What constitutes Proof of Life?

[BenM]

For a given planetary mission, be it Mars, Europa or elsewhere, what evidence would convince you that life had existed or was currently living there?

Perhaps more importantly, what do you think would convince the average public that there was life outside of earth. Would it take a Day the Earth Stood Still moment or would biochemical evidence from a remote robot do?

[The Supreme Canuck]

Biochemical evidence would do it for me as long as it was solid. I'm not an expert (or even approaching being one) in this field, so I couldn't tell you exactly what could be called "solid" evidence.

[Jpax2003]

Meg Ryan and Russell Crowe

[Espritch]

The problem with biochemical evidence is that any biochemical process is a chemical process and might occure without a living system (at least theoretically). Fossiles, or actual sighting of living organisms would be more convincing provided we could actually recognize them as such. But with alien life, such evidence might not be readily recognizable as such.