Reason wrote

>It's still a good assumption. Most planets have reducing atmospheres, being 
>formed that way when the star systems are formed.  It's hard to imagine how 
>a planet can convert to an oxidizing atmosphere without the presence of 
>organisms such as anaerobic bacteria. 

Oxygenated planetary atmospheres can readily form abiogenically. On a newly
formed terrestrial planet, similar to an early Venus, Earth or Mars, where the
planet may have had substantial bodies of water, the water vapor at the top of
the atmosphere can be quite readily UV photodissociated, followed by the
subsequent escape to space of the free hydrogen, leaving an increasingly
oxygen-rich atmosphere over large oceans of water.

This apparently happened on Venus and Mars. Because of the escape of hydrogen
from both Venus and Mars, their water inventory is very low, and the remaining
oxygen has become an oxide of carbon rather than hydrogen in both the soil and
in the atmosphere. Venus is nearly the twin of Earth in every respect, other
than its closer distance to the Sun, but that distance apparently makes all of
the difference. Venus now has only 1/100,000th the water inventory of the
Earth, even though it's believed that the two planets started out nearly
identically. 

Wirt Atmar
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