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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 700nm) such that photons with shorter wavelengths (visible light) are generally absorbed and with longer wavelengths (infrared light) are generally reflected and with sufficient plants across the surface of a world, this can produce a detectable "feature" in its reflectance spectrum, i.e., a jump in power. A feature like this is considered 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 such a feature, constituting additional evidence. Another reason it is a candidate is that good candidates are "few and far between" and a long shot is better than none.

The feature has been found in earthshine as well as in direct measurements of Earth plants. There are theories about the reason for the feature but no explanation is proven. A mechanism such as photosynthesis requires photons within a particular energy range, and on Earth, it is adapted to the light from the Sun allowed through atmospheric windows. Toward more energetic photons (i.e., toward shorter wavelengths), at some point the photon energy will be sufficient to damage the organism's mechanisms, for example, Earth's received ultraviolet light, and the defenses of Earth plants such as biofluorescence. Toward longer wavelengths, which the red edge cuts off, it may simply be the mechanism has no use for the photons and cutoff is mere happenstance given the way photosynthesizing mechanisms must be constructed, or it may be specifically useful to reflect them to reduce their heating effect, reducing instances of heat-induced damage.

On another planet, life might be similar, since naturally-forming amino acids 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.


Referenced by:
spectral feature