A stringent upper limit of the PH3abundance at the cloud top of Venus

R Giles,T K Greathouse,E Marcq,H Sagawa,T Widemann,J Greaves,T Fouchet,T Encrenaz,C Sousa-Silva,B Bézard

doi:10.1051/0004-6361/202039559

Abstract

Aims.Following the announcement of the detection of phosphine (PH3) in the cloud deck of Venus at millimeter wavelengths, we searched for other possible signatures of this molecule in the infrared range.Methods.Since 2012, we have been observing Venus in the thermal infrared at various wavelengths to monitor the behavior of SO2and H2O at the cloud top. We identified a spectral interval recorded in March 2015 around 950 cm−1where a PH3transition is present.Results.From the absence of any feature at this frequency, we derive, on the disk-integrated spectrum, a 3-σupper limit of 5 ppbv for the PH3mixing ratio, which is assumed to be constant throughout the atmosphere. This limit is four times lower than the disk-integrated mixing ratio derived at millimeter wavelengths.Conclusions.Our result places a strong constraint on the maximum PH3abundance at the cloud top and in the lower mesosphere of Venus.

Highlights

The atmospheric chemistry of Venus is driven by the cycles of water and sulfur dioxide (Krasnopolsky 1986, 2007, 2010; Mills et al 2007; Zhang et al 2012)
The water and sulfur dioxide cycles have been extensively monitored over several decades using Pioneer Venus, the Venera 15 spacecraft, Venus Express, and Akatsuki via imaging and spectroscopy in the ultraviolet and infrared ranges
In order to minimize its effect on our analysis as much as possible, we integrated the TEXES data taking into account only the airmasses lower than 1.7 to exclude the limb and the high latitudes contributions

Summary

Introduction

The atmospheric chemistry of Venus is driven by the cycles of water and sulfur dioxide (Krasnopolsky 1986, 2007, 2010; Mills et al 2007; Zhang et al 2012) The clouds, both species are present in relatively large abundances (about 30 ppmv and 130 ppmv, respectively, Bézard & DeBergh 2007; Marcq et al 2013) and, at low latitude, are transported upward by Hadley convection. Clerk Maxwell Telescope (JCMT) and the Atacama Large Millimeter Array (ALMA) facilities (Greaves et al 2020) This result came as a surprise as the presence of phosphine is not expected in an oxidized atmosphere, such as those of the terrestrial planets, if only abiotic processes are considered. We found a spectral region around 955 cm−1 that includes some relatively strong transitions of PH3 and weak transitions of CO2.

Observations

PH3 upper limit

Discussion