The present atmosphere of the Earth is probably not its original atmosphere.
Our current atmosphere is what chemists would call an oxidizing
atmosphere, while the original atmosphere was what chemists would call a
reducing atmosphere. In particular, it probably did not contain oxygen.
Composition of the AtmosphereThe original atmosphere may have been
similar to the composition of the solar nebula and close to the present
composition of the Gas Giant planets, though this depends on the details of how
the planets condensed from the solar nebula. That atmosphere was lost to space,
and replaced by compounds outgassed from the crust or (in some more recent
theories) much of the atmosphere may have come instead from the impacts of comets
and other planetesimals rich in volatile materials.
The oxygen so characteristic of our atmosphere was almost all produced by
or, more colloquially, blue-green algae). Thus, the present composition of the
atmosphere is 79% nitrogen, 20% oxygen, and 1% other gases.
Layers of the AtmosphereThe atmosphere of the Earth may be divided
into several distinct layers, as the following figure indicates.
|Layers of the Earth's atmosphere
The TroposphereThe troposphere is where all weather takes
place; it is the region of rising and falling packets of air. The air pressure
at the top of the troposphere is only 10% of that at sea level (0.1
atmospheres). There is a thin buffer zone between the troposphere and the next
layer called the tropopause.
The Stratosphere and Ozone LayerAbove the troposphere is the
stratosphere, where air flow is mostly horizontal. The thin ozone
layer in the upper stratosphere has a high concentration of ozone, a
particularly reactive form of oxygen. This layer is primarily responsible for
absorbing the ultraviolet radiation from the Sun. The formation of this layer is
a delicate matter, since only when oxygen is produced in the atmosphere can an
ozone layer form and prevent an intense flux of ultraviolet radiation from
reaching the surface, where it is quite hazardous to the evolution of life.
There is considerable recent concern that manmade flourocarbon compounds may be
depleting the ozone layer, with dire future consequences for life on the Earth.
The Mesosphere and IonosphereAbove the stratosphere is the mesosphere
and above that is the ionosphere
(or thermosphere), where many atoms are ionized (have gained or lost
electrons so they have a net electrical charge). The ionosphere is very thin,
but it is where aurora take place, and is also responsible for absorbing the
most energetic photons from the Sun, and for reflecting radio waves, thereby
making long-distance radio communication possible.
The structure of the ionosphere is strongly influenced by the charged
particle wind from the Sun (solar
wind), which is in turn governed by the level of Solar activity. One measure
of the structure of the ionosphere is the free electron density, which is an
indicator of the degree of ionization. Here are electron density contour
maps of the ionosphere for months in 1957 to the present. Compare these
simulations of the variation by month of the ionosphere for the year 1990
(a period of high solar activity with many sunspots) and 1996
(a period of low solar activity with few sunspots):
The adjacent animations simulate the variation by month of the
ionosphere for two different years:
1. The year 1990 (upper image), which was a period of high solar
activity with many (150) sunspots.
2. The year 1996 (lower image),
which was a period of low solar activity with few (10) sunspots.
The plots show electron density contours, which are an indication of
the amount of ionization in the atmosphere. Yellows and reds indicate
larger ionization and blues and greens indicate smaller ionization. Notice
the substantial differences in these two animations, with much stronger
atmospheric ionization in the upper image (the active Sun of 1990) than
the lower image (the quiet Sun of 1996).
The adjacent images are based on these electron density contour
maps of the ionosphere
for months in the year 1957 to the present. Additional animations may be
found in this NOAA
Notice the substantial differences in these two animations, corresponding to
the strong influence of solar activity on the structure of the Earth's