Biosignature
Overview
 
A biosignature is any substance -such as an element, isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

, or molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

 - or phenomenon
Phenomenon
A phenomenon , plural phenomena, is any observable occurrence. Phenomena are often, but not always, understood as 'appearances' or 'experiences'...

 that provides scientific evidence
Scientific evidence
Scientific evidence has no universally accepted definition but generally refers to evidence which serves to either support or counter a scientific theory or hypothesis. Such evidence is generally expected to be empirical and properly documented in accordance with scientific method such as is...

 of past or present life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

. Measurable attributes of life include its complex physical and chemical structures and also its utilization of free energy
Thermodynamic free energy
The thermodynamic free energy is the amount of work that a thermodynamic system can perform. The concept is useful in the thermodynamics of chemical or thermal processes in engineering and science. The free energy is the internal energy of a system less the amount of energy that cannot be used to...

 and the production of biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....

 and wastes
Cellular waste product
Cellular waste products are formed as a byproduct of cellular respiration, a series of processes and reactions that generate energy for the cell, in the form of ATP...

. Due to its unique characteristics, a biosignature can be interpreted as having been produced by living organisms, however, it is important that they not be considered definitive because there is no way of knowing in advance which ones are universal to life and which ones are unique to the peculiar circumstances of life on Earth.
The ancient record on Earth provides an opportunity to see what geochemical signatures are produced by microbial life and how these signatures are preserved over geologic time.
 
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