P092-02
Evaluating possible sources for phosphine on Venus
Friday, 11 December 2020: 10:34
Virtual
William Bains1, Janusz Jurand Petkowski1, Sara Seager2, Sukrit Ranjan3, Clara Sousa-Silva1, Paul Rimmer4, Zhuchang Zhan5, Jane Greaves6 and Anita M. S. Richards7, (1)Massachusetts Institute of Technology, EAPS, Cambridge, MA, United States, (2)Massachusetts Institute of Technology, EAPS, Physics, Aeroastro, Cambridge, MA, United States, (3)Northwestern University, Evanston, United States, (4)University of Cambridge, Department of Earth Sciences, Cambridge, United Kingdom, (5)Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, Cambridge, MA, United States, (6)Cardiff University, Cardiff, CF24, United Kingdom, (7)University of Manchester, Manchester, United Kingdom
Abstract:
The tentative detection of phosphine in the atmosphere of Venus invites exploration of a possible source for this reactive, reducing gas in a hot, oxidizing atmosphere. We present our attempts to find a plausible source for phosphine on Venus. We have explored the likely photochemical destruction paths for phosphine in Venus’ atmosphere, and hence its lifetime and the production rate needed to explain a 5-10 ppb abundance. Phosphorus in Venus’ atmosphere is likely to be oxidized as phosphoric acid or phosphorus pentoxide at cloud level, and so production of phosphine must result from the reduction of these compounds. We show that photochemically derived H and OH species could not produce phosphine in Venus’ atmosphere. We show from thermodynamics that it is unlikely that reducing agents present in Venus’ atmosphere, clouds, surface or subsurface rocks could produce phosphine. We also explore whether P(III) species such as phosphorous acid could be made under Venus conditions, as these could be a source of phosphine, and find that this is also thermodynamically implausible. More exotic sources such as meteoritic phosphides, volcanic phosphides or lightning are also ruled out on quantitative grounds. We conclude that our current knowledge of Venus does not provide a source for phosphine in the predicted abundances that have been tentatively detected in the clouds.
We will discuss the limitations of that knowledge, especially of cloud droplet photochemistry and the interaction of sulfur and phosphorus photochemistry in the atmosphere. Knowledge of reduced phosphorus gas kinetics is lacking, and should be a key target for follow-up study. In the absence of such pathways, it is tempting to speculate regarding biological production. However life also seems a difficult explanation, as the conditions in Venus’ clouds appear strongly inimical to life. We conclude that there is an urgent need to find out more about the phosphorus chemistry of Venus, by atmospheric modelling, re-analyses of existing legacy data, follow-up observations, laboratory experiments on haze and spectral properties of Venusian constituents, and ultimately a mission to Venus
