Science 010 - Fall, 1995

Lecture 2 - Sept 12

The formation, evolution and maintenance of the atmosphere

1. The dimensions of the atmosphere

   The atmosphere is very thin relative to the size of the Earth, and its mass is tiny.

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2. The ancient atmosphere

   a) Isotope dating (Xe - Ar) indicates that the atmosphere formed in the first few hundred million years after the Earth formed.

   b) Early composition deduced by the following logic chain: Presumably it formed by outgassing from the earth's mantle which presumably was cooling during this period. As we will see, there are several plausible scenarios. Very likely it was a reducing (and not an oxidizing) atmosphere. Gases present were either very stable, or the end products of the probable reactions under assumed conditions.

   c) Urey-Miller experiments

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3. Atmospheric evolution

   a) Living organisms create gases as metabolic products. The aggregate activity of the earth's biota is large enough to have had a major impact on the atmosphere's composition. I'll give a brief overview of the metabolic pathways glycolysis, aerobic respiration, and photosynthesis.

   b) The latter is important because it creates oxygen as a byproduct. This has accumulated in the atmosphere since about 2000 million years b.p. (before present). This accumulation has been very important in Earth history and the evolution of the atmosphere (eg ozone). Oxygen and other 'biogenic' gases such as methane and nitrogen are now important constituents of the atmosphere, but it wasn't always this way.

   c) Carbon dioxide has both biotic (respiration) and abiotic sources (e.g. volcanoes), and has both biotic (photosynthesis) and abiotic 'sinks' (weathering). The relative importance of these processes is not entirely clear. It is clear that when you drive your car you are combusting -quite literally - fossilized prehistoric atmosphere.

   d) The ozone layer (briefly)

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4. Mars and Venus

   Our dead (so far as we know) neighbouring planets have atmosphere of almost pure carbon dioxide. Current thinking on how this happened will be outlined. It seems that the lack of life forms is critical (cause and effect unclear though) and plate tectonics may also be important.

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5. Dynamic (non)equilibria

   The measurerments of carbon dioxide (Keeling's curve) made since 1958 show that the atmosphere is very dynamic. I plan to use these measurements to illustrate the notion of of (perturbed) dynamic equilibrium, and to lead into a discussion of the greenhouse effect, emphasis to be on how we know what we think we know. Most of the above material is covered in the first three Sci Am readings. Sample (simple) assignment question: Air pressure at sea level is equivalent to about 1.04 kg cm-2 (probably more familiar as 14 lbs per square inch, but scientists HATE these archaic units.) Calculate the mass of the entire atmosphere if the diameter of the Earth is about 8000 km. Express this as a percentage of the mass of the Ocean, and as a percentage of the mass of the Earth. (Some library work here!)

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