Phosphine in Venus’ atmosphere: Detection attempts and upper limits above the cloud top assessed from the SOIR/VEx spectra

Context. Recent detection of phosphine (PH3) was reported from James Clerk Maxwell Telescope and Atacama Large Millimetre /submillimetre Array observations. The presence of PH3 on Venus cannot be easily explained in the Venus atmosphere and a biogenic source located at or within the clouds was proposed. Aims. We aim to verify if the infrared spectral signature of PH3 is present in the spectra of Solar Occultation at Infrared (SOIR). If it is not present, we then seek to derive the upper limits of PH3 from SOIR spectra. Methods. We analyzed the SOIR spectra containing absorption lines of PH3. We searched for the presence of PH3 lines. If we did not find any conclusive PH3 spectral signatures, we computed the upper limits of PH3. Results. We report no detection of PH3. Upper limits could be determined for all of the observations, providing strong constraints on the vertical profile of PH3 above the clouds. Conclusions. The SOIR PH3 upper limits are almost two orders of magnitude below the announced detection of 20 ppb and provide the lowest known upper limits for PH3 in the atmosphere of Venus