Re-analysis of Phosphine in Venus' Clouds

Abstract: We first respond to two points raised by Villanueva et al. We show the JCMT discovery spectrum of PH3 can not be re-attributed to SO2, as the line width is larger than observed for SO2 features, and the required abundance would be an extreme outlier. The JCMT spectrum is also consistent with our simple model, constant PH3-abundance with altitude, with no discrepancy in line profile (within data limits); reconciliation with a full photochemical model is the subject of future work. Section 2 presents initial results from re-processed ALMA data. Villanueva et al. noted an issue with bandpass calibration. They have worked on a partially re-processed subset of the ALMA data, so we note where their conclusions, and those of Greaves et al., are now superseded. To summarise: we recover PH3 in Venus' atmosphere with ALMA (~5{\sigma} confidence). Localised abundance appears to peak at ~5-10 parts-per-billion (ppb), with suggestions of spatial variation. Advanced data-products suggest a planet-averaged PH3 abundance ~1-4 ppb, lower than from the earlier ALMA processing (which indicated 7+ ppb). The ALMA data are reconcilable with the JCMT detection (~20 ppb) if there is order-of-magnitude temporal variation; more advanced processing of the JCMT data is underway to check methods. Independent PH3 measurements suggest possible altitude dependence (under ~5 ppb at 60+ km, up to ~100 ppb at 50+ km; see Section 2: Conclusions.). Given that both ALMA and JCMT were working at the limit of observatory capabilities, new spectra should be obtained. The ALMA data in-hand are no longer limited by calibration, but spectral ripples still exist, probably due to size and brightness of Venus in relation to the primary beam. Further, spatial ripples are present, potentially reducing significance of real narrow spectral features.

• The paper presents a refined analysis that distinguishes phosphine from sulfur dioxide using distinct spectral features from JCMT data.
• It recalibrates ALMA observations to reduce spectral ripples, revealing a planet-averaged phosphine abundance of 1–4 ppb with local peaks up to 5–10 ppb.
• The study addresses data processing challenges and highlights the role of temporal and spatial variability in advancing Venus atmospheric models.

Re-analysis of Phosphine in Venus’ Clouds

The paper "Re-analysis of Phosphine in Venus’ Clouds" serves as an intricate continuation and defense of prior research published by Greaves et al. in "Nature Astronomy" regarding the presence of phosphine (PH$_3$) in the atmosphere of Venus. The primary intent is to address critiques presented by Villanueva et al., which argue against the conclusive presence of PH$_3$ in Venus' atmosphere, suggesting possible contamination by sulfur dioxide (SO$_2$).

Key Findings and Methodological Analysis

1. Spectral Characteristics and Separation: The authors delineate why the phosphine spectral features observed via the James Clerk Maxwell Telescope (JCMT) cannot be misattributed to SO$_2$. The line widths and spectral data do not support such reassignment without suggesting implausible SO$_2$ concentrations.
2. ALMA Data Processing and Re-evaluation: The paper provides detailed insights into the reanalysis of the Atacama Large Millimeter/submillimeter Array (ALMA) data. The authors rectify some earlier data processing errors by emphasizing that new calibration efforts have reduced spectral ripples, albeit highlighting the need for further spectra. They report a planet-averaged PH$_3$ abundance around 1-4 parts-per-billion (ppb), with observed local peaks up to 5-10 ppb, illustrating spatial variability.
3. Spectral Line Calibration and Interferometric Issues: The discussion of calibration issues, notably those involving bandpass correction and baseline fitting, is extensive. The paper reviews the calibration errors found in the JCMT and ALMA datasets and addresses how these calibration errors were significant in understanding the phosphine detections.
4. Reprocessing Results: Following meticulous reprocessing of ALMA data, a more conservative inference on PH$_3$ abundance was drawn. They identify a much lower signal than previously suggested, requiring temporal and spatial variability to reconcile the findings across different datasets. The confirmed abundance arises with a confidence level of approximately ~4.8σ.
5. Broader Implications: Analysis suggests that given the detection constraints of the observatories, the possibility of temporal fluctuations or altitude-dependent phenomena is likely. These interpretations could have broader implications for atmospheric chemistry models of Venus, indicating active surface-atmosphere interactions or biological processes.

Implications and Future Research Directions

The results put forth significant considerations regarding the methodologies employed when analyzing faint spectral lines from distant celestial objects. The prospective inconsistency between JCMT and ALMA data could denote temporal variations in phosphine abundance, one of the many aspects calling for more rigorous atmospheric models and further observational campaigns. Data from alternative platforms or a combination of spectral and photochemical modeling efforts will be imperative in delineating the phosphine phenomena observed.

Conclusion

This re-analysis paper contributes to an ongoing scientific endeavor to understand Venus' atmospheric chemistry. It highlights the critical role of methodological scrutiny, replication, and reanalysis in the scientific process. Future observational efforts, enhanced spectral analysis methods, and comprehensive photochemical models are essential to unravel the complexities involving phosphine detection in extraterrestrial atmospheres. The pursuit of these initiatives will likely enhance the understanding of Venusian atmospheric processes and aid in distinguishing between biological, chemical, or geophysical sources of phosphine if confirmed.