Quote:
Originally Posted by CraigS
Oxygen can be liberated by photolysis of water, but this is a slow process, (compared with the absorption of oxygen by a rocky crust). I also think Europa has an oxygen atmosphere for this reason.
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True, but the main problem is the reactivity of Oxygen. In conditions like on Europa, the fact that the Oxygen is there is due to the low temps and the relatively dim light. The rates of reaction are very slow. On Earth like planets, whilst photolysis rates are much higher than they would be for your average outer solar system conditions, the fact that Oxygen is so highly reactive and the conditions on these planets drive the reactions rates to substantially higher levels, would mean that photolysis is close to negligible so far as Oxygen production is concerned. To sustain a substantial amount of Oxygen in an atmosphere of an Earth like planet, you would need vast amounts to be continuously supplied to its atmosphere. There's only one way of doing that....life.