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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 modeled 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 nonequilibrium processes. None of these potential phosphine production pathways is sufficient to explain the presence of ppb phosphine levels on Venus. If PH 3 '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.

A long-term goal of exoplanet studies is the identification and detection of biosignature gases. Beyond the most discussed biosignature gas O 2 , only a handful of gases have been considered in detail. In this study, we evaluate phosphine (PH 3 ). On Earth, PH 3 is associated with anaerobic ecosystems, and as such, it is a potential biosignature gas in anoxic exoplanets. We simulate the atmospheres of habitable terrestrial planets with CO 2 - and H 2 -dominated atmospheres and find that PH 3 can accumulate to detectable concentrations on planets with surface production fluxes of 10 10 to 10 14 cm -2 s -1 (corresponding to surface concentrations of 10s of ppb to 100s of ppm), depending on atmospheric composition and ultraviolet (UV) irradiation. While high, the surface flux values are comparable to the global terrestrial production rate of methane or CH 4 (10 11 cm -2 s -1 ) and below the maximum local terrestrial PH 3 production rate (10 14 cm -2 s -1 ). As with other gases, PH 3 can more readily accumulate on low-UV planets, for example, planets orbiting quiet M dwarfs or with a photochemically generated UV shield. PH 3 has three strong spectral features such that in any atmosphere scenario one of the three will be unique compared with other dominant spectroscopic molecules. Phosphine's weakness as a biosignature gas is its high reactivity, requiring high outgassing rates for detectability. We calculate that tens of hours of JWST (James Webb Space Telescope) time are required for a potential detection of PH 3 . Yet, because PH 3 is spectrally active in the same wavelength regions as other atmospherically important molecules (such as H 2 O and CH 4 ), searches for PH 3 can be carried out at no additional observational cost to searches for other molecular species relevant to characterizing exoplanet habitability. Phosphine is a promising biosignature gas, as it has no known abiotic false positives on terrestrial planets from any source that could generate the high fluxes required for detection.

Phosphorus is an essential element for all life on Earth, yet trivalent phosphorus ( e.g. , in phosphines) appears to be almost completely absent from biology. Instead phosphorus is utilized by life almost exclusively as phosphate, apart from a small contingent of other pentavalent phosphorus compounds containing structurally similar chemical groups. In this work, we address four previously stated arguments as to why life does not explore trivalent phosphorus: (1) precedent (lack of confirmed instances of trivalent phosphorus in biochemicals suggests that life does not have the means to exploit this chemistry), (2) thermodynamic limitations (synthesizing trivalent phosphorus compounds is too energetically costly), (3) stability (phosphines are too reactive and readily oxidize in an oxygen (O 2 )-rich atmosphere), and (4) toxicity (the trivalent phosphorus compounds are broadly toxic). We argue that the first two of these arguments are invalid, and the third and fourth arguments only apply to the O 2 -rich environment of modern Earth. Specifically, both the reactivity and toxicity of phosphines are specific to aerobic life and strictly dependent on O 2 -rich environment. We postulate that anaerobic life persisting in anoxic (O 2 -free) environments may exploit trivalent phosphorus chemistry much more extensively. We review the production of trivalent phosphorus compounds by anaerobic organisms, including phosphine gas and an alkyl phosphine, phospholane. We suggest that the failure to find more such compounds in modern terrestrial life may be a result of the strong bias of the search for natural products toward aerobic organisms. We postulate that a more thorough identification of metabolites of the anaerobic biosphere could reveal many more trivalent phosphorus compounds. We conclude with a discussion of the implications of our work for the origin and early evolution of life, and suggest that trivalent phosphorus compounds could be valuable markers for both extraterrestrial life and the Shadow Biosphere on Earth.

We present a new model for the biological production of phosphine (PH 3 ). Phosphine is found globally, in trace amounts, in the Earth's atmosphere. It has been suggested as a key molecule in the phosphorus cycle, linking atmospheric, lithospheric and biological phosphorus chemistry. Phosphine's production is strongly associated with marshes, swamps and other sites of anaerobic biology. However the mechanism of phosphine's biological production has remained controversial, because it has been believed that reduction of phosphate to phosphine is endergonic. In this paper we show through thermodynamic calculations that, in specific environments, the combined action of phosphate reducing and phosphite disproportionating bacteria can produce phosphine. Phosphate-reducing bacteria can capture energy from the reduction of phosphate to phosphite through coupling phosphate reduction to NADH oxidation. Our hypothesis describes how the phosphate chemistry in an environmental niche is coupled to phosphite generation in ground water, which in turn is coupled to the phosphine production in water and atmosphere, driven by a specific microbial ecology. Our hypothesis provides clear predictions on specific complex environments where biological phosphine production could be widespread. We propose tests of our hypothesis in fieldwork., (Copyright © 2018 Elsevier B.V. All rights reserved.)