|One of the
unique features of Planet Earth is the existence of its external layer:
The Atmosphere, which is the result of chemical, physical and biological
evolutionary processes taking place for more than 4.5 billion years. The
Earth's gravitational field plays and important role in the development
of the atmosphere.
The main atmospheric gases are nitrogen
and oxygen, which comprise slightly more than 99 percent of the air we
breathe. The remaining gases include argon, carbon dioxide, and water vapor,
with trace amounts of carbon monoxide, helium, hydrogen, krypton, methane,
neon, ozone, and xenon.
The ancient atmosphere formed by
a process know as degassing, from gases spewing from volcanoes. The early
atmosphere had mixtures of sulfur dioxide, carbon dioxide, nitrogen and
water vapor, with little oxygen. Condensation play a vital role in evolution
of the atmosphere. As it cooled, water vapor from volcanic emissions
condensed and formed oceans.
Once water was present, solar radiation
helped alter the atmosphere still further. Ultraviolet radiation from the
sun broke up water molecules in the air and oceans. Oxygen freed from water
combined with various substances to form, among other things, methane.
Eventually the atmosphere contained large quantities of water, methane,
and ammonia. With more ultraviolet light and time, the atmosphere was transformed
into a mixture of carbon dioxide, nitrogen, water, oxygen, and ozone(O3).
Ozone is an unstable molecule, but
so much was made that it lasted long enough to form a protective layer
above the Earth. There ozone absorbed the ultraviolet radiation, helping
stabilize the water on the planet surface and atmosphere. As life evolved
in the oceans, photosynthesis began.
Photosynthesis is a process by which
plants harness the energy of the sun and convert it into biochemical matter.
Such a process added more oxygen to the atmosphere and oceans where the
plants lived. The oxygen content of the oceans gradually increased until
it could support marine life capable of respiration. That happened about
570 million years ago. It took another 170 million years before there was
enough oxygen in the atmosphere for air breathing land animals to evolve.
The bulk of the Earth's atmosphere
is made up of nitrogen (79 percent) and oxygen (20 percent). The remaining
1 percent of the gases are: argon (0.9 percent), carbon dioxide (0.03 percent),
and trace amounts of ozone, methane, carbon monoxide, hydrogen helium,
neon, krypton, xenon and, of course, water vapor.
has a layered structure. From the earth upward, the layers are the troposphere:
level to 6 km, the stratosphere:
to 45 km (contains the ozone layer), the
to 60 km, the thermosphere:
60 to 500 km, and the exosphere:
500 km, which merges with the thin gases of interplanetary space. The boundaries
between the layers are not sharply defined, and they vary with latitude
One important component missing
from the early atmosphere was free oxygen (or-oxygen that is not combined
with any other elements). Oxygen makes up about 20% of the modern atmosphere.
Evidence that oxygen was absent from the early atmosphere comes from several
sources: (1) The first line of evidence is the formation the socalled banded
iron formations, which is a rock unique to Precambrian times, . These
rocks contain many very thin, interbedded layers of iron and silica. The
iron is in the reduced state (not oxidized, that is, not combined with
oxygen). If free oxygen had been present at the time of this rock's formation,
the iron would have been oxidized (chemcally joined with oxygen to produce
FeO) --- similar to what happens when a garden tools rusts. (2) Oxygen
is not emitted in large quantities from present-day volcanoes, so by analogy,
the volcanoes of Archean time are unlikely to have emitted significant
volumes of oxygen. The oxygen found in the modem atmosphere must
come from somewhere else. Two sources have been identified: (1) One source
of oxygen was photosynthesis by plants. Once plants had evolved, their
photosynthesis processes contributed great quantities of oxygen to the
atmosphere through the breakdown of water. It is known that the addition
of oxygen to the atmosphere was gradual. Although the earliest atmosphere
was devoid of oxygen, by the end of Precambrian time (about 570 my ago),
almost all the oxygen now in the atmosphere was present. (2) The
other source of oxygen was inorganic., chemical reactions that occur in
the upper atmosphere and produce oxygen as an end product. These reactions
are collectively called photochemical dissociation processes.
The absence of oxygen from the atmosphere
had a major role in the evolution of life--an oxidizing environment would
have precluded the formation of cells.