Claimed detection of PH$_3$ in the clouds of Venus is consistent with mesospheric SO$_2$ (2101.09837v1)

Abstract: The observation of a 266.94 GHz feature in the Venus spectrum has been attributed to PH$_3$ in the Venus clouds, suggesting unexpected geological, chemical or even biological processes. Since both PH$_3$ and SO$_2$ are spectrally active near 266.94 GHz, the contribution to this line from SO$_2$ must be determined before it can be attributed, in whole or part, to PH$_3$. An undetected SO$_2$ reference line, interpreted as an unexpectedly low SO$_2$ abundance, suggested that the 266.94 GHz feature could be attributed primarily to PH$_3$. However, the low SO$_2$ and the inference that PH$_3$ was in the cloud deck posed an apparent contradiction. Here we use a radiative transfer model to analyze the PH$_3$ discovery, and explore the detectability of different vertical distributions of PH$_3$ and SO$_2$. We find that the 266.94 GHz line does not originate in the clouds, but above 80 km in the Venus mesosphere. This level of line formation is inconsistent with chemical modeling that assumes generation of PH$_3$ in the Venus clouds. Given the extremely short chemical lifetime of PH$_3$ in the Venus mesosphere, an implausibly high source flux would be needed to maintain the observed value of 20$\pm$10 ppb. We find that typical Venus SO$_2$ vertical distributions and abundances fit the JCMT 266.94 GHz feature, and the resulting SO$_2$ reference line at 267.54 GHz would have remained undetectable in the ALMA data due to line dilution. We conclude that nominal mesospheric SO$_2$ is a more plausible explanation for the JCMT and ALMA data than PH$_3$.

• The paper demonstrates that the 266.94 GHz feature, once attributed to PH3, is more plausibly explained by mesospheric SO2.
• It employs detailed radiative transfer modeling alongside JCMT and ALMA spectral data to reassess Venusian atmospheric chemistry.
• The study advocates for improved observational approaches to accurately distinguish between biological and abiotic atmospheric signatures.

Analysis of the 266.94 GHz Feature in Venus' Atmosphere: \ce{PH3} or \ce{SO2}?

The paper "Claimed detection of \ce{PH3} in the clouds of Venus is consistent with mesospheric SO$_2$" offers a methodical evaluation of the purported detection of phosphine (\ce{PH3}) in Venus' atmosphere, challenging this hypothesis by presenting an alternative explanation centered on sulfur dioxide (\ce{SO2}). Based on spectral data from the James Clark Maxwell Telescope (JCMT) and the Atacama Large Millimeter Array (ALMA), the authors posit that the observed 266.94 GHz feature is more plausibly attributed to \ce{SO2} rather than \ce{PH3}.

Key Findings and Analysis

The identification of \ce{PH3} in Venus' upper atmosphere is contingent upon the interpretation of spectral lines. Phosphine's detected presence at approximately 20 parts per billion (ppb) was initially based on exclusion of \ce{SO2} as a major contributor to the 266.94 GHz spectral line. The original argument posited that, due to a non-detection of a typical \ce{SO2} reference line at 267.54 GHz in ALMA data, \ce{SO2} could not be responsible for the observed absorption feature. However, this paper critically evaluates this methodology and presents an argument heavily fortified by radiative transfer models, which reassess the role of \ce{SO2} under conditions expected in Venus' mesosphere.

Through rigorous modeling encompassing various atmospheric scenarios (Cases A-D), the authors clearly demonstrate that previous assumptions underestimated the complexity of the atmospheric chemistry and radiative transfer in Venus' mesosphere. The key takeaway from their models is that the spectral feature does not originate from the cloud decks of Venus, as previously suggested, but rather from higher altitudes above 80 km. At this altitude, \ce{PH3} would face significant stability challenges given its short chemical lifespan, contradicting the formation and persistence assumptions that supported the initial \ce{PH3} detection claim. The models thus suggest a more realistic vertical distribution of \ce{SO2} supports the observed spectral data.

Additionally, the authors address the non-detection of the \ce{SO2} reference line. They highlight that the non-detection could be attributed to significant line dilution effects inherent in ALMA's observational configuration, which markedly weakens the \ce{SO2} absorption signals compared to their appearance in JCMT data. This presents a convincing alternative perspective which reconciles the apparent discrepancies between the JCMT and ALMA observational outputs without necessitating the introduction of \ce{PH3} into the explanation.

Implications and Future Directions

This paper's evaluation contributes considerably to the discourse on Venusian atmospheric chemistry by refocusing attention on terrestrial sources of ambiguity in observational astrophysics, rather than extraordinary biotic processes. The paper emphasizes the critical need for robust observational strategies and deep understanding of the interplay between atmospheric components before asserting the presence of biosignatures like \ce{PH3}.

Practically, the findings suggest that future spectral observations of Venus should prioritize confirming the presence or absence of \ce{PH3} against a well-characterized backdrop of known sulfur chemistry. Incorporating strategies that minimize dilution effects and improve sensitivity to various atmospheric constituents will be crucial. The authors also propose re-observing the 266.94 GHz line using a combination of single-dish and interferometric observations more sensitive to mesospheric \ce{SO2} in order to resolve the existing debate conclusively.

Overall, while the potential discovery of \ce{PH3} in Venus’ atmosphere drew significant attention due to its implications for biological activity, this evaluation underscores the need for critical assessment of alternative chemical pathways and atmospheric dynamics. The continuation of such pristine methodological investigations will be indispensable in delivering definitive insights into Earth’s enigmatic neighbor and commensurately informing exoplanetary biosignature searches.