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New analysis is presented of the 1.1 mm wavelength absorption lines in Venus' atmosphere that suggested the presence of phosphine. We retrieve a sulphur dioxide observation from the JCMT archive that was simultaneous within a few days of the PH3 1-0 spectrum obtained in 2017 June, and demonstrate via a radiative transfer calculation that contamination of PH3 by SO2 was ≈10 per cent. We also present ALMA 2019 spectra of PH3 1-0 and an SO2 transition acquired simultaneously, and infer that SO2 line-contamination was ≲2 per cent (for the least-noisy half of the planetary disc). The contamination-subtracted ALMA and JCMT spectra (of 6–8σ sigma confidence) are now consistent with similar absorption-depths at the two epochs. The two values span −1.9(±0.2) 10−4 of the continuum signal (which was re-estimated for ALMA), albeit for differing planetary areas. This suggests that the abundance attributed to phosphine in Venus' atmosphere was broadly similar in 2017 and 2019. [ABSTRACT FROM AUTHOR]

The potential detection of ppb levels phosphine (PH3) in the clouds of Venus through millimeter-wavelength astronomical observations is extremely surprising as PH3 is an unexpected component of an oxidized environment of Venus. A thorough analysis of potential sources suggests that no known process in the consensus model of Venus' atmosphere or geology could produce PH3 at anywhere near the observed abundance. Therefore, if the presence of PH3 in Venus' atmosphere is confirmed, it is highly likely to be the result of a process not previously considered plausible for Venusian conditions. The source of atmospheric PH3 could be unknown geo- or photochemistry, which would imply that the consensus on Venus' chemistry is significantly incomplete. An even more extreme possibility is that strictly aerial microbial biosphere produces PH3. This paper summarizes the Venusian PH3 discovery and the scientific debate that arose since the original candidate detection one year ago. [ABSTRACT FROM AUTHOR]

The initial reports of the presence of phosphine in the cloud decks of Venus have led to the suggestion that volcanism is the source of phosphine, through volcanic phosphides ejected into the clouds. Here, we examine the idea that mantle plume volcanism, bringing material from the deep mantle to the surface, could generate observed amounts of phosphine through the interaction of explosively erupted phosphide with sulfuric acid clouds. The direct eruption of deep mantle phosphide is unphysical, but a shallower material could contain traces of phosphide, and could be erupted to the surface. The explosive eruption that efficiently transports material to the clouds would require ocean:magma interactions or the subduction of a hydrated oceanic crust, neither of which occur on modern Venus. The transport of the erupted material to altitudes coinciding with the observations of phosphine is consequently very inefficient. Using the model proposed by Truong and Lunine as a base case, we estimate that an eruption volume of at least 21,600 km3/year would be required to explain the presence of 1 ppb phosphine in the clouds. This is greater than any historical terrestrial eruption rate, and would have several detectable consequences for remote and in situ observations to confirm. More realistic lithospheric mineralogy, volcano mechanics or atmospheric photochemistry require even more volcanism. [ABSTRACT FROM AUTHOR]

A long-term goal of exoplanet studies is the identification and detection of biosignature gases. Beyond the most discussed biosignature gas O2, only a handful of gases have been considered in detail. In this study, we evaluate phosphine (PH3). On Earth, PH3 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 CO2- and H2-dominated atmospheres and find that PH3 can accumulate to detectable concentrations on planets with surface production fluxes of 1010 to 1014 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 CH4 (1011 cm−2 s−1) and below the maximum local terrestrial PH3 production rate (1014 cm−2 s−1). As with other gases, PH3 can more readily accumulate on low-UV planets, for example, planets orbiting quiet M dwarfs or with a photochemically generated UV shield. PH3 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 PH3. Yet, because PH3 is spectrally active in the same wavelength regions as other atmospherically important molecules (such as H2O and CH4), searches for PH3 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. [ABSTRACT FROM AUTHOR]

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 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. [ABSTRACT FROM AUTHOR]