First observations of SO₂ above Venus' clouds by means of Solar Occultation in the Infrared

Solar Occultation in the Infrared (SOIR) is a part of the Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV)/SOIR occultation experiment on board Venus Express dedicated to the study of gaseous and aerosol vertical structure of Venus' mesosphere. SOIR is an echelle spectrometer with acoustooptic selection of diffraction orders operating in the wavelengths range of 2.2–4.3 μ m at high spectral resolution ( λ /Δ λ ∼ 20,000). Detection of minor constituents such as CO, H 2 O, HDO, HCl, HF, and SO 2 , at altitudes between 65 and 130 km has been demonstrated. We report results from a series of six occultations with observations of the 4‐ μ m SO 2 band at latitudes 69°–88°N and 23°–30°N. It is the first time when the vertical distribution of SO 2 is retrieved above the clouds with the help of solar occultation direct method. The sulfur dioxide transmission spectrum is measured on a background of strong CO 2 absorption. Each retrieved vertical profile of SO 2 is characterized by few points; the mixing ratio of SO 2 being ∼0.1 ppm with scale height 1 ± 0.4 km for polar measurements (evening observations) and ∼1 ppm with scale height 3 ± 1 km at low latitudes (morning observations) at the altitude of about 70 km. Upper limits of <∼0.05 ppm are established around 75 km.