Astrobiological exploration is founded upon the premise that biosignatures encountered in space will be recognizable as extraterrestrial life. The usefulness of a biosignature is determined not only by the probability of life creating it but also by the improbability of non-biological (abiotic) processes producing it. Concluding that evidence of an extraterrestrial life form (past or present) has been discovered requires proving that a possible biosignature was produced by the activities or remains of life. As with most scientific discoveries, discovery of a biosignature will require evidence building up until no other explanation exists.
Possible examples of a biosignature include complex organic molecules or structures whose formation is virtually unachievable in the absence of life:
# Cellular and extracellular morphologies
# Biomolecules in rocks
# Bio-organic molecular structures
# Chirality
# Biogenic minerals
# Biogenic isotope patterns in minerals and organic compounds
# Atmospheric gases
# Photosynthetic pigments