  We propose an abiotic geological mechanism that accounts for the abundance of
phosphine detected by Greaves et al., 2020. We hypothesize that trace amounts
of phosphides formed in the mantle would be brought to the surface by
volcanism, and then subsequently ejected into the atmosphere, where they could
react with water or sulfuric acid to form phosphine. To investigate the
plausibility of this hypothesis, we carry out an order of magnitude
calculation. We suggest that active volcanism today could produce a rate
comparable to that required to produce the phosphide-source of the phosphine.
Our hypothesis requires that Venus be currently experiencing a high rate of
basaltic volcanism, one that is consistent with spacecraft observations and
laboratory experiments.
