>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

Well said sir.
One point to add. I wonder if the collision of earth and the Mars-sized
planetoid that led to the origin of our moon - was a deciding factor. The
similar sized (as compared to other planets) earth/moon system may have been -
if not necessary, helpful in the origin of life here.
Comment?
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