Letter to the Editor

A stringent upper limit of the PH₃ abundance at the cloud top of Venus

¹ LESIA, Observatoire de Paris, PSL Université, CNRS, Sorbonne Université, Université de Paris, 92195 Meudon, France
e-mail: therese.encrenaz@obspm.fr
² SwRI, Div. 15, San Antonio, TX 78228, USA
³ LATMOS/IPSL, UVSQ Université Paris-Saclay, Sorbonne Université, CNRS, 78280 Guyancourt, France
⁴ Kyoto Sanyo University, Kyoto 603-855, Japan
⁵ School of Physics and Astronomy, Cardiff University, Cardiff, UK
⁶ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA

Received: 30 September 2020
Accepted: 14 October 2020

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 SO₂ and H₂O at the cloud top. We identified a spectral interval recorded in March 2015 around 950 cm⁻¹ where a PH₃ transition is present.

Results. From the absence of any feature at this frequency, we derive, on the disk-integrated spectrum, a 3-σ upper limit of 5 ppbv for the PH₃ mixing 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 PH₃ abundance at the cloud top and in the lower mesosphere of Venus.