Volcanically extruded phosphides are a plausible source of Venusian phosphine

Abstract:

Phosphine was recently reported in the atmosphere of Venus (Greaves et al., 2020), and various abiotic mechanisms for its generation have been rejected in the published literature, including active volcanism. For an abiotic source to be plausible, the presence of small amounts of phosphides (P(3-) bound in metals such as iron, magnesium etc.) in volcanic dust would be energetically essential, leading to the facile produce of phosphine. We hypothesize that phosphides are brought to the surface by volcanism sourced from a deep mantle and then subsequently ejected into the atmosphere, there they would react with nearly pure sulfuric acid to form phosphine (P(3-) + H2SO4 = PH3). We take issue with the conclusion of Bains et al. (2020), who argued that the volcanic rates would have to be implausibly high. We consider a mantle with the redox state similar to the Earth and magma originating deep in the mantle – a likely scenario for the origin of plume volcanism on Venus (Smrekar & Sotin, 2012), and episodically high but plausible rates of volcanism on a Venus bereft of plate tectonics. We conclude that volcanism could supply an adequate amount of phosphide to produce phosphine. Due to the much less reactive nature of water on Earth compared to sulfuric acid on Venus, and the much shorter lifetime of phosphine on Earth compared to Venus (owing to the much higher concentration of -OH radicals in the Earth’s troposphere), the same volcanic rate to produce 20 ppb in Venusian atmosphere could only generate about 4 ppq of phosphine on Earth, 5x10⁶ times lower than the abundance on Venus, and lower than the lowest concentration of phosphine typically detected on the Earth’s atmosphere of ~ 10 ppq (Sousa-Silva et al., 2020). Our conclusion is supported by remote sensing observations of the Venusian surface that have been interpreted as indicative of currently active volcanism (Smrekar et al., 2010, Ivanov & Head, 2010, Davaille et al., 2017).