Microwave absorptivity by sulfuric acid in the Venus atmosphere: First results from the Venus Express Radio Science experiment VeRa

The Venus Express (VEX) Radio Science experiment VeRa utilizes radio occultation techniques to investigate the Venus atmosphere over a wide range of latitudes. Radio attenuation measurements with the VEX 3.6 cm (X-band) signal provide information on the absorptivity distribution within the Venus cloud deck. The combined results from 6 years of occultation measurements reveal a distinct latitudinal variation in absorptivity in the altitude range from 50 to 55 km. Enhanced absorptivity is observed at equatorial and mid-latitudes (0–50°S), exceeding 0.008 dB/km on the dayside and 0.01 dB/km on the nightside of the southern hemisphere. Poleward of 50°S latitude a decrease in the absorptivity is observed, reaching minimal values at polar latitudes (>70°S), where the absorptivity did not exceed 0.005 dB/km on the dayside and 0.004 dB/km on the nightside. The main absorber of radio waves in the Venus atmosphere, gaseous sulfuric acid, can serve as a tracer for atmospheric motions. The inferred absorptivity was used to determine the abundance of gaseous sulfuric acid. Abundances of about 1–2 ppm are found between 0°S and 70°S latitude in the altitude range from 50 to about 52 km, sometimes increasing to values of about 3 ppm on the dayside and 5 ppm on the nightside near 50 km. The abundance at polar latitudes (>70°S) did not exceed 1 ppm within the considered altitude range. The absorptivity and gaseous sulfuric acid height profiles are compared with previous measurements.