In September 2025, NASA reported the first tantalising hints of a potential biosignature – a scientifically testable indicator of past or present extraterrestrial life – on the surface of Mars.
The discovery by NASA’s Perseverance rover of leopardskin-like spots on a rock from the Red Planet’s Jezero Crater sparked great public excitement: it was the first persuasive evidence of biological activity on a world outside our own.
But NASA cautioned that the find required “further study” before conclusions could be reached, raising the question: what might count as proof of alien life?
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Looking for the signs of life
The scientific community tends to employ ‘confidence of life’ detection scales: a multi-step ‘ladder’ that begins, as with Perseverance’s discovery, with the initial detection of possible biosignatures.
Other biosignatures may be observed indirectly, perhaps through the disequilibrium of gases in a planet’s atmosphere.
On Earth, life’s waste products have modified our atmosphere in a manner that would be difficult to artificially replicate.
Methane abundance, for example, is several orders of magnitude above equilibrium because of continual emissions from living organisms.
‘Proving’ the authenticity of potential alien biosignatures is a painstaking task.
Scientists must first ensure the sample has not been contaminated and that biology is possible under the known conditions.
They then need to rule out all non-biological possibilities, search for additional supporting biosignatures and exhaust all competing hypotheses.
Only then can a find be regarded as irrefutable evidence of extraterrestrial life.
