Decadal Variations of the H 2 SO 4 Gas Mixing Ratio in the Venusian Low‐Latitude Region From Venus Express and Akatsuki Radio Occultation Data

Abstract By analyzing radio occultation data obtained by Venus Express in 2006–2013 and Akatsuki in 2016–2023, we investigated long‐term variations in the sulfuric acid vapor mixing ratio and temperature within and below the lower cloud layer (42–54 km altitude) at low latitudes (35S–35N) on Venus. The results show that the sulfuric acid vapor mixing ratio above the cloud base (∼48 km) remained nearly constant from 2006 to 2008, decreased between 2008 and 2018, and increased afterward. A similar long‐term trend was found in temperature. Annual variations of temperature were positively correlated with those of vapor mixing ratio likely because the sulfuric acid vapor mixing ratio within the cloud layer is roughly comparable to the saturation mixing ratio, which is temperature‐dependent. Our findings suggest that the Venusian cloud structure has long‐term variations that are driven by changes in sulfuric acid vapor mixing ratio and temperature. Because sulfuric acid clouds constitute the core of the Venusian climate system, decadal‐scale climate variability may be present on Venus.