A biosignature (sometimes called chemical fossil or molecular fossil) is any substance – such as an element, isotope, molecule, or phenomenon – that provides scientific evidence of past or present life on a planet or moon. Measurable attributes of life include its physical or chemical structures, its use of free energy, and the production of biomass and wastes.
The field of astrobiology uses biosignatures as evidence for the search for past or present extraterrestrial life. Candidate biosignatures strongly indicate some of the earliest known life forms, aid studies of the origin of life on Earth as well as the possibility of life on Mars, Venus and elsewhere in the universe.