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, abbreviated VRE) 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. Clouds or seasons might produce an observable a change in the effects of the feature, constituting additional evidence. There are not many plausible exoplanet biosignature candidates capable of being observed at stellar distances and all remain of interest.

The feature is evident in direct measurements of Earth plants, and has also shown in earthshine. There are theories regarding the reason for the red edge 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 red edge 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 plausibly form there seem a reasonable "starting point" for the formation of organisms. A planet orbiting within the habitable zone of a similar star might receive electromagnetic radiation with a similar spectral energy distribution, inducing the development of similar mechanisms. Dissimilar stars might result in a similar feature but with a different cutoff wavelength.

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Referenced by pages:
spectral feature