Discovery of a 150 day period in the Venus condensational clouds

We have analyzed the long-term regional and global variation of emitted radiance in 1.74 µ m and 2.30 µ m near infrared spectral windows in the Venus atmosphere using the medium resolution, infrared channel of the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS-M-IR) on the Venus Express spacecraft. We find a periodic variation in the 1.74 µ m  radiance that is most pronounced at mid-latitudes (30∘−60∘ latitude). This oscillation has a period of approximately 150 days, and is shown to be unlikely to be driven by variations in viewing geometry, nor by variations in instrumental characteristics such as spectrometer temperature. The oscillation has an amplitude at least as large as that of the typical day-to-day variations observed. The decay and recovery timescales are consistent with response of the cloud vertical structure to the radiative-dynamical feedback. It remains unclear why this variation would be seen only at mid-latitudes, but some hypotheses to guide future modeling testing are suggested.