Phosphine On Venus Cannot Be Explained By Conventional Processes

A model for biological production of phosphine on Venus. The favored path for reduction of atmospheric phosphorus species to phosphine is reduction of phosphoric acid to phosphine (upper reaction pathway on the schematic above). Correspondingly, the reduction of phosphite to phosphine is disfavored, because of the low concentration of the phosphite reactant (lower reaction pathway on the schematic above). If the concentration of phosphite is allowed to rise in the cell, then reduction of phosphate to phosphite becomes less energetically favorable, and reduction of phosphite to phosphine correspondingly more favorable. It is plausible to suggest, though it is speculative, that phosphite would accumulate in cells to a level where its reduction to phosphine was thermodynamically neutral, allowing a multi-step reduction pathway for phosphate. HX: biological reducing agent, such as NADH.

The recent candidate detection of 20 ppb of phosphine in the middle atmosphere of Venus is so unexpected that it requires an exhaustive search for explanations of its origin. Phosphorus-containing species have not been modelled for Venusian atmosphere before and our work represents the first attempt to model phosphorus species in Venusian atmosphere.

We thoroughly explore the potential pathways of formation of phosphine in a Venusian environment, including in the planet's atmosphere, cloud and haze layers, surface, and subsurface. We investigate gas reactions, geochemical reactions, photochemistry, and other non-equilibrium processes. None of these potential phosphine production pathways are sufficient to explain the presence of ppb phosphine levels on Venus.

The presence of PH3, therefore, must be the result of a process not previously considered plausible for Venusian conditions. The process could be unknown geochemistry, photochemistry, or even aerial microbial life, given that on Earth phosphine is exclusively associated with anthropogenic and biological sources. The detection of phosphine adds to the complexity of chemical processes in the Venusian environment and motivates in situ follow up sampling missions to Venus.

William Bains, Janusz J. Petkowski, Sara Seager, Sukrit Ranjan, Clara Sousa-Silva, Paul B. Rimmer, Zhuchang Zhan, Jane S. Greaves, Anita M. S. Richards

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Chemical Physics (physics.chem-ph)
Cite as: arXiv:2009.06499 [astro-ph.EP] (or arXiv:2009.06499v1 [astro-ph.EP] for this version)
Submission history
From: Janusz Petkowski
[v1] Mon, 14 Sep 2020 15:03:01 UTC (5,395 KB)

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