Venus, clouds and sulphur
The sulphur cycle in the atmosphere of Venus has been an object of study and debate for decades. Sulphurdioxide can be observed using Earth-bound and spacecraft instruments and it is detected by instruments on entry probes into the atmosphere of Venus.
On 3 December 2025 Europlanet organised a special webinar to discuss new observations and insights into this topic. Speakers Thérèse Encrenaz (Paris Observatory, France) and Rakesh Mogul (California State Polytechnic University) presented their work followed by an extended and lively discussion with the 50-people large audience. The full session can be viewed here.
Thérèse Encrenaz and collaborators have been been patiently collecting a unique dataset using the same (!) instrumentation to observe Venus for almost 15 years. It is the TEXES (Texas Echelon Cross-Echelle Spectrograph) imaging spectrometer (5-25 micron wavelength range) mounted at the NASA InfraRed Telescope Facility (IRTF, Mauna Kea Observatory). They have published a series of seven research papers to date. From their data the abundance and spatial distribution of H2O and SO2 can be inferred at around 60km altitudes, around the cloud tops. The main observation is that SO2 is very heterogeneously distributed spatially and the abundances are varying significantly over time. These time variations occur on short (hours) and mid (months) and long (years) time scales. Water on the other hand is much more homogeously distributed with little variation over time.
The cause of the temporal variations of SO2 are not clear. Since SO2 parttakes in photochemical processes in the clouds a correlation with solar flux could be expected. Part of the observations seem to hint at such a correlation. But when taking into account all the observations, this correlation is a lot less clear. Other possible sources such as volcanism or impacts cannot be ruled out, but are as yet difficult to prove.
Rakesh Mogul and co-authors shed a new light on the chemical composition of the clouds in their recent paper on the re-analysis of Pioneer Venus Entry Probe data. Pioneer Venus released its Large Probe into the atmosphere in 1978. The observations from the Neutral Mass Spectrometer and the Gas Chromatograph instruments showed features that were never well understood. Though it was clear from the first analysis of the data in the early 1980s that cloud particles had clogged the inlets of these instruments during part of the descent, the new analysis results in a much better understanding of the effects of this clogging, and the subsequent unclogging by evaporation of the clogs as the probe was diving deeper into the hotter layers of the atmosphere. Mogul describes a very elaborate and detailed analysis, from which it becomes clear that the data recorded during the phase when cloud particles clogged the instruments is sounding the chemical composition of these particles, much more than that of the atmosphere. With that in mind, a new bulk (!) composition for the cloud particles can be derived. This composition is rather different from what has been assumed to date, especially with respect to the total water and iron content. It turns out that water is accounts for more than 60% in terms of bulk weight, whereas ferric sulphates and sulpheric acid account for about 20% each. Most of the water however is present as hydrated iron and magnesium sulphates, not as volatile. It is the volatile fraction that comes into play when the clouds are observed via remote sensing, from which a high acidity of over 75% weight % H2SO4 has been derived and known for many decades.
The big question is how to fit the results of these works and many other work by other researchers on this topic into a complete picture. How do the new insights into the cloud particle chemical composition lead to a better understanding of the overall chemistry and cycles in the clouds? Ongoing discussion between researcher groups is much needed. I wonder whether other (old) sets should be reevaluated with this new information in mind.