HDO and SO₂ thermal mapping on Venus

VII. SO₂ variations with altitude and influence of the solar cycle

¹ LESIA, Observatoire de Paris, PSL University, CNRS, Sorbonne Université, Université de Paris, 92195 Meudon, France
² SwRI, Div. 15, San Antonio, TX 78228, USA
³ LATMOS/IPSL, UVSQ Université Paris-Saclay, Sorbonne Université, CNRS, 78280 Guyancourt, France
⁴ Department of Astrophysics and Atmospheric Science, Kyoto Sangyo University, 603-8555 Kyoto, Japan
⁵ Department of Space Research and Technology, Technical University of Denmark, Denmark

^★ Corresponding author.

Received: 4 July 2025
Accepted: 17 October 2025

Abstract

Context. Sulfur dioxide and water are two key minor species of Venus’ atmosphere which drive its chemical evolution. However, the long-term variations in the SO₂ abundance at the cloud top, measured since 1978, remain unexplained.

Aims. In order to address this question, since 2012, we have performed a ground-based campaign to monitor the SO₂ and H₂O abundances in the region of the Venus upper cloud, using the TEXES (Texas Echelon Cross-Echelle Spectrograph) imaging spectrometer at the NASA InfraRed Telescope Facility (IRTF, Mauna Kea Observatory).

Methods. Observations were recorded in three spectral ranges: (1) the 1342–1348 cm⁻¹ (7.4 µm) spectral range, where SO₂, CO₂ and HDO (used as a proxy for H₂O) transitions are observed at an altitude of about 62 km, defined in our model as the cloud top; (2) the 529–530 cm⁻¹ range (18.9 µm), where SO₂ and CO₂ are probed within the clouds a few kilometers below the cloud top; (3) the 1160–1165 cm⁻¹ range (8.6 µm) where the weak SO₂ v₁ band is used to probe a few kilometers above the cloud top.

Results. We present here the data recorded from July 2023 to February 2025. As was reported in our previous analyses, the SO₂ maps show evidence for the formation of SO₂ plumes, mostly located around the equator, with a typical lifetime of a few hours; large variations in the SO₂ disk-integrated abundance also appear on a timescale of a few months. In contrast, the H₂O abundance is remarkably uniform over the disk and shows moderate variations as a function of time. The present dataset shows for the first time the detection of the SO₂ v₁ band above the cloud top. The simultaneous analysis of the three SO₂ bands allows us to constrain the SO₂ volume mixing ratio within and above the cloud. In addition, we have re-analyzed the long-term evolution of the SO₂ and H₂O mixing ratios at the cloud top. Between 2012 and 2023, the SO₂ abundance is clearly anti-correlated with the solar activity, which suggests that photochemistry, activated by the solar UV flux, is the main mechanism driving the SO₂ abundance. After 2023, SO₂ exhibits strong variations with a timescale as short as two months, which suggest that other mechanisms are involved. Finally, we have re-analyzed the distribution of SO₂ at the cloud top as a function of the local time, using three different data subsets (2012–2017, 2021–2022, 2023–2025). In spite of the differences, there is a general trend toward a minimum value around 12:00 and maxima around 5:00 and 17:00.

Conclusions. Our data suggest that the solar cycle plays an important role in the long-term evolution of SO₂ and H₂O at the cloud top. In addition, in some cases, other mechanisms are probably also involved, possibly associated with dynamical motions, implying sublimation/condensation processes and/or gas-aerosol conversion.