
degrees Celsius in this region
Chemicals are in an excited state, as
they absorb energy from the sun
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Ionosphere
Altitude range: 62 - 124 miles.
This region is characterized by the
presence of plasma.
Its boundaries vary according to solar
activity.
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Thermosphere
Altitude range:
124 - 310 miles (200 - 500 km)
Temperatures increase with altitude
due to the sun's energy, reaching as
high as 1,727 degrees Celsius
Auroras, caused by the sun's particles
striking the earth's atmosphere, occur
at this level
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Exosphere
Altitude range:
310 - 434 miles (500 - 700 km)
The region begins at the top to the
thermosphere and continues until it
merges with interplanetary gases, or
space
The prime components, hydrogen and
helium, are present at extremely low
densities
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About Ozone
Increasing evidence
atmospheric ozone is being
destroyed
Agents of destruction are
industrial compounds called
CFCs (chlorofluorocarbons)
Each year, a large ozone
forms above the Antarctic continent
By now, the ozone loss has progressed into the temperate
zone
The production of CFCs has been banned by
international agreement
These chemicals are however destroyed slowly and are
still often released in the atmosphere
One can thus expect further reduction of the ozone layer in
the next century
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Weather and Climate
All planets with atmospheres have weather
Weather is simply the name given to the
circulation of air through the atmosphere
Climate is a term used to describe the
evolution of weather through long periods of
time: decades or centuries
Changes in climate are typically difficult to
detect over short periods of time. However,
their accumulating effects can be sizeable
and sometimes quite dramatic
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About the Weather
The energy that power this motion is derived
primarily from the sunlight that heats the
Earth's surface
As the planet rotates, and orbits the Sun, the
slower seasonal changes cause variations in the
amount of heat of sunlight striking the different
parts of the planet
The heat then proceeds to redistribute itself
from warmer to cooler areas giving rise to
various weather patterns
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Hurricane Elena
In the Gulf of Mexico on Sept 1, 1985
Wind speeds were in excess of 110
miles per hour
Eventually made landfall near Gulfport,
Mississippi
Origin of Life
Early Earth atmosphere is believed to contain
abundant carbon dioxide but no oxygen gas
In the absence of oxygen, many complex
chemical reactions are possible that lead to
the production of amino acids, proteins, and
many other chemical building blocks of life
Genetic studies of the many million species
that now live on Earth suggest that they are
related to one another
This led to the idea that all terrestrial life
descends from a single common microbial
ancestor
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Evolution of Life
Blue-algae consume carbon dioxide
and produce oxygen as a waste
product
They use the energy from sunlight, in a
process called photosynthesis to develop
and grow
They are thought to have proliferated and
eventually evolved into what we know
today as plants
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Appearance of Oxygen in
the Atmosphere
Studies suggest that oxygen started to
accumulate in the atmosphere some 2 billion
years ago
Led to formation of the Earth's ozone layer
Layer produced a shield under which more
complex life could evolve and develop
Life is believed to arise from the vast oceans
and venture into solid grounds
In this scenario, as animals evolved in
environment increasingly rich in oxygen, they
were able to develop techniques for breathing
oxygen directly from the atmosphere (primitive
lungs appeared)
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Role of Carbon Dioxide (CO2)
Sunlight striking the surface is absorbed
heats the surface layers
re-emitted as infrared/heat radiation
Atmospheric CO2 transparent to visible light
does not impede sunlight to reach the surface
CO2 opaque to infrared energy
behaves as a blanket, trapping the heat in the
atmosphere and impeding the flow back of energy
back to space
This is called the greenhouse effect
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Greenhouse Effect
On average, as much
heat reaches Earth’s
surface from the
atmospheric greenhouse
effect as from direct
sunlight
This explains why
night-time temperatures are only slightly
lower than daytime temperatures
life is actually possible on this planet
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Global Warming (1)
Estimated that greenhouse effect elevates
the surface temperature by about 23°C on
the average
Without this effect, the average temperature
would be below freezing
Earth would be covered with ice
Global ice age
An increase of atmospheric CO2 implies that
the atmospheric temperature would rise to
much higher average values
and then endanger life on our planet
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Global Warming (2)
Modern society increasingly depends on energy
Energy production is accomplished by burning fossil
fuels which when burned release carbon dioxide
The problem is exacerbated by ongoing destruction
of tropical forests in Asia, Africa, and South America
Atmospheric CO2 has increased by about 25% in the
last 100 years
It is rising at a frightening pace of 0.5% per year
CO2 level will soon reach twice the value it had before the
industrial revolution
Consequences are complex, not completely known
Sophisticated and elaborate computer models are
used
Conclusions are not firm at this point
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Craters
Why is there no clear evidence of craters on Earth?
Suggested Answer
Geological activity!
Earth Craters
Evidence of fairly recent
impacts can be found on
our planet's surface
The best studied case
took place on June 30, 1908, near the
Tunguska River in Siberia, Russia
8 km above the ground
Flattened more than a thousand square
kilometers of forest
Blast wave spread around the world and was
recorded by instruments designed to record
changed in atmospheric pressure
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Arizona Meteor Crater
Impact thought to have occured
50,000 years ago
Iron-nickel meteorite
Hurtling at about 40,000
miles per hour
Northern Arizona
Explosive force greater than 20
million tons of TNT
Estimated size of meteor about
150 feet across
Weigh several hundred
thousand tons
Crater 700 feet deep, 4000 feet
across
Today: crater is 550 feet deep,
and 2.4 miles in circumference
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