Volcanically extruded phosphides as an abiotic source of Venusian phosphine

Abstract

We hypothesize that trace amounts of phosphides formed in the mantle are a plausible abiotic source of the Venusian phosphine observed by Greaves et al. [Nat. Astron., https://doi.org/10.1038/s41550-020-1174-4 (2020)]. In this hypothesis, small amounts of phosphides (P3− bound in metals such as iron), sourced from a deep mantle, are brought to the surface by volcanism. They are then ejected into the atmosphere in the form of volcanic dust by explosive volcanic eruptions, which were invoked by others to explain the episodic changes of sulfur dioxide seen in the atmosphere [Esposito, Science 223, 1072–1074 (1984)]. There they react with sulfuric acid in the aerosol layer to form phosphine (2 P3− + 3H2SO4 = 2PH3 + 3SO42-). We take issue with the conclusion of Bains et al. [arXiv:2009.06499 (2020)] that the volcanic rates for such a mechanism would have to be implausibly high. We consider a mantle with the redox state similar to the Earth, magma originating deep in the mantle—a likely scenario for the origin of plume volcanism on Venus—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. Our conclusion is supported by remote sensing observations of the Venusian atmosphere and surface that have been interpreted as indicative of currently active volcanism.

Constraints on the Production of Phosphine by Venusian Volcanoes

“…We describe several constraints on the production of phosphine by Venusian volcanoes, and determine quantitatively under what conditions Venusian volcanism could be a valid source of PH 3 . We build on the baseline case of [16] using the approach described in [11]. We first discuss why the mineralogy (chemical species) of erupted material is necessarily determined by the upper mantle and lithospheric minerology (Section 3.1), which leads to the detailed modelling of the abundance of phosphides in the mantle and lithosphere (Section 3.2).…”

“…We then discuss why explosive volcanism is likely to be rare on Venus, (Section 3.3), why all volcanism is less likely to produce large ash clouds on Venus (Section 3.4), and some other limits on the production of phosphine from phosphide (Section 3.5), before reviewing the rate at which phosphide must be delivered to the clouds (Section 3.6). Section 4 integrates these calculations to show that the minimum rate of eruption that explains 1 ppb phosphine in the clouds is 21,600 km 3 /year, not 0.03 km 3 /year as suggested by [16], even given the most optimistic assumptions.…”

“…We can therefore say that, even if the crust has an oxygen fugacity of QIF/IM (which, as mentioned above, is unlikely), phosphides will comprise only a very small fraction of erupted phosphorus. Therefore, to produce phosphine from plume volcanism, a substantially larger amount of rock would have to erupt than is assumed by [16]. We present a quantitative calculation of how much magma would have to erupt in Section 4 below.…”

“…Fragmentation is needed for two reasons; to allow the material to be lifted to high altitude and to allow it to react with sulfuric acid efficiently. Truong and Lunine's model [16] postulates that explosive volcanism lifts erupted material tens of kilometers into the Venusian atmosphere so that it can react with sulfuric acid to make PH 3 . The most obvious site of the reaction is the sulfuric acid droplets in the clouds, but in principle phosphides, could also react with sulfuric acid vapor below the clouds, down to 35 km, below which H 2 SO 4 is predominantly dissociated from H 2 O and SO 3 [70].…”

“…Subsequently, Truong and Lunine [16] revisited the concept that plume volcanism could be a source of PH 3 . Specifically, they propose a model in which plume volcanism could bring the P(-3) species (presumed to be metal phosphides) present in the deep mantle to the surface; explosive volcanism could lift them to the cloud layer, and the sulfuric acid in the clouds could then react with phosphides to make PH 3 .…”