Suppose that, instead of obsessing over colors that indicate the right conditions for life, we look for the colors of life itself? On modern Earth, the dominant light-harvesting mechanisms for life employ a set of pigments we call chlorophylls. These molecules preferentially absorb light in the blue and red, reflecting back greenish tones. As a result, near the surface of Earth’s oceans, microscopic photosynthetic organisms can create vast green blushes, mixed with the scattered blue light of pure water. On dry land, plant life can also produce great swathes of green, blending with the inorganic hues of the planet.
If we could see these signatures in Earthshine—the summed up, one-shot, reflected light of the whole planet—we might be able to tease out the signs of life, and then look for those same signs in distant planets. But evaluating Earthshine is difficult because we seldom have the right spectrographic instruments and telescopes sitting off world. So astronomers have resorted to ingenious methods that measure Earthshine as it reflects from the night side of the moon. It’s a technique that was used from the 1920s to the 1960s to evaluate Earth’s reflectivity, or albedo, but then largely forgotten.
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