text: A biosignature (sometimes called chemical fossil or molecular fossil) is any substance &ndash; such as an element, isotope, molecule, or phenomenon &ndash; that provides scientific evidence of past or present life on a planet. Measurable attributes of life include its complex physical or chemical structures, its use of free energy, and the production of biomass and wastes.
  The field of astrobiology uses biosignatures as evidence in the search for past or present extraterrestrial life.