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标题：Production of ammonia makes Venusian clouds habitable and explains observed cloud-level chemical anomalies
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作者：William Bains ; Janusz J. Petkowski ; Paul B. Rimmer 等
期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2021
卷号：118
期号：52
DOI：10.1073/pnas.2110889118
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Significance This research provides a transformative hypothesis for the chemistry of the atmospheric cloud layers of Venus while reconciling decades-long atmosphere anomalies. Our model predicts that the clouds are not entirely made of sulfuric acid, but are partially composed of ammonium salt slurries, which may be the result of biological production of ammonia in cloud droplets. As a result, the clouds are no more acidic than some extreme terrestrial environments that harbor life. Life could be making its own environment on Venus. The model’s predictions for the abundance of gases in Venus’ atmosphere match observation better than any previous model, and are readily testable. The atmosphere of Venus remains mysterious, with many outstanding chemical connundra. These include the unexpected presence of ∼10 ppm O 2 in the cloud layers, an unknown composition of large particles in the lower cloud layers, and hard to explain measured vertical abundance profiles of SO 2 and H 2O. We propose a hypothesis for the chemistry in the clouds that largely addresses all of the above anomalies. We include ammonia (NH 3), a key component that has been tentatively detected both by the Venera 8 and Pioneer Venus probes. NH 3 dissolves in some of the sulfuric acid cloud droplets, effectively neutralizing the acid and trapping dissolved SO 2 as ammonium sulfite salts. This trapping of SO 2 in the clouds, together with the release of SO 2 below the clouds as the droplets settle out to higher temperatures, explains the vertical SO 2 abundance anomaly. A consequence of the presence of NH 3 is that some Venus cloud droplets must be semisolid ammonium salt slurries, with a pH of ∼1, which matches Earth acidophile environments, rather than concentrated sulfuric acid. The source of NH 3 is unknown but could involve biological production; if so, then the most energy-efficient NH 3-producing reaction also creates O 2, explaining the detection of O 2 in the cloud layers. Our model therefore predicts that the clouds are more habitable than previously thought, and may be inhabited. Unlike prior atmospheric models, ours does not require forced chemical constraints to match the data. Our hypothesis, guided by existing observations, can be tested by new Venus in situ measurements.
关键词：Venus; clouds; atmospheric chemistry; astrobiology; habitability