HDO and SO2 thermal mapping on Venus. VI. Anomalous SO2 behavior during late 2021

Since January 2012, we have been monitoring the behavior of sulfur dioxide and water on Venus using the Texas Echelon Cross-Echelle Spectrograph (TEXES) imaging spectrometer at the NASA InfraRed Telescope Facility (IRTF, Mauna Kea Observatory). We present here new data recorded in 2021 and 2022, after an 18-month interruption due to the Covid pandemic. Most of the observations were recorded in two spectral ranges: the 7.4 µm (1345 cm⁻¹) range, where SO₂, CO₂, and HDO (used as a proxy for H₂O) transitions are observed at the cloud top, at an altitude of about 62 km, and the 19 µm (530 cm⁻¹) range, where SO₂ and CO₂ are probed within the clouds at an altitude of about 57 km. We recently added the 8.6 µm (1162 cm⁻¹) range, which probes a few kilometers above the cloud top (z = 67 km). As in our previous studies, the volume mixing ratio of SO₂ is estimated using the SO₂/CO₂ line depth ratio of weak transitions; the H₂O volume mixing ratio is derived from the HDO/CO₂ line depth ratio, assuming a D/H ratio of 200 times the Vienna Standard Mean Ocean Water (VSMOW). As reported in our previous analyses, the SO₂ mixing ratio shows strong variations with time and also over the disk, showing evidence of the formation of SO₂ plumes. These local maxima appear sporadically on the SO₂ maps and stay visible over a few hours. In contrast, the H₂O abundance is remarkably uniform over the disk and shows moderate variations as a function of time. The present dataset shows significant differences with respect to the 2012-2019 dataset: (1) the SO₂ mixing ratio at the cloud top has decreased by a factor of about 3 with respect to the maximum value observed in July 2018; (2) the long-term anti-correlation between SO₂ and H₂O previously observed between 2014 and 2019 is no longer visible; (3) a very high SO₂ plume activity was observed in Nov. 2021, in spite of the low SO₂ mixing ratio at the cloud top. In contrast, the distribution of the SO₂ plume appearance over the disk is confirmed, with a maximum along the equator and around the morning terminator. Information on the SO₂ vertical gradient is retrieved from the simultaneous analysis of SO₂ at 7.4 µm and 19 µm. The gradient is constant in most cases with a few exceptions, especially in November 2021 when the plume activity was high. Finally, the temperature distributions retrieved from the continuum maps in September and November 2021 show an unusual pattern possibly associated with gravity waves, as previously observed by the longwave infrared camera camera aboard the Akatsuki spacecraft.