Astrophysics (Index)About

vegetation red edge

(VRE, red edge)
(wavelength cut-off point in the reflectivity of typical plants)

The vegetation red edge (or just red edge) is a feature of the reflectance of Earth vegetation: there is a limit (at a point near wavelength 700 nm) such that photons with shorter wavelengths (visible light) are generally absorbed and those with longer wavelengths (infrared light) are generally reflected. Sufficient such plants across the surface of a world can produce a detectable "feature" in its reflectance spectrum, i.e., a jump in power. Such a feature is a possible candidate for a biosignature for life on an extra-solar planet, in part because clouds or seasons might show a change in the strength of the feature, constituting additional evidence. Plausible exoplanet biosignature candidates are "few and far between" and even if this is a long shot, it is one of only a few.

The feature is evident in direct measurements of Earth plants, and has also shown in earthshine. There are theories about the reason for the feature but no proven explanation. A mechanism such as photosynthesis requires photons within a particular energy range, and on Earth, such mechanisms are adapted to the light from the Sun that is allowed through atmospheric windows. Above some photon energy (i.e., sufficiently short wavelength), EMR damages the organism's photosynthesis mechanism (for example, Earth's received ultraviolet light) and Earth plants have mechanisms to defend themselves, e.g., with biofluorescence. UV's damaging mechanism does not apply to wavelengths longer than necessary, which is what the red edge cuts off. The cutoff may be mere happenstance given the way photosynthesizing mechanisms are constructed. Or it may be to lessen the chance of heat damage: photosynthesis does not process the energy of the longer wavelength IR and if absorbed, it merely heats the plant.

On another planet, life might be similar, since abiotically-produced amino acids which could form there seem a reasonable "starting point" for the formation of organisms. A star with a similar habitable zone and planet might produce a similar spectral energy distribution of received electromagnetic radiation, inducing the development of similar mechanisms. Dissimilar stars might result in a similar feature but with a different cutoff wavelength.

Further reading:

Referenced by pages:
spectral feature