Phosphine on Venus Cannot be Explained by Conventional Processes

TL;DR

The detection of phosphine in Venus' atmosphere cannot be explained by known chemical or geological processes, suggesting the possibility of unknown phenomena or life-related sources, and highlighting the need for further exploration.

Contribution

First comprehensive modeling of phosphorus species in Venus' atmosphere, exploring all known formation pathways and concluding they are insufficient to explain phosphine presence.

Findings

Conventional processes cannot produce observed phosphine levels.

Detection suggests unknown geochemical, photochemical, or biological processes.

Results motivate in situ missions and biosignature investigations.

Abstract

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.