Biosignatures

Biosignatures are indicators that life, past or present, may have influenced a planet or other object. They encompass chemical, isotopic, and morphological features that stand out from purely abiotic processes. In astronomy and astrobiology, biosignatures include atmospheric compositions, surface or subsurface textures shaped by organisms, and isotopic patterns resulting from biological activity.

Atmospheric biosignatures arise when gases produced by life accumulate in a planetary atmosphere and persist in a state of chemical disequilibrium. The most discussed examples are oxygen and ozone, which on Earth are tied to photosynthesis, as well as methane and nitrous oxide, which can have biological and abiotic sources. The interpretation of atmospheric signals depends on the planet’s temperature, alien chemistry, geology, and sunlight environment, since nonbiological processes can also generate gases under certain conditions. Detecting multiple gases in plausible combination strengthens a biosignature claim.

Surface and geological biosignatures include microfossils, stromatolites, and other biogenic minerals that record past life, as well as isotopic fractionations of carbon, sulfur, or nitrogen that reflect biological processing. These signatures are often preserved in rocks, ice, or minerals and require careful context to distinguish from abiotic analogs.

Detection and challenges: In exoplanets, biosignatures are sought through spectroscopy and, where possible, direct imaging. False positives from nonbiological chemistry and the need for corroborating evidence emphasize that robust claims rely on multiple lines of evidence within a coherent planetary context.