No evidence of phosphine in the atmosphere of Venus by independent analyses

Abstract: The detection of phosphine (PH3) in the atmosphere of Venus has been recently reported based on millimeter-wave radio observations (Greaves et al. 2020), and its re-analyses (Greaves et al. 2021a/b). In this Matters Arising we perform an independent reanalysis, identifying several issues in the interpretation of the spectroscopic data. As a result, we determine sensitive upper-limits for PH3 in Venus' atmosphere (>75 km, above the cloud decks) that are discrepant with the findings in G2020 and G2021a/b. The measurements target the fundamental first rotational transition of PH3 (J=1-0) at 266.944513 GHz, which was observed with the James Clerk Maxwell Telescope (JCMT) in June 2017 and with the Atacama Large Millimeter/submillimeter Array (ALMA) in March 2019. This line's center is near the SO2 (J=309,21-318,24) transition at 266.943329 GHz (only 1.3 km/s away from the PH3 line) which represents a potential source of contamination. The JCMT and ALMA data, as presented in G2020, are at spectral resolutions comparable to the frequency separation of the two lines. Moreover, the spectral features identified are several km/s in width, and therefore do not permit distinct spectroscopic separation of the candidate spectral lines of PH3 and SO2. We present the radiative transfer modelling we have performed and then discuss the ALMA and JCMT analyses in turn.

• The paper reanalyzes JCMT and ALMA data to reassess prior claims of phosphine detection in Venus’ atmosphere.
• Methodological concerns, including spectral line overlap with sulfur dioxide, are identified as key sources of error.
• The study establishes an upper limit of <1 ppbv for phosphine, refocusing research on Venusian atmospheric chemistry.

Critical Analysis of Phosphine Detection in Venus' Atmosphere

This paper focuses on the contentious debate surrounding the potential detection of phosphine (PH$_3$) in Venus' atmosphere. Previous studies reported the detection of phosphine based on radio observations from the James Clerk Maxwell Telescope (JCMT) and the Atacama Large Millimeter/submillimeter Array (ALMA), leading to significant scientific interest. The current study undertakes an independent reanalysis of the original data, uncovering methodological concerns and ultimately rejecting the earlier claims of phosphine presence.

Summary of Findings

The authors perform a detailed reanalysis of the spectroscopic data originally used to assert phosphine detection. Their analysis reveals several inconsistencies and potential sources of error in the prior studies. Key findings include:

1. Spectral Resolution and Line Contamination: The spectral resolution achieved in the past studies did not allow for a clear separation between the phosphine line and the nearby sulfur dioxide (SO$_2$) line. The proximity of these lines at approximately 1.3 km/s in the frequency domain suggests the potential for cross-contamination, which was not sufficiently mitigated in previous analyses.
2. ALMA Data Reanalysis: In reevaluating the ALMA data, the authors employed updated calibration scripts and scrutinized various methodological approaches to assess the spectroscopic signatures. Across multiple analytical frameworks—ranging from traditional channel-to-channel calibrations to advanced methodologies developed independently by three separate groups—no statistically significant phosphine signature was detected. Instead, an upper limit of less than 1 parts per billion by volume (ppbv) was established for phosphine's presence.
3. Interpretation of JCMT Data: The analysis of JCMT data corroborated the challenges identified in differentiating between phosphine and SO$_2$. The JCMT data's spectral features could be entirely attributed to potential mesospheric SO$_2$ contamination, further invalidating the claim of an independent phosphine signature.

Implications and Future Directions

The findings of this paper have important implications for planetary science and astrobiology. From a scientific perspective, this study illustrates the complexities and challenges in interpreting radio spectroscopic data, especially in distinguishing signals from similar yet distinct molecular species. The lack of evidence for phosphine in Venus' atmosphere diminishes the initial excitement regarding potential biosignatures and emphasizes the need for cautious analysis and interpretation in exoplanetary science.

For future research, these results accentuate the necessity of high-resolution, multi-instrument observational campaigns for accurate atmospheric characterization. Further methodological improvements, particularly in calibration and line separation techniques, will aid in reducing uncertainties in future studies. In the context of astrobiology, these findings redirect the focus towards alternative hypotheses regarding Venus' atmospheric chemistry.

The study also underscores the role of rigorous independent verification and exemplifies the scientific process in action, where conclusions evolve in light of new evidence and refined analysis. Future observational campaigns, perhaps involving more precise missions or instruments, could provide clearer insights into the molecular phenomena occurring in Venus' atmosphere.

Conclusion

By presenting a comprehensive and independent analysis of both JCMT and ALMA data, this paper effectively addresses and refutes earlier claims of phosphine detection on Venus. Through meticulous examination of potential sources of spectral contamination and variance in data processing methodologies, the authors establish a robust upper limit on phosphine's presence, contributing a critical voice to the ongoing scientific discourse regarding Venusian atmospheric composition.