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Vegetation's Red Edge: A Possible Spectroscopic Biosignature of Extraterrestrial Plants

机译：植被的红边：外星植物可能的光谱生物特征

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摘要

Earth's deciduous plants have a sharp order-of-magnitude increase in leaf reflectance between approximately 700 and 750 nm wavelength. This strong reflectance of Earth's vegetation suggests that surface biosignatures with sharp spectral features might be detectable in the spectrum of scattered light from a spatially unresolved extrasolar terrestrial planet. We assess the potential of Earth's step-function-like spectroscopic feature, referred to as the "red edge," as a tool for astrobiology. We review the basic characteristics and physical origin of the red edge and summarize its use in astronomy: early spectroscopic efforts to search for vegetation on Mars and recent reports of detection of the red edge in the spectrum of Earthshine (i.e., the spatially integrated scattered light spectrum of Earth). We present Earthshine observations from Apache Point Observatory (New Mexico) to emphasize that time variability is key to detecting weak surface biosignatures such as the vegetation red edge. We briefly discuss the evolutionary advantages of vegetation's red edge reflectance, and speculate that while extraterrestrial "light-harvesting organisms" have no compelling reason to display the exact same red edge feature as terrestrial vegetation, they might have similar spectroscopic features at different wavelengths than terrestrial vegetation. This implies that future terrestrial-planet-characterizing space missions should obtain data that allow time-varying, sharp spectral features at unknown wavelengths to be identified. We caution that some mineral reflectance edges are similar in slope and strength to vegetation's red edge (albeit at different wavelengths); if an extrasolar planet reflectance edge is detected care must be taken with its interpretation.
机译：地球上的落叶植物在大约700至750 nm波长之间的叶片反射率急剧增加。地球植被的这种强反射性表明，在空间上未解析的太阳系外行星的散射光的光谱中，可以检测到具有清晰光谱特征的表面生物特征。我们评估了地球类似阶梯函数的光谱特征（称为“红边”）作为天体生物学工具的潜力。我们回顾了红边的基本特征和物理起源，并总结了其在天文学中的用途：早期在火星上寻找植被的光谱努力以及最近在大地光谱中检测到红边的报告（即空间整合的散射光）地球光谱）。我们介绍了来自Apache Point天文台（新墨西哥）的Earthshine观测结果，以强调时间可变性是检测弱表面生物特征（例如植被红色边缘）的关键。我们简要讨论了植被红边反射率的进化优势，并推测尽管地外“采光生物”没有令人信服的理由显示与陆生植被完全相同的红边特征，但它们在不同波长下的光谱特征可能与陆上相似植被。这暗示着未来的以地球行星为特征的太空任务应该获得能够识别未知波长下随时间变化的清晰光谱特征的数据。我们提醒您，某些矿物反射边缘在坡度和强度上与植被的红色边缘相似（尽管波长不同）。如果检测到太阳系外行星反射边缘，则必须注意其解释。

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