What would that indirect chemical evidence look like? Perhaps an example you have in mind? Outside of non-committal elements like Uranium or something showing up, I'm not sure what that evidence would look like?
No, but in keeping with the "earth-like" avenue of search... If there are other similarities to earth's chemistry (like having water), and we know earth's oxygen comes from breathing... Not evidence, but a good enough reason to look for some. How that happens, idk.
Yeah O2 is an input of cellular respiration. The presence of O2 is more likely an indicator of plant-life than it is an indicator of animal life. But plant life is still life.
You do not need animals to have plants. Plants came before animals here on earth. But I’m not sure of a way for animals to exist without plants. So yeah a stronger indicator of photosynthesis than cellular respiration.
Methane. It is only really transiently generated by geological means and decays quickly when exposed to cosmic radiation. It is also very likely made by any carbon based life ecosystem.
The absence of evidence is not evidence of absence, and so on. We don't know for sure there isn't life on Titan; we just haven't seen enough evidence yet.
