This sharp contrast in absorbance in the red and reflectance in the NIR is termed the “vegetation red edge” or simply the “red edge”. While surface vegetation slightly reflects green photons in the visible spectrum, the strongest spectral reflectance occurs just beyond the visible in near-infrared (NIR) wavelengths. Abundant, green surface vegetation is a prominent example of such productivity. Oxygenic photosynthetic organisms dominate primary productivity on the modern Earth (e.g., Field et al., 1998). By constraining the absorbance spectra of alien, photosynthetic organisms, future observations may be better equipped to detect the weak spectral signal of red edge analogs. Our predicted absorbance spectra for photosynthetic surface organisms depend on both the stellar type and planetary atmospheric composition, especially atmospheric water vapor concentrations, which alter the availability of surface photons and thus the predicted pigment absorption. These predictions are consistent with previous, qualitative estimates of pigment absorptance. Around the coolest M type stars, these organisms may preferentially absorb in the near-infrared, possibly past one micron. We find that peak photon absorption for photosynthetic organisms around F type stars tends to be in the blue while for G, K, and early M type stars, red or just beyond is preferred. In this model, cellular energy production is maximized when pigments are tuned to absorb at the wavelength that maximizes energy input from incident photons while minimizing energy losses due to thermal emission and building cellular photosynthetic apparatus. Using a numerical model that predicts the absorbance spectrum of extant photosynthetic pigments on Earth from Marosvölgyi and van Gorkom (2010), we calculate the absorbance spectrum for pigments on an Earth-like planet around F through late M type stars that are adapted for maximal energy production. Thus, knowing the wavelengths at which photosynthetic organisms preferentially absorb and reflect photons is necessary to detect red edge analogs on other planets. On planets orbiting different stellar types, red edge analogs may occur at other colors than red. This edge-like signature occurs at wavelengths of peak photon absorbance, which are the result of adaptations of the phototroph to their spectral environment. On the modern Earth, this spectral reflectance is characterized by a steep increase in reflectance between the red and near‐infrared wavelengths, a signature known as the “red edge”. In the search for life on other planets, the presence of photosynthetic surface vegetation may be detectable from the colors of light it reflects. 3Goddard Institute for Space Studies (NASA), New York, NY, United States.2University of Washington, Seattle, WA, United States.1Exobiology Branch, Ames Research Center, National Aeronautics and Space Administration (NASA), Moffet Field, CA, United States.
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