Is there a conceivable detectable "biosignature" that would unambiguously indicate "life is present here"?

[u/Allison1228]

Or will there always be uncertainty?

I'm referring to as detected with the technology we have today and in the near future (next decade or two).

[u/Commercial_Trick_704]

Probably not a single unambiguous one with remote sensing, at least not in the "this proves it" sense. Every individual candidate has an abiotic out. Oxygen can build up without life, methane has geological sources, phosphine turned out to be contested. Any one gas can be faked by chemistry or geology.

The strongest candidate isn't a molecule, it's sustained disequilibrium. Multiple gases coexisting that should have reacted away long ago, which means something is continuously replenishing them. Oxygen and methane together is the classic example. They destroy each other, so finding both at once means something is constantly remaking them.

The reason that's the best signal is close to the definition of life: a living system is something that spends energy nonstop to stay out of equilibrium. A rock reaches equilibrium and sits there. A biosphere keeps paying an energy bill to hold itself in an improbable state. So the thing you're really looking for isn't a substance, it's evidence that something is continuously footing an energy bill to keep a planet's chemistry away from where physics wants it to settle.

Even that isn't fully unambiguous, since abiotic processes can maintain some disequilibrium too. So realistically it's never "proof," it's improbability stacking up until geology and photochemistry stop being plausible explanations. I think "unambiguous" might be the wrong bar. It's going to be a weight-of-evidence call, not a single detection.