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An atmospheric gas or gas combination whose abundance and environmental context can support a biological interpretation. Detectability, a plausible production flux and exclusion of significant abiotic alternatives matter together. Modern-Earth molecular oxygen and nitrous oxide have predominantly biological sources, but an oxygen biosignature false positive illustrates why extrapolation to another planet requires context.

Nonbiological molecular oxygen can accumulate when photochemistry and planetary redox sinks allow it, for example after water photolysis and hydrogen loss. Ozone produced from that oxygen is not independent proof of a biological source. The stellar spectrum, water inventory, atmospheric escape and other gases constrain these interpretations.

A triatomic form of oxygen, produced photochemically from molecular oxygen. The ozone layer absorbs ultraviolet radiation; ozone can trace atmospheric oxygen without being directly emitted by metabolism.

An ideal atmospheric biosignature gas has a strong, distinguishable spectral signature, can accumulate to a detectable abundance, and has a biologically plausible production flux. Its abiotic sources should be small or identifiable from the planet's environmental context; its lifetime must be long enough for detection but compatible with continuing replenishment. A useful diagnosis may be a disequilibrium combination of gases rather than a single molecule.

A primary metabolic byproduct comes from reactions needed for energy generation, growth or biomass synthesis. Examples include methane from methanogenesis and oxygen released by oxygenic photosynthesis. A secondary metabolic byproduct results from specialized functions such as chemical defense, signaling or stress responses; dimethyl sulfide and chloromethane are examples. Secondary products can be chemically more distinctive but are often produced in much smaller amounts.

For modern Earth, molecular oxygen (O2) and nitrous oxide (N2O) are characteristic, predominantly biologically maintained atmospheric gases: oxygenic photosynthesis maintains the former, and microbial nitrogen cycling produces much of the latter. Ozone (O3) is also a classic remote biosignature, but is made photochemically from O2 rather than being a second independent metabolic product. If O2/O3 are counted as a two-gas observational pair, they diagnose the same oxygen reservoir.

The word “unique” needs qualification: no one gas is guaranteed to be biogenic on every planet. An oxygen biosignature false positive can arise from water loss or CO2 photochemistry in suitable environments, and nonbiological N2O production is possible. The modern terrestrial source attribution does not remove the need for context when interpreting another world.

Conversion of light energy into chemical energy used to build biological material. Oxygenic photosynthesis uses water as an electron donor and releases molecular oxygen; other forms do not necessarily produce oxygen. Its atmospheric oxygen source is a major component of the modern terrestrial atmospheric biosignature gas interpretation.

A product released by reactions needed for energy acquisition, growth or biomass construction. Methane from energy-yielding microbial reactions and molecular oxygen from oxygenic photosynthesis are examples. Abiotic chemistry can make some of the same molecules, so production mechanism and environmental abundance matter.