| Summary: | The recent candidate detection of ~1 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 Venus' atmosphere before and our work represents the first attempt
to model phosphorus species in the 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. If PH3's presence in Venus' atmosphere is confirmed,
it therefore is highly likely to 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. Our analysis provides a template for investigation of
phosphine as a biosignature on other worlds.
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