HDO and SO₂ thermal mapping on Venus

V. Evidence for a long-term anti-correlation

¹ LESIA, Observatoire de Paris, PSL University, 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 Sangyo University, Kyoto 603-8555, Japan
⁵ LMD/IPSL, Sorbonne University, ENS, PSL University, Ecole Polytechnique, University Paris Saclay, CNRS, 75252 Paris Cedex 05, France
⁶ Planetary Science Laboratory, University of Michigan, Ann Arbor MI 48109-2143, USA
⁷ Zentrum fuer Astronomie und Astrophysik, Technische Universitaet Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
⁸ Jet Propulsion Laboratory, Pasadena, CA 91109, USA
⁹ Hokkaido Information University, Hokkaido 069-8585, Japan
¹⁰ Department of Earth and Planetary Sciences, University of California, Santa Cruz, CA 95064, USA

Received: 14 February 2020
Accepted: 19 May 2020

Abstract

Since January 2012, we have been monitoring the behavior of sulfur dioxide and water on Venus, using the Texas Echelon Cross-Echelle Spectrograph imaging spectrometer at the NASA InfraRed Telescope Facility (IRTF, Mauna Kea Observatory). Here, we present new data recorded in February and April 2019 in the 1345 cm⁻¹ (7.4 μm) spectral range, where SO₂, CO₂, and HDO (used as a proxy for H₂O) transitions were observed. The cloud top of Venus was probed at an altitude of about 64 km. As in our previous studies, the volume mixing ratio (vmr) of SO₂ was estimated using the SO₂/CO₂ line depth ratio of weak transitions; the H₂O volume mixing ratio was derived from the HDO/CO₂ line depth ratio, assuming a D/H ratio of 200 times the Vienna standard mean ocean water. As reported in our previous analyses, the SO₂ mixing ratio shows strong variations with time and also over the disk, showing evidence for the formation of SO₂ plumes with a lifetime of a few hours; in contrast, the H₂O abundance is remarkably uniform over the disk and shows moderate variations as a function of time. We have used the 2019 data in addition to our previous dataset to study the long-term variations of SO₂ and H₂O. The data reveal a long-term anti-correlation with a correlation coefficient of −0.80; this coefficient becomes −0.90 if the analysis is restricted to the 2014–2019 time period. The statistical analysis of the SO₂ plumes as a function of local time confirms our previous result with a minimum around 10:00 and two maxima near the terminators. The dependence of the SO₂ vmr with respect to local time shows a higher abundance at the evening terminator with respect to the morning. The dependence of the SO₂ vmr with respect to longitude exhibits a broad maximum at 120–200° east longitudes, near the region of Aphrodite Terra. However, this trend has not been observed by other measurements and has yet to be confirmed.