Life on Earth would be impossible in the absence of the atmosphere - the thin layer of gas that envelops our globe, writes William Reville. So what is the origin of the atmosphere and how did it establish conditions hospitable to the origin of life?
The diameter of the Earth is 12,756 kilometres. It is difficult to say exactly how thick the atmosphere is because it fades away gradually in its upper reaches. If you ascended vertically from sea level you would run out of breathable air at a height of nine kilometres and you would reach the top of the troposphere (the top layer in which weather occurs) at a height of 15 kilometres. If the planet were a football, the breathable atmosphere would represent a very thin surface skin about 0.25 millimetres thick.
The atmosphere contains a mixture of gases, principally nitrogen (78 per cent) and oxygen (21 per cent). The remaining 1 per cent is composed of water vapour (variable) and carbon dioxide (0.035 percent) and several other gases. High in the atmosphere is another thin layer of a special form of oxygen called ozone. The gaseous composition of the atmosphere has varied considerably over the Earth's history.
The Earth was formed about 4.6 billion years ago. Sedimentary rocks were forming 3.8 billion years ago and traces of life are discernible in these rocks from 3.5 billion years ago. Since sedimentary rocks are formed only under water, liquid water must have been plentiful on Earth 3.8 billion years ago.
At that time, the sun was producing only 75 per cent of its current heat output. In the absence of any mechanism to retain the modest heat that reached here from the sun, the Earth would have been an icy ball with a mean temperature well below the freezing point of water. This frozen state could have persisted indefinitely, despite the sun warming up as it aged, because ice is a good reflector of heat and might never have absorbed enough warmth to melt.
The greenhouse effect (GE) is the mechanism whereby a gaseous atmosphere keeps a planet warmer than it would otherwise be. It explains how the early Earth harboured oceans of liquid water. The term GE comes from the analogy with the garden greenhouse in which the radiant energy of the sun passes unabsorbed through the glass walls and ceiling to warm the ground inside. This warms the air inside the greenhouse, which cannot escape because of the walls and roof.
In the same way, radiant energy from the sun, mostly as visible light, passes almost unabsorbed through the atmosphere and warms the surface of the Earth. The heat is radiated back into the atmosphere as longer wavelength infra-red radiation, but this radiation is largely absorbed by the carbon dioxide and water vapour in the atmosphere. This in turn heats the bottom layer of the atmosphere, triggering the weather systems of the world and preventing the heat from leaking back into outer space.
WHEN the Earth first formed as a mass of molten rock it had no atmosphere. As the Earth cooled, a crust formed. Gases escaped from cracks in the crust and from volcanoes. Volcanic gases included water vapour, nitrogen, ammonia. There was also an enormous amount of carbon dioxide in the early atmosphere.
The water vapour condensed and fell to Earth as liquid, forming the oceans. Sunlight split ammonia into nitrogen and hydrogen. Hydrogen is very light and escaped from earth into space. Nitrogen is a heavy unreactive gas and built up in the atmosphere.
The GE caused by the carbon dioxide-rich early atmosphere kept earth warm enough to support watery oceans. Carbon dioxide is soluble in water and began to dissolve in the oceans, reducing its concentration in the atmosphere and weakening the GE as the sun grew warmer. The dissolved carbon dioxide in the oceans was laid down as carbonate in sedimentary rocks. If we were to release all the carbon dioxide now trapped in rocks we would increase the atmospheric pressure by 60 times.
The atmospheric blanket of air around the Earth spreads heat and smooths temperature differences. On the moon, which has no atmosphere, daytime temperature soars over 100°C and night-time temperature plummets to -150°C. The average temperature of the moon is -18°C whereas the average temperature of the Earth is 15°C, even though both bodies are just about the same distance from the sun.
The early atmosphere on Earth allowed life to begin, but once life flourished it began to change the composition of the atmosphere. Life began as a simple type of bacteria. These cells learned how to harness the energy of the sun to combine the elements carbon, hydrogen and oxygen into the biomolecules that make up their own fabric (photosynthesis).
The early organisms got their oxygen from carbon dioxide in the oceans and their hydrogen from water, breaking the H2O molecule and releasing oxygen as a by-product into the atmosphere. By one billion years ago the composition of the atmosphere began to resemble what it is today - mostly nitrogen and oxygen and just traces of everything else, including carbon dioxide.
William Reville is associate professor of biochemistry and director of microscopy at UCC.)