The photosynthesis red edge (PRE; also called the vegetation red edge, VRE) is a sharp step-like increase in spectral reflectance near ~700 nm produced by chlorophyll-bearing land vegetation on Earth. It is detectable in disk-integrated reflectance spectra of Earth and is the canonical surface biopigment biosignature target for future visible-to-NIR direct-imaging missions, including the Habitable Worlds Observatory.
Detection history
- Galileo spacecraft flyby of Earth (1990) detected the PRE in disk-integrated Earth spectrum — landmark calibration observation.
- Earthshine spectroscopy has reproduced PRE detection from Earth-as-exoplanet observations.
Known mineral false positives (2026)
According to Parkinson, Kaltenegger et al. (2026), the following abiotic surface materials produce PRE-like reflectance steps in existing mineral spectral databases:
- Sulfide minerals (multiple species)
- Tectosilicate minerals (multiple species)
- Potassium ferrocyanide — a prebiotically relevant cyanide salt
This expands the false-positive-biosignatures taxonomy from gas-phase abiotic O₂ (src-oxygen-false-positive-biosignatures-2026-03) to surface mineralogy. The authors argue PRE biogenicity must be evaluated in atmospheric context plus an explicit Bayesian prior comparing biological vs non-biological surface hypotheses.