1 Department of Astronomy and Astrobiology Program, University of Washington , Seattle, Washington.
2 NASA Astrobiology Institute-Virtual Planetary Laboratory , USA .
Astrobiology. 2017 Oct;17(10):1022-1052. doi: 10.1089/ast.2016.1578. Epub 2017 Apr 26.
Oxygenic photosynthesis is Earth's dominant metabolism, having evolved to harvest the largest expected energy source at the surface of most terrestrial habitable zone planets. Using CO and HO-molecules that are expected to be abundant and widespread on habitable terrestrial planets-oxygenic photosynthesis is plausible as a significant planetary process with a global impact. Photosynthetic O has long been considered particularly robust as a sign of life on a habitable exoplanet, due to the lack of known "false positives"-geological or photochemical processes that could also produce large quantities of stable O. O has other advantages as a biosignature, including its high abundance and uniform distribution throughout the atmospheric column and its distinct, strong absorption in the visible and near-infrared. However, recent modeling work has shown that false positives for abundant oxygen or ozone could be produced by abiotic mechanisms, including photochemistry and atmospheric escape. Environmental factors for abiotic O have been identified and will improve our ability to choose optimal targets and measurements to guard against false positives. Most of these false-positive mechanisms are dependent on properties of the host star and are often strongest for planets orbiting M dwarfs. In particular, selecting planets found within the conservative habitable zone and those orbiting host stars more massive than 0.4 M (M3V and earlier) may help avoid planets with abundant abiotic O generated by water loss. Searching for O or CO in the planetary spectrum, or the lack of HO or CH, could help discriminate between abiotic and biological sources of O or O. In advance of the next generation of telescopes, thorough evaluation of potential biosignatures-including likely environmental context and factors that could produce false positives-ultimately works to increase our confidence in life detection. Key Words: Biosignatures-Exoplanets-Oxygen-Photosynthesis-Planetary spectra. Astrobiology 17, 1022-1052.
需氧光合作用是地球占主导地位的新陈代谢，它的进化是为了在大多数可居住的陆地行星的表面收获最大的预期能源。利用预期在可居住的类地行星上丰富且广泛存在的 CO 和 H₂O 分子，需氧光合作用作为一种对全球有影响的重要行星过程是可行的。由于缺乏已知的“假阳性”（也可能产生大量稳定的 O 的地质或光化学过程），光合作用产生的 O 长期以来一直被认为是可居住的系外行星上生命的特别可靠标志。O 作为生物特征还有其他优势，包括其在大气柱中的高丰度和均匀分布，以及在可见光和近红外光中的独特、强烈吸收。然而，最近的建模工作表明，大量氧气或臭氧的假阳性可能是由非生物机制产生的，包括光化学和大气逃逸。已经确定了非生物 O 的环境因素，这将提高我们选择最佳目标和测量方法以防止假阳性的能力。这些假阳性机制中的大多数都依赖于宿主恒星的特性，并且对于围绕 M 矮星运行的行星通常最强。特别是，选择位于保守的可居住带内的行星和围绕质量大于 0.4 M（M3V 及更早类型）的宿主恒星运行的行星，可能有助于避免因水损失而产生大量非生物 O 的行星。在行星光谱中搜索 O 或 CO，或缺乏 H₂O 或 CH₄，可能有助于区分 O 或 O 的生物和非生物来源。在新一代望远镜问世之前，对潜在生物特征进行彻底评估——包括可能的环境背景和可能产生假阳性的因素——最终有助于提高我们对生命探测的信心。关键词：生物特征-系外行星-O-光合作用-行星光谱。天体生物学 17, 1022-1052.
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