Download Earth`s Magnetic Field, Atmosphere and Geology

Earth
The Model Planet
If you encountered another planet,
what would you want to learn about
it?
• Basic physical parameters
• How old is the planet? How was it
formed?
• How is it structured internally? Externally?
Learning more
• Does it have a magnetic field? How
strong is it? How is it structured? What
does it tell us about the planet’s interior?
• How is its atmosphere structured? Of
what is it made? What are its weather
patterns? How does the atmosphere help
control the planet’s energy budget?
Learning more (2)
• Has the atmosphere always been the
same as it is now? How has the
atmosphere interacted with the surface?
• What kinds of physical processes have
produced the planet’s landforms?
• Is there life on the planet? How does it
interact with the various ecosystems?
Physical Properties
Diameter: 12,756 km
at the equator
Why specify “at the
equator”?
Because the
polar diameter is
only 12,697 km
Physical Properties
The bulging at the equator and flattening at
The poles is called OBLATENESS.
It’s due to the rotation of the planet.
Earth is the most spherical (least oblate)
of all the planets.
Physical Properties
• Volume: 1.1 trillion cubic kilometers (km3)
Mt. Everest is about than 2400 km3
http://experts.about.com/q/Geography-1729/Volume-Mount-Everest-1.htm
• Mass: 5.97 x 1024 kilograms
This is equal to about 81 moons!
Physical Properties
• Density: 5500 kg/m3 or 5.5 g/cm3
– We compare the density of materials
like rocks & metals to the density
standard: WATER!
• Water’s density is 1.0 g/cm3
Physical Properties
• Most rocks have densities between
2.5 and 4.0 g/cm3
• Most metals have densities greater
than 6.0 g/cm3
– Iron is 7.8 g/cm3
– Nickel is 8.9 g/cm3
Physical Properties
• The density of an object reflects its
composition.
• What does Earth’s density tell you about
its composition?
Earth must be made of a
combination of rocks
and metals.
How old is the Earth?
• About 4.6 billion years.
– The oldest rocks are found on the north
slope of Canada, the Canadian Shield.
These rocks are 4.0 billion years old. They
were dated from the radioactive decay of
uranium into lead.
• The other 0.6 billion years is an estimate
of how long it took the earth to form.
How was the earth formed?
•
•
•
•
Accretion
Differentiation
Interior Structure
Evidence, or “How sure are we?”
•A rotating cloud of gas & dust:
a nebula
•Rotation causes the nebula to flatten
•A star ignites in the center and a
temperature gradient forms.
•Solid chunks, called planetissimals,
begin to condense close to the star.
Accretion
• Planetissimals (dust-sized to small moonsized solid bodies) begin to form near the
sun.
• Gravity attracts planetissimals into larger
and larger bodies. Planets begin to grow.
• Most of the “stuff” of planetissimals is rock
(silicates: Na, Ca, Mg, O, Si) and heavy
metals (Fe, Ni).
Differentiation
• As earth grows in size, its gravity grows
too
• It begins to pull in other planetissimals,
which impact on its surface.
• What happens when you repeatedly hit a
piece of metal with a hammer?
Differentiation
• All the heat generated by
planetissimals impacting on the
surface, plus the heat generated by
radioactive decay in the young earth’s
interior causes the entire earth to
melt.
Differentiation
• When the entire earth is molten, the heavy
elements (iron, nickel) sink to the interior.
• The lighter materials (granite-type rocks)
rise to the surface.
• The medium density rocks (basalt-type)
ends up in the middle.
• Layers form: core, mantle, crust.
Molten Earth, heated
by impacts & radioactive
decay.
Differentiated (layered)
Earth after cooling.
Interior Structure
• Inner Core (kept solid by the immense
pressure of all the material on top of it.)
• Outer Core (less pressure allows it to be a
liquid.)
• Mantle
– Asthenosphere
– Lithosphere
• Crust
Solid Inner Core
2400 km diameter
Iron & Nickel
Mantle
2900 km thick
Basaltic rocks:
Olivine, Pyroxene
Liquid Outer Core
2270 km thick
Iron & Nickel
Crust
20-100 km thick
Granitic rocks:
Feldspars
Interior section of mantle is a thick fluid called the
asthenosphere. The outer mantle + the crust are rigid
and are collectively called the lithosphere.
More about the layers
• The difference between the mantle
and the crust is based on chemical
composition.
• The difference between the
asthenosphere and the lithosphere is
based on viscosity (the ability to flow
under pressure.)
How do we know?
What evidence do we really have that the
interior of the earth is the way we think it is?
• Deep mines are hot!
• Heat and molten material escape from
volcanos and geysers.
• Earthquake waves
Earthquake Waves
• When an earthquake occurs it
produces 2 types of waves:
– P or primary waves. These are waves
of compression of the rock. They travel
fastest.
– S for secondary or shear waves. The
rock moves up & down or sideways.
Earthquake waves
• P waves are capable of traveling through
both liquids and solids, so they travel
through mantle and cores.
• S waves can’t travel through liquids, so they
stop when they hit the outer core.
• Look closely at the next diagram.
How big’s that core?
• With the right placement of
seismometers around the earth’s
surface, we get a good estimate of
the size of the outer core.
• The size of the inner core is
calculated from theory.
Earth’s Magnetic Field
• Why does Earth have a global or worldwide magnetic field, while other similar
planets either have no magnetic fields or
very different kinds of fields?
• Why should we care about Earth’s
magnetic field? What does it do for us?
Earth’s Magnetic Field
• Magnetic fields are made wherever there
is an electric current, that is the movement
of electrons.
• In a regular bar magnet, the magnetic field
comes from the electrons orbiting around
the nuclei of the iron atoms. All the
electrons orbit in the same direction.
Earth’s Magnetic Field
• In the earth, electrical currents run through
the molten iron core. Friction within the
molten, flowing iron knocks electrons off
iron atoms.
• The molten iron flows at about 0.8 inches
per second, but the electrical currents can
flow faster.
Earth’s Magnetic Field
• The electrical currents within the earth
cause earth to act like a gigantic
electromagnetic generator.
• This is called the Dynamo Theory of
magnetic field generation.
A little more info…
• The earth’s magnetic field isn’t strong
enough for us to feel, but many animals
can sense it and even use it to navigate.
It’s only about 0.4 Gauss, much weaker
than a small magnet you can hold in your
hand.
• On average, the North & South poles “flip”
every 390,000 years. There have been 9
flips in the past 3.5 million years.
The poles “flip” ?
• No one knows how long the process
takes, maybe a few years, maybe a few
minutes.
• Every so often, what was the North
magnetic pole suddenly becomes the
South magnetic pole.
• Lava that cools quickly on the sea floor
records these flips and lets us date them.
Stripes of different magnetic polarity
form in the rocks as the lava from the
mid-ocean ridge cools.
Strange Things Going On
• Earth’s magnetic field is NOT aligned with
its rotational axis.
• The magnetic field is tilted 12o to the
rotational axis, and doesn’t even pass
directly through the center of the earth.
• Does this mean that the electrical currents
don’t flow evenly and uniformly inside the
earth? Is there turbulence inside?
Magnetic Fields in Space
• Earth’s magnetic field extends 7-10 times
the earth’s diameter outward from the
earth.
• The earth’s magnetic field would be
spherical, but the solar wind compresses it
on the side closest to the sun, and
stretches it out into a long tail on the side
opposite the sun.
• Overall, it’s kind of tadpole shaped.
Magnetic Field Structure
• The whole magnetic field is called the
magnetosphere.
• On the side closest to the sun, where the
solar wind compresses the field, there is a
“bow shock”, just like a boat pushes some
water out of the way at its bow as it sails
forward.
Why do we care?
• Earth’s magnetic field isn’t just “there” with
no purpose. Without it, you and I and
every living thing on this planet would be
dead (including the cockroaches!)
• The magnetic field channels away the
solar wind.
• It also prevents erosion of the atmosphere.
Solar Wind
• So what is the solar wind anyway?
– It’s radiation: extremely hot, high-energy, fastmoving charged particles (ions) given off by
the sun. Most of these particles are protons.
– If you were exposed to it for just a few hours
without protection, your skin and every organ
in your body would be burned, and you’d have
a fatal dose of radiation poisoning.
How does the magnetic field
protect us?
• The magnetic field captures the solar wind
and channels much of it into a donut of
radiation around the earth.
• This donut (actually 2 layers – one inside
the other) is called the Van Allen Radiation
Belt (V.A.R.B.)
Van Allen Radiation Belts
• Satellites must orbit either below or above
the V.A.R.B., or their electronics would be
fried.
• The problem is even worse when we send
manned missions into space. The ship
must pass through the radiation belts as
quickly as possible or the crew is toast !
Where does the radiation go?
• Since the sun continually supplies new
solar wind, where does the solar wind go
that the earth has already captured?
• The magnetic field channels some of it into
our atmosphere at the north & south poles.
Here it ionizes oxygen and nitrogen atoms,
causing the beautiful northern and
southern lights.
Northern Lights?
• The northern lights are properly called the
“aurora borealis.” They’re nothing more
than a very large fluorescent light display
(without the fluorescent tube!)
• The northern lights are sometimes seen as
far south as Florida, especially when the
sun is very active.
This aurora was photographed in Tennessee in October, 2002.
Where does the rest of the
radiation go?
• Much of it flows through the magnetic field,
around the earth, and “drips” off the tail of
the magnetic field. The tail is called the
“magnetotail.”
• Without our “Teflon-coating” of magnetic
field, the earth would have been cooked
many billions of years ago.
Switch Gears…!
• Let’s switch topics, from the magnetic field
to the atmosphere.
• Earth’s atmosphere is unlike any other
planet’s in chemical composition, but it is
like every other planet’s in the processes
that go on within it.
Chemical Composition
• Our current atmosphere is:
 78% nitrogen (N)
 21% oxygen (O)
 1% argon (Ar), helium (He), carbon dioxide
(CO2), water vapor (H2O), and about 20 other
rare gases.
Chemical Composition
• The % of water in the atmosphere can
vary from near 0% over deserts to 0.5% in
the tropics.
• The % of carbon dioxide has doubled in
the past 300 years, from 150 parts per
million (ppm) to about 340 ppm today.
• This means that our atmosphere is
evolving! Could it have evolved in the
past?
Atmospheric Pressure
• Pressure is the downward push of the
column of air above you.
• At earth’s surface, the air (barometric)
pressure is 14.7 pounds / square inch.
• Other units are 29.92 inches of mercury in
a barometer, and 1013 millibars.
The Atmosphere’s Structure
• Earth’s atmosphere has both vertical and
horizontal structure.
• Vertically, the atmosphere is divided into 4
layers.
• Horizontally, the atmosphere is divided
into 6 circulation cells, 3 in the northern
hemisphere & 3 in the southern.
4 Layers
• Troposphere, the weather layer. From
the earth’s surface to 10 km up. It gets
colder the higher up you go within this
layer.
• Stratosphere, the circulation layer. The
jet stream and ozone layer that protects us
from UV light are in this layer. Extends
from 10 to 40 km up. Temperature rises
as you go up within this layer.
4 Layers continued
• Mesosphere, a middle layer, up to 75 km.
Here the air pressure is only 1/10,000th of
the pressure at the earth’s surface. The
temperature again falls as you go up
within this layer.
Thermosphere, the hot layer, up to 120
km. This is the outer edge of earth’s
atmosphere. Here, the temperature
equalizes with the temperature of the hot
solar wind. This is where auroras form.
What sets
off one
layer from
the next
is the way
the
temperature
varies
within it.
How does the atmosphere affect
the surface?
• …in 4 ways:
• 35% of the sunlight that hits the
atmosphere is reflected back into space
by clouds.
• The % of visible light reflected by a planet is called
its albedo. Earth’s albedo is 0.35.
• Clouds, ice, deserts all increase albedo.
• A high albedo generally means that the planet has
a cold surface (lots of ice.)
How does the atmosphere affect
the surface?
• 33% of the sunlight is absorbed by gases
and dust, but then is re-radiated as
infrared (heat). Much of this infrared light
goes back into space and is lost.
• The absorption of light is called attentuation or
extinction (just like the dinosaurs!)
• Greenhouse gases (water vapor, carbon dioxide,
methane or CH4) help to trap the heat and prevent
it from going back into space.
• Without the greenhouse gases, earth’s surface
would be about -18oC.
How does the atmosphere affect
the surface?
• Dust in the atmosphere also causes
reddening, a process where the blue light
is scattered, but red light is allowed to
pass straight through.
• We see the scattered blue light as the blue of our
sky. We also see the red of sunset as it passes
straight through the atmosphere.
• Reddening also happens in space when starlight
passes through dusty nebulas.
• By the way, only 32% of the sunlight makes it to
the surface.
The
reddening
effect.
Atmospheric Circulation
• Earth’s atmosphere has 3 circulation cells
in each hemisphere (called Hadley cells
on other planets).
• The northern-most is the polar cell, from 90o to 60o
north latitude
• We live in the temperate cell, from 60o to 30o north
latitude.
• The southern-most is the tropical cell, from 30o
north to the equator.
Atmospheric Circulation
• How does the air circulate?
Warm air rises at the equator, cools off at high
altitude, then falls back to the surface at 30o north
latitude. It eventually circulates back to equator.
The polar cell operates by cold air falling at the north
pole, flowing away from the pole, warming and
rising at about 60o north latitude.
The temperate circulation cell is just caught in the
middle between the tropical & polar cells.
Atmospheric Circulation
• If the earth didn’t rotate, the air in the
circulation cells would simply move north
and south.
• However, the earth’s rotation causes the
Coriolis Effect. This causes moving wind
to turn or deflect to the right in the northern
hemisphere. The circulation cells turn
into tubes, allowing the winds to move all
the way around the globe.
Click here for an animation
of the Coriolis Effect
Other Planets
• How are other planet’s atmospheres
different? Other planets rotate faster or
slower, are hotter or cooler.
• How would rotating faster affect the
atmosphere? It might turn circulation cells
into bands, where the winds simply move
from west to east or east to west.
• Hotter temperatures could be expected to
make the wind speeds higher.
East – west bands of winds result from a very rapid rate of rotation.
Where did the atmosphere come
from?
• Some water and gases were contributed
by comets, meteors, and other
planetissimals impacting on earth’s
surface.
Where did the atmosphere come
from?
• But most of the atmosphere came from
volcanic outgassing. Volcanoes release
over 100 billion kilograms of water vapor
and gases into the atmosphere every year.
• Over 4.5 billion years, 5 x 1020 kilograms
of water and gases have been released.
This is enough for the atmosphere and the
oceans!
Did the atmosphere change?
• Earth has had 3 atmospheres.
• The first atmosphere was hydrogen (H)
and helium (He) from the original solar
nebula.
• Since these gases are light and the earth
was hot way back then, most of the H and
He was eventually lost to space.
2nd atmosphere
• After the H and He escaped into space,
only the gases that were too heavy to be
lost were left behind: nitrogen (N2) and
carbon dioxide (CO2).
• We still have the N2 today, but where did
the CO2 go?
• Most of it dissolved in the oceans,
combined with calcium (Ca) and was
turned into limestone. Some was
absorbed by photosynthetic bacteria.
The layer of limestone below
formed by the chemical equation:
CO2 + CaO
CaCO3
3rd atmosphere
• As the photosynthetic bacteria began to
use the CO2, it began to produce oxygen
(O2).
• Some of the bacteria evolved into
photosynthetic plants which increased the
rate of O2 production.
• Our present atmosphere is N2 and O2 in
about a 4:1 ratio.
First came the photosynthetic bacteria,
then the green plants. Both added
oxygen (O2) to our atmosphere.
Earth’s Geology
• Earth’s surface changes by processes that
are similar to some of the other planets:
• Wind blows particles that cause erosion and build
up structures like sand dunes and dust fields.
• Flowing water cuts canyons and river beds, and
transports material. Floods can erode huge
channels.
• Flowing ice or lava can act much like flowing
water.
• Subsurface movements cause hills, mountains,
volcanoes, huge cracks and rift valleys.
Earth’s Geology
• Earth also has larger-scale processes that
other planets don’t have: plate tectonics.
• Earth’s crust is divided up into about 20
large pieces or plates of rigid crust that
float on top of flowing mantle.
• Where these plates come together or pull
apart is where we get mid-ocean ridges,
chains of volcanoes and mountains, and
earthquakes.
Earth’s Geology
• The movement of plates is driven by hot
currents of magma welling up from deep
within the mantle of the earth.
• Look at the following diagrams closely.
Look especially for places where the
plates are separating or crushing together,
because we’ll be looking for similar
features on other planets too!
A plume of hot magma is welling up from the deep mantle. Notice
how it pushes up the crust. A mid-ocean ridge may form here.
The next time you think of the
earth, remember all of the
parts that make it up.
We’ll use this
information
again, when
we start
looking at
the other
planets!
Photo & Audio Credits
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NASA
U.S. Geological Survey
Donald E. Davis – NASA
Cornell University
B. Tissue