1C EVOLUTION of the EARTH'S ATMOSPHERE
Doc Brown's Chemistry - Earth Science & Geology Revision Notes for KS4 Science, GCSE, IGCSE & O Level Courses
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
These revision notes on the evolution of the Earth's atmosphere and origin of life on Earth should prove useful for the existing GCSE science courses and the NEW AQA GCSE chemistry, Edexcel GCSE chemistry & OCR GCSE chemistry (Gateway & 21st Century) GCSE (9–1), (9-5) & (5-1) science courses.
More detailed pages on the chemistry of the atmosphere
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 so 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.
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.
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).
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.
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.
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).
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.
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.
Gathering the evidence to justify the
theory of how the atmosphere evolved
Geologist examine rock formations and make various deductions e.g.
Appendix The Origin of Life
More detailed pages on the chemistry of the atmosphere
GCSE/IGCSE/O Level KS4 Earth Science-Geology ANSWER-REVISION-NOTES
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