Any observable feature or substance that provides evidence of past or present life, such as oxygen in an atmosphere, specific molecular combinations, or patterns that indicate biological processes. These signatures can be detected in exoplanet atmospheres or on planetary surfaces.
Compound word from Greek 'bios' (life) and 'signature' (Latin 'signatura' meaning a sign or mark). The term emerged in astrobiology in the 1990s as scientists developed methods to detect life on other worlds remotely.
Oxygen in Earth's atmosphere is actually a biosignature—it's so reactive that it would disappear in millions of years without life constantly replenishing it! Future space telescopes will search for biosignature combinations like oxygen plus water vapor plus methane, which together strongly suggest biological activity.
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