Biosignatures: Structures & Vocabulary

A biosignature is an observable feature whose presence or abundance may indicate biological activity, but robust interpretation requires ruling out plausible abiotic alternatives. No single molecule should automatically be treated as proof of life.

Life can alter an environment by producing gases faster than nonbiological processes remove them, maintaining strong chemical disequilibrium, fractionating isotopes, or generating complex structures. Abiotic photochemistry and geochemistry can mimic some of the same signals.

Earth provides the only confirmed biosphere for calibration. Remote Earth spectra show oxygen, methane, water, and vegetation-related signatures, while Solar-System studies demonstrate multiple abiotic pathways for organics and atmospheric gases.

Candidate signatures include atmospheric gases, disequilibrium, isotopic ratios, surface pigments, morphology, complex organics, and temporal changes. Each has different persistence and ambiguity.