Quote:
Originally Posted by whswhs
What could the resulting atmospheres be like? If you start out with, say, a hydrogen based atmosphere, and life forms native to that atmosphere do something analogous to photosynthesis, breaking up an available liquid or gas, what are the waste products, and what kind of closed chemical cycle might they create? Would the atmosphere be as different from a gas giant atmosphere as Garden is from Ocean? What if you start out with, oh, an ammonia world? Are there new possible atmospheric types?
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The purpose of photosynthesis is after all to extract the stuff you build your body from out of the environment. Since life is going to be made mostly of carbon, the likely atmospheric interactions are inhale CO2 and exhale oxygen or inhale methane (or some other light alkane) and exhale hydrogen (or water vapor if you have an oxygen source). I suppose you might extract nitrogen from ammonia, exhaling hydrogen, or oxygen from water vapor (again exhaling hydrogen), sulfur from H2S (exhaling hydrogen). There aren't really very many plausible atmospheric gases that you could break apart and dump a gas other than oxygen or hydrogen, and hydrogen is probably generally uninteresting - an atmosphere that doesn't already have *lots* of it probably means a world that doesn't have enough gravity to hold onto it long enough to change the atmosphere dramatically.
The reverse, respiration - burning atmospheric gases with something else for energy - might get you a handful of other options - letting you consume ammonia (exhaling hydrogen and nitrogen) or perhaps sulfur oxides exhaling H2S.
Not that *small* amounts of gaseous waste products can't change an atmosphere in ways that matter to human explorers. It doesn't take a lot of HF or HCN or O3 or halogens for humans to decide it's a different atmosphere type, even if it isn't enough for the rocks to care much.