Hi ASA
The following link covers work by James Kasting on the Earth's earliest atmosphere...
http://www.psu.edu/ur/NEWS/news/snowballearth.html
According to his figures 0.01% of the current levels of atmospheric oxygen would have oxidised methane in the early atmosphere. This caused a large cooling episode for the Earth because the continents of the time were around the equator and a reduction in the greenhouse effect caused by the methane meant large sea-ice caps formed. These spread unhindered to the equator and froze the Earth. A similar sequence of events occurred perhaps four times between 750 - 570 million years ago.
The relevance? Well many concordist scenarios require obscuration of the heavens to explain day/night prior to sun/moon in Genesis, but a layer of continuous cloud cover is unlikely to have persisted for billions of years. Methane in the upper atmosphere however would've been UV processed into a layer of "smog" similar to that which covers Saturn's moon Titan, and this would've done the job. Its demise is correlated to the appearance of continents collected at the equator and life arising on land almost exactly the sequence recorded in Genesis...
coincidence?
Adam
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