Activity 2: The Earth

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Activity 2 The Earths Atmosphere
Magnetic Field
Module 7 Home Planet the Earth
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Summary

• In this Activity, we will investigate
• (a) the composition of the Earths atmosphere,
• (b) layers temperatures in the Earths
  atmosphere,
• (c) the effects of sunlight,
• (d) the greenhouse effect,
• (e) the ozone layer,
• (f) atmospheric circulation, and
• (g) the structure of the Earths magnetic field.

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(a) The composition of the Earths
atmosphere
The Earths atmosphere has played a pivotal part
in our evolution, and as we shall see in the
next Module, it toohas evolved dramatically with
time.
Other planets have atmospheres too, though none
quitelike ours! Well be comparing their
atmospheres to oursin later Modules.
We can study an atmosphere in various ways. The
easiestway to start is with its bulk (i.e.
average) properties
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The bulk composition of the Earths atmosphere
is approximately

• 78 molecular nitrogen
• 20 molecular oxygen
• 0.03 carbon dioxide,
• and
• up to 2 water vapour.

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(b) Layers of the Earths Atmosphere
? 400 km
Ionosphere
90 km
Mesosphere
50 km
Ozone Layer
Stratosphere
18 km
Tropopause
14 km
Troposphere
0 km
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Temperature variations in the Earths
atmosphere
? 400 km
Ionosphere
90 km
Mesosphere
50 km
Ozone Layer
Stratosphere
18 km
Tropopause
? 220oK
? 280oK
14 km
Troposphere
0 km
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• The troposphere is mainly heated by infrared
  radiation re-emitted by the ground, so the
  temperature in the troposphere decreases with
  altitude.

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? 400 km
Ionosphere
90 km
Mesosphere
50 km
? 260oK
Ozone Layer
Stratosphere
18 km
Tropopause
14 km
Troposphere
0 km
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• The ozone layer is in the upper stratosphere and
  lower mesosphere.
• The ozone absorbs ultraviolet radiation from the
  Sun, and this process heats up the neighbouring
  layers
• - which are therefore warmer than the upper
  troposphere.

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? 400 km
Ionosphere
? 190oK
90 km
Mesosphere
50 km
Ozone Layer
Stratosphere
18 km
Tropopause
14 km
Troposphere
0 km
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• The temperature gradually drops again as we go up
  in altitude through the mesosphere,

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? 400 km
? 270oK
Ionosphere
90 km
Mesosphere
50 km
Ozone Layer
Stratosphere
18 km
Tropopause
14 km
Troposphere
0 km
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• until we reach the ionosphere (sometimes called
  the thermosphere) which is heated by absorbing
  energy from energetic X-rays from the Sun, so the
  temperature there can be quite high

- but the density is extremely low.
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(c) The Effects of Sunlight

• The Earths atmosphere is profoundly affected by
  another member of our Solar System .

the Sun.
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1.37 kJ of energy per square metre arrivesat
the Earths orbit every second from the Sun!
The solar energy incident on the daytime side of
the Earthis eventually reflected or absorbed.
The absorbed part heats the Earths atmosphere
and surface. Without its atmosphere, the Earths
surface temperature would vary more widely, and
its average would be well below freezing.

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Components of the Earths atmosphere and surface
absorb and reflect the suns radiation by
differing amounts
land absorbs well
clouds reflect well
sea absorbs the most
ice reflects the most
The ability of a surface to reflect light is
called its albedo.
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The formal definition of albedo is the fraction
of incident light reflected from a surface, and
so has a value between 0 and 1
no light reflected
all light reflected
What do you expect the average albedo of the
Earth to be - smaller or greater than 0.5?
The Earths average albedo is 0.37. This makes
sense - both sea and land have very low albedos.
Clouds reflect well, but clouds cover only part
of the Earth on average.
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We will see in later Modules that Mercury, a
planet with no cloud, has a very low albedo
(0.12) - whereas Venus, which is permanently
shrouded in cloud, has a very high albedo (0.76).

• There are actually several different types of
  albedo, including
• monochromatic albedo, which is simply the ratio
  of incident energy to reflected energy at any
  particular wavelength (e.g. in the optical)
• Bond albedo is the ratio of the total radiation
  reflected from a surface to the total light
  incident from the Sun averaged over all
  wavelengths

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(d) The Greenhouse Effect
All planets absorb energy from the Sun, but long
agothey reached equilibrium - that is, the
amount of energy they absorb per second is equal
to the amount per second they re-radiate out
into space.
Incident sunlight
Reflected sunlight
Re-radiated energy
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Water vapour, methane and (to a lesser extent)
carbon dioxide and certain other gases in our
atmosphere are good absorbers of infrared
radiation, so they trap much of the re-radiated
energy inside the atmosphere.
Most of the re-radiated infrared radiationis
trapped within the atmosphere
The result is that the Earth is
significantlywarmer than it would be without an
atmosphere.
Water vapour, carbon dioxide and methaneare
examples of greenhouse gases.
To find out why the term greenhouse is (mis)used,
click here.
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Water vapour is the main greenhouse gas. Its
levelsin the Earths atmosphere vary from time
to time, but remain roughly constant on average.
The current scientific debate about the
greenhouseeffect centres on the rising levels of
carbon dioxide and methane in the Earths
atmosphere, due to sources such as the burning of
fossil fuels and effects such as deforestation
and increased agricultural activities.
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The average atmospheric temperature of the Earth
appears to be rising somewhat - is this due to
increased levels of greenhouse gases due to human
activities, or other natural effects such as
long-term fluctuations in the Earths weather?
This is a continuing (and important) scientific
politicaldebate. From whatever cause, geological
evidence suggests that the Earth is now as warm
as it has ever been in the last 150,000 years,
and the Earths global temperature increased by
about 0.6C in the 20th Century. Unfortunately
we cant afford to watch for the few hundred
years it would take to establish a firm trend,
andthe cause and effects, conclusively either
way!
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As we will see, there is one place in the Solar
System where we can see the effects of a runaway
greenhouse effect Venus.
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(e) The Ozone Layer
Part of the incident sunlight striking our
atmosphere ismade up of ultraviolet (UV)
radiation.
Incident sunlight,including UV radiation
Earths upper atmosphere
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The UV radiation breaks up oxygen molecules (O2),
with the result that they recombine as ozone
molecules (O3).
Incident sunlight,including UV radiation
These ozone molecules accumulatein a 30km thick
layer (starting about25km above the Earths
surface) -the Ozone Layer.
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Ozone is a strong absorber of UV radiation, so
the ozonelayer protects the oxygen in the lower
atmosphere from most of the suns UV rays.
Incident sunlight,including UV radiation
The ozone layer
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In recent times the ozone layer appears to be
thinningout. For example, the ozone
concentration over the Antarctic dropped by a
factor of two from the 1950s to the 1980s.
The prime suspects are chlorofluorocarbons
(CFCs),released from old-style refrigerant
systems spray cans.Each chlorine atom is
capable of breaking up approximately 100,000
ozone molecules.
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The Antarctic and Arctic regions are particularly
at risk, because, in the polar winters, the
stratosphere in those regions becomes cold enough
to form water ice and nitric acid ice particles,
which act as catalysts to accelerate the
production of chlorine molecules.
Once summer returns to these regions, sunlight
breaks (photodissociates) the chlorine
molecules up into chlorine atoms, which then in
turn attack the ozone layer.
International efforts are now taking place to
reverse this trend. With prompt action, the
levels of ozone in the ozone layer can be built
up again.
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(f) Atmospheric Circulation
As we who live here well know, the Earths
atmosphere is not static. Winds storms are
regular features on this and other planets.
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As sunlight warms the surface of the Earth, it
warms the layer of air directly abovethe
surface.
Warm air expands,becoming less dense and
lighter than the airabove it.
Therefore it rises,
and heavier air above falls down to take its
place.
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The layer of air that had risen starts to cool
down, becomingdenser again.
The layer of air that is now directly above the
surface warms up and rises in turn.
- so the whole cycle repeats itself.
These air currents are convective currents.
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Different areas of the Earths surface - e.g.
land water - reach different equilibrium
temperatures.
At the waters edge on a hot summers day, for
example, warm air rises over the land and cooler
air from over the ocean takes its place -
providing a cooling onshorebreeze.
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In these ways convective currents are set up in
the Earths lower atmosphere.
The Earths rotation
twists the convective currents to establish
global atmospheric circulation patterns.

In the Activities on the Jovian planets,we will
compare their atmosphericcirculation to that of
the Earth.
Jupiter, the Dominant Gas Giant Planet, and The
Other Jovians
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The Earthsatmosphericcirculationpatterns
aretraced byits cloud patterns.
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The Earths atmospheric circulation patterns are
complicated by the presence of significant
amounts of water vapour.
Water vapour is the only gas in the Earths
atmospherewhich can change to a liquid (in
clouds) and fall to the surface (as rain).
When water vapour turns to rain, the local air
pressuredrops somewhat, providing local
variations in the aircurrents and making the
atmospheric circulation morecomplex.
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(g) The Structure of the Earths Magnetic Field
The Earth acts much like a bar magnet, possessing
a magnetic field which deflects compasses on
theEarths surface to point northwards.
We represent the magnetic field at any point on
or above the Earths surface by a line pointing
in the direction a compasswould point.
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rotation axis
The magnetic field axisis tilted at 12 to the
axisof rotation of the Earth.
magnetic field axis
Careful study of the magnetic structure of
ancient rocks suggests that the Earthsmagnetic
field has reversedits direction several
timesover the Earths history - though exactly
how this reversal mechanism works is not
understood
Click here to see an animation of the earths
Magnetic Field
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It is known, however, that the magnetic poles are
constantly on the move. The location of the
magnetic north pole has been recorded for over
170 years and has been steadily moving north by
an average 10 km per year.
The global magnetic field strength has also
weakened by about 10 since the 19th century.
The jury is still out as to whether this means we
are due for another field reversal or not.
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To understand magnetic reversal, we first need to
understand what actually drives the Earths
magnetic field.
Magnetic fields are created by moving electric
charges. The Earths magnetic field is thought to
be produced by the motion of charged particles in
the convective currents of the metallic liquid
outer core.
liquid outer core
The theory of the Earths self-generating
magnetic field is called the dynamo effect,
though the exact details are not fully
understood.
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For a planet to have a magnetic field, it needs a
region where charged particles are moving in
convective currents. The planets rotation is
also important in helping generate its magnetic
field.
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The Earths magnetic field acts to protect life
on Earth from cosmic ray particles coming from
the Sun and from deep space.
magnetic field axis
Cosmic rays are mostly deflected by the Earths
magnetic field, some spiraling around it till
they reach the atmosphere over the poles.
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When the number of cosmic rays is high, the
energy they release when striking the atmosphere
is seen at the polar regions as the northern and
southern lights, or aurorae.
Aurora are rapidly varying colourful displays
that shimmer across large regions of the sky.
The different colours are mainly due to excited
oxygen (green and red) and nitrogen (blue) atoms
and molecules in the upper atmosphere.
We will discuss aurorae again in the Activity on
High Energy Astronomy.
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Summary

• In this Activity, we have looked at the average
  properties of the Earths atmosphere, including
  its composition and structure. The effects of
  the Sun on our atmosphere and atmospheric
  circulation were also investigated, and we
  introduced the Earths magnetic field.
• In the next Module, we will investigate how the
  Earth has evolved since its formation over 4.5
  billion years ago.

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Image Credits
NASA View of Australia http//nssdc.gsfc.nasa.gov
/image/planetary/earth/gal_australia.jpg NASA
Monsoon over India http//earth.jsc.nasa.gov/lores
.cgi?PHOTOSTS51F-31-0069 NASA View of the
Mid-Pacific Ocean http//nssdc.gsfc.nasa.gov/image
/planetary/earth/gal_mid-pacific.jpg NASA The
Northern Lights http//www.athena.ivv.nasa.gov/cur
ric/space/solterr/aurora.html NASA World Cloud
Cover Pattern http//www.hq.nasa.gov/office/ese/
gallery/Originals/cloud.jpg Natural Resources
Canada Movement of Earths north magnetic
pole http//www.geolab.nrcan.gc.ca/geomag/images/n
mppath2001.gif

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• Now return to the Module home page, and read more
  about the Earths atmosphere and magnetic field
  in the Textbook Readings.

Hit the Esc key (escape) to return to the Module
7 Home Page
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The Greenhouse Effect

• Greenhouses maintain a higher temperature than
  their surroundings - which is why delicate plants
  are kept in them in cold winters.

They achieve this due to their glass (or plastic)
walls, which let light in which is largely
absorbed by the plants and surfaces inside the
greenhouse. These reradiate infrared radiation,
which warms up the air in the greenhouse.
This sounds pretty similar to the situation of
the Earth andits atmosphere, which is why the
term greenhouse effectis used.
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• There is an important difference though. Although
  the air and walls of a greenhouse do absorb
  infrared radiation, the main reason that a
  greenhouse stays warmer than its surroundings in
  winter is that its walls trap the warm air,
  preventing cooling drafts.

So the Earths atmosphere is not exactly like a
greenhouseit has no walls. Our atmosphere is
relatively warm becauseit traps re-radiated
infrared radiation by absorbing most of it
before it reaches space.
Back to the Activity!
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