Water and sulfur dioxide thermal mapping on Venus : Long-term monitoring and vertical distribution of SO2 within and above the clouds

Sulfur and water play a key role in the chemistry, dynamics, and radiative transfer taking place in the atmosphere of Venus. Temporal and local variations in SO₂ and HDO, monitored within or above the clouds, can be indicators of photochemical/thermochemical processes or dynamical changes. Since 2012, ground-based monitoring of the SO₂ ν₂ and ν₃ bands, centered at 7.4 mm and 18.9 mm respectively, has been performed with TEXES at the IRTF to probe different atmospheric levels atop and within the clouds ; according to the model, the cloud top, probed at 7.4 mm, is at z = 62 km; the lower level, probed at 19 mm, is at z = 57 km. HDO (used as a proxy of H₂O) was simultaneously recorded at 7.4 mm. Since 2021, a third measurement has been systematically added at 8.6 mm to probe a higher atmospheric level (z = 67 km) in the n₁ SO₂ band.

 

As reported in our previous analyses, the SO₂ mixing ratio shows strong variations as a function of time but also over the disk, indicating the formation of SO₂ plumes. These local maxima appear sporadically on the SO₂ maps and stay visible over a few hours, but less than a day. In contrast, the H₂O abundance is uniform over the disk and shows moderate variations as a function of time (Encrenaz et al. A&A 674, A199, 2023).

 

Data recorded since 2021 have led to two main results:

 

1) The long-term variations of H₂O and SO₂ abundances at the cloud top were found to be anticorrelated between 2014 and 2019, but do not show this anticorrelation after 2021. While the disk-integrated H₂O abundances are more or less constant around 750 ppbv, the disk-integrated SO₂ abundances show variations with time by a factor up to 5 (from 100 to 500 ppbv) on a time scale of 2 months.

 

2) The n₁ SO₂ band observed at 8.6 mm, probing around z = 67 km, is now detected on all datasets for which SO₂ is higher than 300 ppbv at the cloud top.  From these data, we infer that the SO₂ volume mixing ratio (vmr) at 67 km is about ten times lower than its value at the cloud top (z = 62 km in our model). The SO₂ vmr gradient is also retrieved from the data at 7.4 mm (z = 62 km) and 18.9 mm. It is found to be close to 0 in most occasions (implying a constant vmr between 57 and 62 km), with a few exceptions where it can be either negative or positive ; the strongest negative values have been found at a period of high plume activity.