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by TeMPOraL
1002 days ago
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The first one. It can be produced through non-biological processes and is fairly stable, but is also necessary for "life as we know it". Gaseous oxygen would be 2 - it's highly reactive, so if you detect it in an atmosphere, it means there must be some kind of activity going on that replenishes it. On Earth, it's what life does. |
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