1C EVOLUTION of the EARTH'S ATMOSPHERE
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1C
The evolution of the Earth's primordial atmosphere, theories of the origin of
life
See also
Section 1A
OUR
ATMOSPHERE - composition, determining % oxygen, uses of gases in air
Section 1B
RECYCLING
OF GASES - carbon cycle, global warming, oxygen
balance and photosynthesis,
and
past ancient atmospheres, changes due to
man's activities
INDEX OF EARTH SCIENCE PAGES
More detailed pages on the chemistry of the atmosphere
(i)
Air pollution,
incomplete combustion, carbon monoxide & soot
(ii)
Greenhouse
effect, global warming, climate change,
carbon footprint from fossil fuel burning
(iii)
Air pollution, sulfur oxides, nitrogen oxides,
acid rain

1C
The EVOLUTION of the EARTH'S ATMOSPHERE
Theories about the composition of the Earth’s
early atmosphere and how the atmosphere was formed have changed and developed
through the 20th century and continue into the 21st century. Unfortunately
evidence for the early atmosphere is quite limited because of the time scale of
4.6 billion years, that is the current estimate of the age of planet Earth and
the time over which the Earth's atmosphere has been evolving.
The current Earth's atmosphere has
been around for about 200 million years but this section describes how we think
the Earth's atmosphere evolved into its current composition. So, the evolution
of our atmosphere? How did our atmosphere evolve after the formation of planet
Earth?
Phase 1: Volcanic action and gas release
 When
the Earth was first formed any gaseous atmosphere would boil away despite the
Earth's gravity pull. After the Earth's original hot molten surface had cooled
sufficiently to form a crust, volcanoes formed and have continued erupting until
the present time, if quite not as frequently as millions of years ago!
From intense volcanic
action it is believed that the early Earth's atmosphere consisted mainly
of carbon dioxide and water vapour and virtually no oxygen.
It is worth noting that the atmospheres
of Mars and Venus are mainly carbon dioxide with little or no oxygen gas,
and may represent what their original atmospheres, and that of Earth were like?
From intense volcanic activity nitrogen would have built up along with water
vapour and
probably small
amounts of ammonia and methane (mainly in the first billion years!). As the Earth continued to cool down
forming a thicker crust, there would be far more volcanic activity that
we see today and far more gases spewed out from volcanoes forming the initial 'stable'
atmosphere and the first seas and oceans from condensed water vapour.
There would be little or no oxygen and the Earth's early atmosphere was probably
mainly carbon dioxide, with some water vapour, rather like the atmospheres on Mars and Venus
today.
Some texts also refer to small amounts of
other gases e.g. hydrogen, nitrogen, carbon monoxide and sulfur dioxide.
Note: We can't be absolutely
sure about the history of the Earth's first atmosphere, but study of the atmospheres of other planets are helping
our understanding
of the Earth's early atmosphere, and all of these gases exist on one or more of the
planets. The initial surface of the Earth after its formation would have been extremely
hot and molten. Therefore much of the hydrogen, helium and other gases originally present would have been
'boiled' off as the Earth's gravity would not be strong enough to hold these
fast moving molecules at high temperatures!
Phase 2: Formation of oceans and the
evolution of the first forms of life
Seas and oceans would
form from condensed water vapour as the
early Earth surface cooled down even more, to the extent that water now
covers 2/3rds of the Earth's surface. Carbon dioxide and ammonia would dissolve
in this water as would some of the minerals of the Earth's crust, so the formation
of the oceans would considerable decrease the concentration of the gases in air.
When the oceans formed and carbon dioxide dissolved in the water,
carbonates were then precipitated producing sediments on the seabed, reducing the amount of
carbon dioxide in the atmosphere. The carbon dioxide
could form soluble sodium carbonate, sodium hydrogencarbonate or calcium
hydrogencarbonate as well as insoluble calcium carbonate and other
carbonate deposits. Some limestone deposits are hundreds of millions of
years old.
The seas and oceans contain large
quantities of dissolved salts which were once part of rock formations, now
weathered, eroded and washed away in rivers. These salts do NOT evaporate, unlike the water, so the
oceans have gradually become more concentrated in salts. Much later some salts are
removed as shells and skeletons of marine organisms (rich in carbonates) which
evolved later (see next paragraph onwards!). Some chemical reactions produce
precipitates which form part of the sea-floor sediments, and crystallisation to
form salt deposits e.g. in high concentration warm parts of the world like the
Dead Sea and enclosed seas/lakes may completely dry up to give 'rock salt' and
'potash' sedimentary rock formations.
Primitive bacterial life evolved
about 3500 million years ago (3.5 billion years ago) and
the first green algae like plants evolved about
2700 million years ago (2.7 billion years ago).
It is believed that the first bacteria
were cyano bacteria and were able to use sunlight energy, not by later plant
photosynthesis but utilised particular light absorbing pigments and can
absorb carbon dioxide and release oxygen.
Later green algae (and other marine
organisms), primitive land plants and later more complex plants thrived in
the carbon dioxide rich atmosphere because much of the carbon dioxide would
dissolve in the seas and oceans. These organisms could use the carbon
dioxide in photosynthesis - another step in evolution.
The first marine animals evolved and
their shells and skeletons used carbonates from the ocean.
When plants, plankton and marine animals
died the debris falls to the seabed and get buried by layers of sediment. In
time over millions of years these sediments become highly compressed to form
sedimentary rocks like chalk and limestone. Sometimes oil and gas are
trapped within these sedimentary layers thereby trapping carbon and keeping
the levels of carbon dioxide lower.
Phase 3: Increase in atmospheric oxygen
levels enabled more complex life to exist
All these cyanobacteria, algae and green plants were
absorbing, and therefore removing carbon dioxide from the atmosphere, and producing oxygen in the process of
photosynthesis. So the atmospheric oxygen concentration steadily increased as it
replaced carbon dioxide (decreasing) in the atmosphere and this increase
in oxygen levels facilitates greater evolution of more diverse animal life.
So the evolution of plants brings about a most dramatic
change in the Earth's atmosphere by replacing carbon dioxide with oxygen,
summarised in the photosynthesis equation ...
water + carbon
dioxide == sunlight energy ==> glucose sugar + oxygen
6H2O
+ 6CO2 ====> C6H12O6 + 6O2
After algae had first introduced oxygen
into the atmosphere about 2.7 billion years ago, over the next billion years
plants evolved and the percentage of oxygen gradually increased to
a level that enabled animals to evolve.
When these marine organisms died
and became buried under layers of sediment, the carbon dioxide 'locked up'
by photosynthesis in organic material or shells, ends up in sedimentary
rocks, e.g. insoluble carbonate rocks like limestone or fossil fuels like
coal and oil.
So, both photosynthesis in algae and
plants and sedimentary rock formation decreased the level of carbon dioxide
and increased the oxygen content of the atmosphere.
The increasingly successful evolution of green
photosynthesizing plants
colonising both land and water, produced an increasingly oxygen richer atmosphere
and in so doing removed most of the carbon dioxide from the original early
atmosphere (remember now, oxygen is 21% and carbon dioxide 0.04% of the present
atmosphere, a complete reversal from over 2-3 billion years ago).
This 'oxygenated' atmosphere would be 'polluting' and 'toxic' to
many microorganisms which could not tolerate oxygen, having evolved in a non-oxygen
environment, so many species of organism would have become extinct, but oxygen
tolerant organisms would survive.
AND, by 1000 million (1 billion years) years ago, there was
sufficient oxygen to allow the evolution of respiring animal life.
Respiration of course uses oxygen, now abundant in the Earth's atmosphere.
'A rock note':
Limestone is a sedimentary rock, mainly
calcium carbonate, formed from the shells and skeletons of marine organisms
- these rocks formed to the extent that very little carbon dioxide
survives in the atmosphere - but its presence of only 0.04% is NOT
insignificant.
Coal is a sedimentary rock formed from thick plant deposits that were buried
and compressed over millions of years.
The remains of plankton were
deposited in muds on the sea floor and were covered over and compressed over
millions of years, producing crude oil and natural gas that became trapped
in the rocks.
So, appreciate that
the remains of dead algae, rotting plants, skeletons and shells of marine
creatures became buried under layers of sediment to form sedimentary rocks
like coal (mainly carbon), limestone (carbon dioxide to metal carbonates) and in oil and gas deposits
(hydrocarbons).
Ammonia would be converted to
soluble nitrates mainly by nitrifying bacteria or, to a small extent, ammonia
would be directly
oxidised to nitrogen gas by the newly formed oxygen.
The nitrates are absorbed by plants
to form proteins or converted to atmospheric nitrogen by denitrifying bacteria.
Methane would be oxidised to carbon dioxide and water by the new 'oxygenated'
atmosphere, which is why there is only the tiniest trace left, from volcanoes
and cows!
Ozone (O3) would now be formed as
the oxygen levels in the atmosphere increased and this oxygen would absorb and
filter out much of the ultraviolet light that is harmful to many organisms
(including ourselves).
Ozone is formed in the upper atmosphere and is split back
into ordinary oxygen molecules (O2) when hit by high energy uv
photons - that's the chemistry of the ultraviolet protective action of the ozone
layer.
3O2 (g) ==> 2O3
(g)
As mentioned above, many early organisms
could not cope with rising levels of oxygen and many species would become
extinct - killed off, BUT, (i) the ozone uv filtering and (ii)
increased oxygen in the atmosphere, would then allow a much wider evolutionary development of
much more complex plant and animal
organisms.
Phase 4: Final phase - to date!
By about 200 million years ago the
atmosphere composition is believed to be approximately that of today
(importantly - 21% oxygen), though
we now have rising carbon dioxide and small amounts of gases we regards as
pollutants due to the anthropogenic effects of pollution and fossil fuel burning.
Anthropogenic means any
environmental change or pollution due to human activity.
The greenhouse gases in the atmosphere,
such as water vapour, carbon dioxide and methane, all help maintain temperatures
on Earth high enough to support life.
See also
More on ozone
APPENDIX 1 Gathering the evidence to justify the
theory of how the atmosphere evolved
Geologist examine rock formations and make
various deductions e.g.
The age of rocks can be estimated from
the ratio of various isotopes of metals in a particular rock formation.
Rock formations containing iron
compounds, which are NOT iron oxides, must have been formed before there
were significant amounts of oxygen around. These rock formations are very
ancient suggesting the early atmosphere had little oxygen in it.
The earliest fossils are derived from
tiny organisms e.g. the cyanobacteria, which could survive without any
oxygen in the atmosphere.
The fossils from more complex organisms,
particularly animals, only appear later in the fossil record after the
atmosphere had become much richer in oxygen.
Appendix
2
The Origin of Life
-
There are several theories of how primitive bacterial life evolved
about 3500 million years ago (3.5 billion years).
-
We can do experiments which
provide tantalising clues as to how the basic molecules of life can be
formed.
-
However, there is no accepted
scientific theory that adequately explains the origin of life, all that we
can be sure about is that life has evolved from some kind of primitive cell
microorganisms and that through the course of evolution diverse and more
complex forms of life increasingly inhabited the Earth's surface.
-
The 'primordial soup theory'
is based on the chemistry of what can happen to mixtures of gases when
exposed to high temperatures, uv light or high voltage electrical discharges
(lightning).
-
Some of the first
experiments were done by Miller and Urey who used a mixture of
water, methane, ammonia, carbon monoxide and hydrogen and applied a high
voltage electrical charge to them.
-
Many organic compounds were
formed eg amino acids and sugars and so the electrical spark certainly
sparked some chemical ideas on the origin of life and its subsequent
evolution.
-
These were significant
experimental results because organic compounds form the basis of life on
Earth as we know it.
-
BUT, how these relatively
simple molecules somehow combined to give a self-replicating organism i.e.
the simplest possible living cell, is a complete mystery!
-
Since then many other
experiments done using different mixtures of gases, increasing the range of
organic compounds that could have been formed in the early history of the
Earth offering all sorts of chemical pathways to much more complex
molecules, BUT ....!!!!???? who knows how a self-replication life form
evolved!
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More detailed pages on the chemistry of the atmosphere
(i)
Air pollution,
incomplete combustion, carbon monoxide & soot
(ii)
Greenhouse
effect, global warming, climate change,
carbon footprint
(iii)
Air pollution, sulfur oxides, nitrogen oxides,
acid rain
[SEARCH
BOX]
TOP OF PAGE
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