The Color of Life, on Earth and on Extrasolar Planets
"Apparently the vegetable kingdom in Mars, instead of having green for a dominant colour, is of a vivid blood-red tint."
— H.G. Wells, The War of the Worlds, 1898
Although we now know that Mars has no surface vegetation (we have not yet ruled out other life), H.G. Wells was nonetheless prescient in speculating that photosynthetic organisms on another planet might evolve to have a different dominant color than the green that is prevalent on Earth. Over 200 giant planets in other solar systems have been discovered since 1993. Future space telescope missions are being designed not only to detect Earth-size planets but to resolve them spectrally. The prime motivation for these missions is to look for signs of life outside our Solar System. But to do so, scientists need to be able to discern the spectra of life that has evolved on planets orbiting parent stars very different from our Sun.
Except for small ecosystems around hydrothermal vents and in deep mines, photosynthesis is the basis for nearly all life on Earth, providing food, fuel, and oxygen. Its distinct impacts on the spectral signature of our planet are, most significantly, oxygen in the atmosphere and the surface reflectance spectrum of land plants. The latter is notable not only for a "green bump" but also a "red edge", the steep contrast between absorbance by chlorophyll in the red and high reflectance of plant leaves in the near-infrared (NIR). However, purple bacteria perform photosynthesis with NIR radiation and produce no oxygen, and lichens do not have a strong red edge. Scientists still puzzle over why plants are green, because it seems this wastes the light where our Sun produces the most energy.
Scientists at GISS and the Virtual Planetary Laboratory (VPL) of the NASA Astrobiology Institute (NAI) conducted an extensive survey of photosynthetic organisms on Earth and found some rules for how photosynthesis is adapted to a planet's parent star. Photosynthetic pigments harvest light over a range of wavelengths and funnel that energy in a cascade of pigments absorbing at longer (i.e., redder) wavelengths in a process known as "resonance transfer". The energy cascade ends at "trap" chlorophylls, which perform photosynthesis at the longest wavelength. Since photosynthesis relies on the number of photons, not on total energy, the color of pigments can be explained by the photon flux spectrum reaching Earth's surface, rather than by the energy flux spectrum. That light spectrum is a result of atmospheric filtering, which is itself a result of biogenic gases emitted to the atmosphere, in particular, oxygen.
The three parts of Figure 1 each shows the spectral photon flux density (PFD) at the top of Earth's atmosphere (dark blue curve) and at the surface after filtering by the atmosphere (brown curve). Additionally, part (a) shows the absorbance spectra of pigments of land plants, part (b) the PFD at various water depths along with the spectra of bacterichlorophyll, and part (c) the whole organism reflectance spectra for several plant types.
The transmittance windows in Figure 1 are defined by absorption bands of water and, significantly, of oxygen. The photon flux spectra peak much more toward the red than do the energy flux spectra. Furthermore, Rayleigh scattering and the broad band of oxygen absorbance over ~500-700 nm depress transmittance in the blue to yellow and push the photon flux peak to the red. The oxygen A-band (761 nm) and B-band (687.5 nm) bound the red edge (Figure 1c). Water bands demarcate transmittance windows for bacteriochlorophylls.
Thus, given resonance transfer and the surface photon flux spectrum, it appears optimal for chlorophyll to have its absorbance peaks in the blue and red, with the reaction core in the red. It is not suboptimal to reflect in the green, which is not where the surface PFD peaks. Although cyanobacteria and algae, the ancestors of land plants, have more numerous colors than just green, the greenness of land plants may be a result of the oxygenation of the atmosphere prior to the emergence of photosynthesis on land 460 million years ago. The oxygenated atmosphere provided ultraviolet (UV) protection and favored green algae over other colors.
Thus, we propose that photosynthetic pigments will evolve with a planet's atmosphere and parent star to have their absorbance peaks at the following wavelength categories: 1) at the bluest edge of an atmospheric transmittance window for light harvesting, 2) at the reddest edge of an atmospheric transmittance window for energy trapping, and 3) at the peak photon flux for maximum light capture.
These rules allow us to narrow the possibilities for photosynthetic spectra on extrasolar planets. We took previously simulated atmospheres for Earth-like planets in the habitable zone around F, K, and M stars and calculated the spectral PFDs incident at the surface. The M stars included a range of quiescent star temperatures, as well as the spectrum of the star AD Leo, which is a young, active star with UV flares. Scenarios included both Earth-like surface fluxes of biogenic trace gases as well as negligible atmospheric oxygen, in case anoxygenic photosynthesis dominates. The resulting surface PFDs are shown in Figure 2. Critical peaks and edges of transmittance windows are indicated; these wavelengths are where scientists should first look for absorbance peaks by extrasolar photosynthetic pigments.
Planets around F stars, which are hotter than our G-class Sun, have peak PFD in the blue. K-star planets peak in the red-orange. Because of their abundance visible radiation, F- and K-star planets are likely to have very similar photosynthesis to that on Earth, with slight variation in the dominant visible color. M-star planets, however, peak in the near infrared and have very little visible light. Because mature M stars past their UV flaring stage will not produce UV radiation at damaging levels to organisms, anoxygenic photosynthesis in the NIR could have the competitive advantage on land.
M-star planets still have enough visible light such that Earth-like organisms could survive. However, their productivity would be a tenth to half that on Earth. Anoxygenic photosynthetic productivity of photosynthetic organisms on planets around M stars could be comparable to or exceed that of land plants on Earth, if utilizing photons over 400-1100 nm. The 1100 nm may be an upper wavelength cut-off to useful photons, because at longer wavelengths underwater radiation is strongly limited, and it may be difficult to achieve other than vibrational energy. Photosynthesis is likely to develop first underwater, even around M stars, since young stars still produce UV flares. The depth of water at which early photosynthesizers would need UV protection would still provide PFDs an order of magnitude or higher than that required for photosynthesizers to survive.
News Release: NASA Predicts Non-Green Plants on Other Planets
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Please address all inquiries about this research to Dr. Nancy Kiang.