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Wouldn't it be more correct to say "the presence of oxygen in the atmosphere helps retain water on planets"?

We don't really know how well presence of oxygen correlates with presence of life yet, do we? :)

Well, sort of. It takes a lot of free oxygen to overcome all the various reducing agents usually found on planets (iron, hydrogen, aluminum, lithium, silicon, etc). The only process we know of capable of generating a continuous long-term flow of oxygen is (aerobic) life. The presence of an oxidizing atmosphere (rather than the standard reducing atmosphere) is thus considered an indicator of life.

Happened that way of Earth, anyway. See http://en.wikipedia.org/wiki/Great_Oxygenation_Event

While it's possible, I suppose, that there could be places with such naturally abundant oxygen that there could be free oxygen left over to create a static oxidizing atmosphere, even in spite of geologic churn, astronomy and cosmology says it's unlikely.

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