A stringent upper limit of the PH$_3$ abundance at the cloud top of Venus (2010.07817v1)

Abstract: Following the announcement of the detection of phosphine (PH$_3$) in the cloud deck of Venus at millimeter wavelengths, we have searched for other possible signatures of this molecule in the infrared range. Since 2012, we have been observing Venus in the thermal infrared at various wavelengths to monitor the behavior of SO$_2$ and H$_2$O at the cloud top. We have identified a spectral interval recorded in March 2015 around 950 cm$^{-1}$ where a PH$_3$ transition is present. From the absence of any feature at this frequency, we derive, on the disk-integrated spectrum, a 3-$\sigma$ upper limit of 5 ppbv for the PH$_3$ mixing ratio, assumed to be constant throughout the atmosphere. This limit is 4 times lower than the disk-integrated mixing ratio derived at millimeter wavelengths. Our result brings a strong constraint on the maximum PH$_3$ abundance at the cloud top and in the lower mesosphere of Venus.

• The paper presents infrared observations setting a stringent 3-sigma upper limit of 5 ppbv for phosphine volume mixing ratio at Venus's cloud tops.
• Utilizing TEXES infrared spectroscopy, the derived limit is four times lower than previous estimates from millimeter observations, challenging the earlier detection claims.
• This discrepancy suggests the need to re-evaluate phosphine vertical distribution models and emphasizes the necessity for multi-wavelength observations and independent verification.

Limitations on Phosphine Detection in the Atmosphere of Venus

The paper "A stringent upper limit of the PH$_3$ abundance at the cloud top of Venus" examines the potential presence of phosphine (PH$_3$) in the atmosphere of Venus, offering a critical evaluation in light of previous controversial reports of PH$_3$ detection. Utilizing ground-based observations with the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility, the authors aim to detect possible spectral signatures of PH$_3$ in the infrared spectrum of Venus.

Observational Approach and Results

The methodology involved a targeted search for the spectral signals of PH$_3$ in the thermal infrared around 950 cm$^{-1}$, recorded in March 2015. Using imaging spectroscopy, the authors observed Venus to identify any transitions indicating the presence of PH$_3$. The dataset exploited included previously recorded observational spectra which allowed for monitoring key chemical constituents like sulfur dioxide (SO$_2$) and water (H$_2$O) at the cloud tops.

From these analyses, the researchers derived a 3-$\sigma$ upper limit, setting the PH$_3$ volume mixing ratio at a maximum of 5 parts per billion by volume (ppbv). Notably, this upper limit is significantly lower, by a factor of four, than the mixing ratio suggested by earlier millimeter wavelength observations conducted by other researchers.

Implications and Discussion

The delineation of a stringent upper limit significantly challenges the previous claims of a detectable PH$_3$ presence, as reported in the controversial 2020 detection using JCMT and ALMA facilities. This discrepancy may stem from differing altitudinal regions probed by millimeter and infrared observations. While millimeter wavelength observations probe deeper within the cloud layers, the TEXES infrared observations primarily sample the upper reaches of Venus's cloud decks and lower mesosphere, suggesting a need for reconsideration of the PH$_3$ vertical distribution models on Venus.

Further debates arise concerning the formation of narrow millimeter line cores and potential baseline ripple effects that could lead to an overestimation of phosphine quantities in previous studies. The concerns extend to possible spatial and temporal variability of PH$_3$, analogous to other sulfur-related species, which demand confirmation through secondary detections in various spectral ranges.

Conclusion

The findings prompt a reevaluation of the presence and implications of phosphine in Venus’s atmosphere, underscoring the necessity for additional independent verifications with various spectroscopic techniques. Continued research is imperative to conclusively ascertain the role and dynamics of phosphine in the Venusian environment. Future work should also accommodate the vertical atmospheric structure, highlighting the synergy between multi-wavelength observations and robust atmosphere modeling to elucidate the true chemical processes dictating the atmospheric composition of Venus.