Low levels of sulphur dioxide contamination of Venusian phosphine spectra

TL;DR

This study refines the analysis of Venusian atmospheric spectra, demonstrating that sulphur dioxide contamination of phosphine signals is minimal, leading to more accurate estimates of phosphine abundance consistent over multiple years.

Contribution

The paper provides a detailed assessment of SO2 contamination in Venusian phosphine spectra, improving the accuracy of phosphine detection and abundance estimates.

Findings

SO2 contamination of phosphine was ~10% in 2017

SO2 contamination was ~2% in 2019

Phosphine abundance estimates are consistent across years

Abstract

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 June 2017, 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 percent (for the least-noisy half of the planetary disc). The contamination-subtracted ALMA and JCMT spectra (of 6-8 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.