Transcript The Earth`s Atmosphere & Magnetic Field

Module 7: Home Planet
– the Earth
Activity 2:
The Earth’s
Atmosphere
& Magnetic Field
Summary
In this Activity, we will investigate
(a) the composition of the Earth’s atmosphere,
(b) layers & temperatures in the Earth’s atmosphere,
(c) the effects of sunlight,
(d) the greenhouse effect,
(e) the ozone layer,
(f) atmospheric circulation, and
(g) the structure of the Earth’s magnetic field.
(a) The composition of the Earth’s
atmosphere
The Earth’s atmosphere has played a pivotal part in our
evolution, and as we shall see in the next Module, it too
has evolved dramatically with time.
Other planets have atmospheres too, though none quite
like ours! We’ll be comparing their atmospheres to ours
in later Modules.
We can study an atmosphere in various ways. The easiest
way to start is with its “bulk” (i.e. average) properties:
The bulk composition of the Earth’s atmosphere
is approximately:
78% molecular nitrogen
20% molecular oxygen
0.03% carbon dioxide,
and
up to 2% water vapour.
(b) Layers of the Earth’s Atmosphere
 400 km
Ionosphere
90 km
Mesosphere
Ozone Layer
50 km
Stratosphere
Tropopause
18 km
Troposphere
14 km
0 km
Temperature variations in the Earth’s
atmosphere:
 400 km
Ionosphere
90 km
Mesosphere
50 km
Ozone Layer
Stratosphere
Tropopause
Troposphere
 220ooK
 280 K
18 km
14 km
0 km
The troposphere is mainly heated by infrared radiation
re-emitted by the ground, so the temperature in the
troposphere decreases with altitude.
 400 km
Ionosphere
90 km
Mesosphere
Ozone Layer
 260oK
50 km
Stratosphere
Tropopause
18 km
Troposphere
14 km
0 km
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.
 400 km
Ionosphere
 190oK
90 km
Mesosphere
Ozone Layer
50 km
Stratosphere
Tropopause
18 km
Troposphere
14 km
0 km
The temperature gradually drops again as we go up in
altitude through the mesosphere,
 270oK
 400 km
Ionosphere
90 km
Mesosphere
Ozone Layer
50 km
Stratosphere
Tropopause
18 km
Troposphere
14 km
0 km
… 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.
(c) The Effects of Sunlight
The Earth’s atmosphere is profoundly affected by
another member of our Solar System ….
… the Sun.
1.37 kJ of energy per
square metre arrives
at the Earth’s orbit every
second from the Sun!
The solar energy incident on the daytime side of the Earth
is eventually reflected or absorbed. The absorbed part
heats the Earth’s atmosphere and surface.
Without its atmosphere, the Earth’s surface temperature
would vary more widely, and its average would be well
below freezing.
Components of the Earth’s atmosphere and surface absorb
and reflect the sun’s 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.
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
1
0
sea land
all light
reflected
clouds polar caps
What do you expect the average albedo of the Earth to be
- smaller or greater than 0.5?
The Earth’s 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.
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
(d) The Greenhouse Effect
All planets absorb energy from the Sun, but long ago
they 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
this re-radiated energy is
infrared radiation
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 reradiated energy inside the atmosphere.
Most of the re-radiated infrared radiation
is trapped within the atmosphere
The result is that the Earth is significantly
warmer than it would be without an
atmosphere.
Water vapour, carbon dioxide and methane
are examples of “greenhouse gases”.
To find out why the term greenhouse is (mis)used, click here.
Water vapour is the main greenhouse gas. Its levels
in the Earth’s atmosphere vary from time to time, but
remain roughly constant on average.
The current scientific debate about the greenhouse
effect centres on the rising levels of carbon dioxide and
methane in the Earth’s atmosphere, due to sources
such as the burning of fossil fuels and effects such as
deforestation and increased agricultural activities.
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
Earth’s weather?
This is a continuing (and important) scientific & political
debate. 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 Earth’s global
temperature increased by about 0.6°C in the 20th Century.
Unfortunately we can’t afford to watch for the few
hundred years it would take to establish a firm trend, and
the cause and effects, conclusively either way!
As we will see, there is one place in the Solar
System where we can see the effects of a runaway
greenhouse effect: Venus.
(e) The Ozone Layer
Part of the incident sunlight striking our atmosphere is
made up of ultraviolet (UV) radiation.
Incident sunlight,
including UV
radiation
Earth’s upper atmosphere
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 accumulate
in a 30km thick layer (starting about
25km above the Earth’s surface) the Ozone Layer.
Ozone is a strong absorber of UV radiation, so the ozone
layer protects the oxygen in the lower atmosphere from
most of the sun’s UV rays.
Incident sunlight,
including UV
radiation
The ozone layer
In recent times the ozone layer appears to be thinning
out. 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.
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.
(f) Atmospheric Circulation
As we who live here well
know, the Earth’s
atmosphere is not static.
Winds & storms
are regular features on
this and other planets.
As sunlight warms the
surface of the Earth,
it warms the layer of air
directly above
the surface.
Warm air expands,
becoming less dense
and lighter than the air
above it.
Therefore it rises,
and heavier air above falls down to take its place.
The layer of air that had risen
starts to cool down, becoming
denser 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.
Different areas of the Earth’s surface - e.g. land & water reach different equilibrium temperatures.
At the water’s edge on a hot summer’s day, for example,
warm air rises over the land and cooler air from over the
ocean takes its place - providing a cooling onshore
breeze.
In these ways convective currents are set up in the
Earth’s lower atmosphere.
The Earth’s rotation
twists the convective currents
to establish global atmospheric
circulation patterns.
In the Activities*on the Jovian planets,
we will compare their atmospheric
circulation to that of the Earth.
* Jupiter, the Dominant Gas Giant Planet, and The Other Jovians
The Earth’s
atmospheric
circulation
patterns are
traced by
its cloud
patterns.
The Earth’s atmospheric circulation patterns are
complicated by the presence of significant amounts
of water vapour.
Water vapour is the only gas in the Earth’s atmosphere
which can change to a liquid (in clouds) and fall to
the surface (as rain).
When water vapour turns to rain, the local air pressure
drops somewhat, providing local variations in the air
currents and making the atmospheric circulation more
complex.
(g) The Structure of the Earth’s Magnetic Field
The Earth acts much like a
bar magnet, possessing
a magnetic field which
deflects compasses on the
Earth’s surface to point
northwards.
We represent the
magnetic field at any
point on or above the
Earth’s surface by
a line pointing in
the direction a compass
would point.
magnetic field
axis
rotation
axis
The magnetic field axis
is tilted at 12° to the axis
of rotation of the Earth.
Careful study of the magnetic
structure of ancient rocks
suggests that the Earth’s
magnetic field has reversed
its direction several times
over the Earth’s history though exactly how this
reversal mechanism works is
not understood
Click here to see an animation
of the earths Magnetic Field
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 magnetic pole’s
northward journey
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.
To understand magnetic reversal, we first need to
understand what actually drives the Earth’s magnetic field.
Magnetic fields are created by moving electric charges.
The Earth’s 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 Earth’s
self-generating magnetic field
is called the “dynamo effect”,
though the exact details are
not fully understood.
For a planet to have a magnetic
field, it needs a region where
charged particles are moving in
convective currents. The planet’s
rotation is also important in
helping generate its magnetic
field.
magnetic field
axis
The Earth’s magnetic field acts to protect life
on Earth from cosmic ray particles coming
from the Sun and from deep space.
Cosmic rays are mostly
deflected by the Earth’s
magnetic field, some
spiraling around it till they
reach the atmosphere
over the poles.
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.
Summary
In this Activity, we have looked at the average properties
of the Earth’s atmosphere, including its composition and
structure. The effects of the Sun on our atmosphere
and atmospheric circulation were also investigated, and
we introduced the Earth’s magnetic field.
In the next Module, we will investigate how the Earth
has evolved since its formation over 4.5 billion years
ago.
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?PHOTO=STS51F-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/curric/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 Earth’s north magnetic pole
http://www.geolab.nrcan.gc.ca/geomag/images/nmppath2001.gif
Now return to the Module home page, and read more
about the Earth’s atmosphere and magnetic field in
the Textbook Readings.
Hit the Esc key (escape)
to return to the Module 7 Home Page
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 and
its atmosphere, which is why the term greenhouse effect
is used.
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 Earth’s atmosphere is not exactly like a greenhouse:
it has no walls. Our atmosphere is relatively warm because
it traps re-radiated infrared radiation by absorbing most of it
before it reaches space.
Back to the Activity!