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Phosphine on Venus : No detected traces in infrared spectra

16 October 2020

An international team, coordinated by researchers from Paris Observatory - PSL, at LESIA, has searched for the signature of PH3 on Venus in infrared spectra. The value obtained is much lower than that recently announced in the press by the British team of J. Greaves. The study is to be published at the end of October 2020 in the Astronomy & Astrophysics journal.

Following the recent announcement by J. Greaves et al. of the detection of phosphine PH3 in the clouds of Venus from radio measurements, an international team, led by scientists from Paris Observatory, has searched for the signature of PH3 in infrared spectra of Venus recorded in March 2015 with the TEXES (Texas Echelon Cross Echelle Spectrograph) imaging spectrometer mounted at the IRTF (InfraRed Telescope Facility), at Maunakea Observatory (Hawaii).

The team has observed a wavelength (10,47 microns) corresponding to vibrational transition of PH3, well isolated from other possible bands, and have found no trace of the line. They have deduced an upper limit of 5 ppbv (par per billion in volume) for the partial pressure of PH3 at the cloud top of Venus.

The disk-integrated spectrum of Venus (black error bars, 3-sigma, recorded with the TEXES instrument on March 28, 2015, compared with three models computed with different abundances of phosphine PH3. A line of carbon dioxide, very abundant in the atmosphere of Venus, is also visible in the spectrum. The slope of the TEXES spectrum is due to the absorption of the Venus spectrum by a water vapor line of the terrestrial atmosphere. There is no trace of phosphine in the Venus spectrum at the location of the PH3 transition.

This value is four times lower than the value found by Greaves et al. (20 ppbv), assuming a constant PH3 mixing ratio above the clouds.

This new measurements brings a strong constraint on the maximum abundance of phosphine at the cloud top. The two results can be reconciled only if PH3 is present only in the upper mesosphere of Venus, at a level which cannot be probed by the infrared radiation, or if the phosphine abundance is variable with time.

In conclusion, a detection of phosphine at other wavelengths is definitely needed to confirm its presence in the atmosphere of Venus.

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