How can I tell if an atmosphere is suitable for life as we know it?

I've seen a couple questions before (like this one and this one) that ask about the habitability of a planet, with a key part of that being if the atmosphere is survivable. It seems like we should have one question that covers all the basics of atmospheric habitability.

So, what does the composition of an atmosphere need to look like in order for it to be suitable for life as we know it? Which gases need to be present, and in what concentrations? Which gases can be present without significantly affecting habitability (other than not leaving enough room for necessary gases)? Which (common) gases should not be present in significant amounts?

Keep in mind that for a lot of gases, it is the partial pressure that is important. For example, at 1 atm you should have at least 16% and (much?) less than 60% oxygen in the atmosphere, but at 0.5 atm you'd need 32% oxygen as the minimum (and 100% oxygen might be safe long term?)

posted over 7 years ago

Rob Watts‭

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