Is there phosphine in Venus’s atmosphere?

Venus’s extreme greenhouse effect pushes surface temperatures to 460 °C—hot enough to melt lead. But the planet’s dense clouds, located at an altitude of about 50–70 km, are a balmy 20 °C. In 2020 Cardiff University’s Jane Greaves and colleagues presented the first evidence for the presence of phosphine (PH₃) gas in Venus’s upper atmosphere. Phosphine has been discussed as a potential biomarker for the presence of life on planets, and the putative discovery led to a reappraisal of the possible sources and sinks of atmospheric phosphorus-bearing gases.

The evidence presented by Greaves and colleagues was based on two measurements—a tentative detection of the molecule’s spectral line using the James Clerk Maxwell Telescope (JCMT) in Hawaii and a more convincing detection of the same line using Chile’s Atacama Large Millimeter/Submillimeter Array (ALMA). Armed with those data, the researchers interpreted the abundance at 20 parts per billion ; a later recalibration of the data suggested a range of 1–7 ppb. But three subsequent investigations found no significant evidence for PH₃ absorption in the same JCMT and ALMA data.

To help resolve the debate about the possible presence of the gas on Venus, Martin Cordiner of the NASA Goddard Space Flight Center and colleagues performed a search for PH₃ using the German Receiver for Astronomy at Terahertz Frequencies instrument, shown here, aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) aircraft. At its operating altitude of around 13 km above Earth’s surface, SOFIA has a unique window to high-frequency submillimeter and IR emission, which is not accessible using ground-based astronomy. The team’s aim was to detect a set of the stronger, higher-energy rotational transitions of collisionally excited PH₃ to augment prior observations.

After analyzing the measurements acquired during three flights conducted in November 2021, the team found no evidence for atmospheric PH₃. The observations indicate an upper limit for PH₃ concentration of 0.8 ppb across a range of latitudes, in Venus’s mid to upper atmosphere.

Although PH₃ appears even less likely to exist on Venus than before, the planet is still full of phosphorus oxides and phosphoric acids. Those reactive molecules are likely generated by the condensation of volcanic vapors. (M. A. Cordiner et al., Geophys. Res. Lett., 2022, doi:10.1029/2022GL101055 .)