Claimed Detection of PH₃ in the Clouds of Venus Is Consistent with Mesospheric SO₂

Abstract

The observation of a 266.94 GHz feature in the Venus spectrum has been attributed to phosphine (PH3) in the Venus clouds, suggesting unexpected geological, chemical, or even biological processes. Since both PH3 and sulfur dioxide (SO2) are spectrally active near 266.94 GHz, the contribution to this line from SO2 must be determined before it can be attributed, in whole or part, to PH3. An undetected SO2 reference line, interpreted as an unexpectedly low SO2 abundance, suggested that the 266.94 GHz feature could be attributed primarily to PH3. However, the low SO2 and the inference that PH3 was in the cloud deck posed an apparent contradiction. Here we use a radiative transfer model to analyze the PH3 discovery, and explore the detectability of different vertical distributions of PH3 and SO2. We find that the 266.94 GHz line does not originate in the clouds, but above 80 km in the Venus mesosphere. This level of line formation is inconsistent with chemical modeling that assumes generation of PH3 in the Venus clouds. Given the extremely short chemical lifetime of PH3 in the Venus mesosphere, an implausibly high source flux would be needed to maintain the observed value of 20 ± 10 ppb. We find that typical Venus SO2 vertical distributions and abundances fit the JCMT 266.94 GHz feature, and the resulting SO2 reference line at 267.54 GHz would have remained undetectable in the ALMA data due to line dilution. We conclude that nominal mesospheric SO2 is a more plausible explanation for the JCMT and ALMA data than PH3.

Planetary Mass Spectrometry for Agnostic Life Detection in the Solar System

“…Venus has recently fueled significant interests from the astrobiology community, with the putative discovery of phosphine (PH 3 ) by remote spectroscopy; PH 3 is a type of biosignature gas which has been attributed to Terran anaerobic life (Greaves et al, 2020), although admittedly few investigations have been undertaken to explore potential abiotic processes that could also produce this compound. This surprising detection has subsequently given rise to an ongoing debate centered around the detection and interpretation of potential PH 3 signatures in the atmosphere of Venus (Encrenaz et al, 2020;Snellen et al, 2020;Lincowski et al, 2021). The origin, evolution, and composition of the Venusian atmosphere is an enigma that will soon be revealed by DAVINCI.…”

The Case (or Not) for Life in the Venusian Clouds

“…More recently, the detection has been corrected down to peak concentrations of 5-10 ppb and a global concentration of 1-4 ppb by the authors from the original paper [42]. However, concerns about the spectral analysis [43], in particular that the PH 3 detection could be misidentified sulfur dioxide, have been raised [44][45][46], mostly because the absorption lines of PH 3 (266.94 GHz) and SO 2 (~267.5 GHz) are close. The original authors continued to insist that their analytical methods are correct, and that the phosphine detection is valid [47].…”

“…However, this is not the spectral range Greaves et al [2] used to detect phosphine. As the phosphine detection has been marred in controversy [42][43][44][45][46][47][48], an independent verification of the claimed PH 3 detection is warranted. Especially insightful would be if a different methodology could be employed, such as trying to detect phosphine in the infrared range of the spectrum.…”

Introducing the Venus Collection—Papers from the First Workshop on Habitability of the Cloud Layer

“…Phosphine, proposed as a potential biosignature for exoplanets (Sousa-Silva et al, 2020), was reported as having been discovered in the venusian atmosphere (Greaves et al, 2020a(Greaves et al, , 2020b(Greaves et al, , 2020c(Greaves et al, , 2021. A spirited debate about the detection (Encrenaz et al, 2020;Snellen et al, 2020;Akins et al, 2021;Lincowski et al, 2021) has ensued. Much of the debate has centered on the analysis of the observations, the altitude range over which phosphine was detected, the instability of phosphine in the hydrogen-poor venusian atmosphere, and the lack of known mechanisms of its production (Bains et al, 2021b).…”