The Geochemistry of the Earth - Thanatia: The Destiny of the Earth's Mineral Resources - page 107

5.3 The atmosphere
The atmosphere is the colourless, odourless and tasteless gaseous envelope sur-
rounding the Earth. Its relative mass compared to the other spheres of the Earth
is minuscule (see Table 5.1). Nevertheless, it is a crucial geochemical reservoir,
providing conditions essential for sustaining life by supplying O 2 , CO 2 , moisture
and many nutrients. The atmosphere also plays a very direct role in controlling the
Earth's climate reducing the temperature extremes between day and night and via
the absorption and scattering of sunlight including infrared radiation.
The atmosphere is made up of several layers with different qualities (Turekian,
1969; Kendall et al., 1974; Strahler, 1975):
The troposphere is the lowest atmospheric layer. It begins at the planetary
surface and extends between 7 to 17 km. It contains over 75% of all the at-
mospheric gases and vast quantities of water and dust. Almost all weather and
climatic phenomena that physically affect Man take place within the tropo-
sphere, caused by the churning of its mass. The troposphere is the region in
which the infrared radiation is absorbed mainly by water vapour which is an
important factor in surface temperature.
The stratosphere extends from the troposphere about 50 km. This thin layer
constitutes 19% of the atmospheric gases and a small quantity of water vapour.
Temperature increases with height because of the absorption of ultraviolet light
by the ozone layer.
The mesosphere extends from about 50 km to the range of 80 to 85 km. The
gases in the mesosphere are too thin to absorb much of the sun's radiation
but are thick enough to slow down meteorites. In this case, the temperature
decreases with height.
The thermosphere begins at 80-85 km and ends at more than 640 km. The gases
of this sphere are even thinner than those of the mesosphere, but they absorb
ultraviolet light from the sun and as a consequence, temperature increases with
height.
The ionosphere forms part of the thermosphere and is made up of electrically
charged gas particles ionised by solar radiation. It plays an important role since
it influences radio propagation to distant places on Earth. Furthermore, it is
responsible for auras.
And finally the exosphere, which is the outermost layer of the atmosphere and
extends from 500 to 1000 km up to 10,000 km. It is composed of free-moving
particles that may migrate into and out of the magnetosphere. In this layer,
gases get thinner and thinner until they drift off into space.
The stability of the physical and geochemical conditions of the atmosphere is
being altered by the action of Man through air pollution. The greatest source of
emissions is created through the burning of fossil fuels, generating huge quantities