Phosphine in the Venusian Atmosphere: A Strict Upper Limit From SOFIA GREAT Observations

The presence of phosphine (PH₃) in the atmosphere of Venus was reported by Greaves et al. (2021, https://doi.org/10.1038/s41550-020-1174-4), based on observations of the J = 1-0 transition at 267 GHz using ground-based, millimeter-wave spectroscopy. This unexpected discovery presents a challenge for our understanding of Venus's atmosphere, and has led to a reappraisal of the possible sources and sinks of atmospheric phosphorous-bearing gases. Here we present results from a search for PH₃ on Venus using the German REceiver for Astronomy at Terahertz Frequencies instrument aboard the Stratospheric Observatory for Infrared Astronomy aircraft, over three flights conducted in November 2021. Multiple PH₃ transitions were targeted at frequencies centered on 533 and 1,067 GHz, but no evidence for atmospheric PH₃ was detected. Through radiative transfer modeling, we derived a disk-averaged upper limit on the PH₃ abundance of 0.8 ppb in the altitude range 75-110 km, which is more stringent than previous ground-based studies.