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.

3O_{2 (g)} ==> 2O_{3 (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!

    • basic organic molecules == ???????? ===> self-replicating living organism

• 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

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