wrote in message news:cnjpel$2jap$1{at}darwin.ediacara.org...
> A related question. What determines the density of earths atmosphere
> and the proportion of oxygen? Or rather how much can it vary between
> planets?

One factor often overlooked is combustion.  If you could magically remove
nitrogen from the earth's atmosphere, you might think that the O2
proportion of the atmosphere would increase.  But then forest fires would
be much more severe.  The O2 percentage would drop back to ~20%, with CO2
taking the place of N2 as the inert buffer.  We would end up with a lower
oxygen partial pressure than we started with.

But give us a thicker atmosphere by magically adding CO2 and N2, and you
may end up with more O2 in the atmosphere, not in terms of percentage but
in terms of partial pressure.
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