The abundance of H2SO4 and SO2 in the Venus atmosphere derived from radio occultation measurements and observed radio scintillations

Radio occultation measurements at Venus were applied to derive vertical profiles of temperature, pressure, number density and absorptivity in Venus’ atmosphere. For this purpose, an orbiter sounded the planet’s atmosphere with radio waves which were recorded at ground stations on Earth. While profiles of temperature, pressure and number density were derived below about 100 km from variations of the signal phase, the additional decrease of the signal amplitude allowed to determine the frequency dependent absorptivity. Venus Express sounded the atmosphere of Venus with radio waves at 13 cm (S-band) and 3.6 cm (X-band) wavelengths between the years 2006 and 2014. Absorptivity profiles were used to determine the abundance of gaseous sulfuric acid between about 40 and 55 km altitude and sulfur dioxide near the cloud base. The orbit of VEX allowed to sound the atmosphere over a wide range of latitudes and local times providing a global picture of the H2SO4(g) and SO2 distribution [1]. The resolution of the obtained profiles is in the order of the Fresnel size, i.e., up to a few hundred meters. Variations on smaller scales are the so-called radio scintillations. Those are the result of increased refractive index variations which are caused by small-scale density variations. A possible source for the latter are vertical propagating internal gravity waves. The frequency dependent radio scintillations were observed at altitude regions of enhanced atmospheric stability where the propagation of gravity waves is supported. Gravity waves provide therefore a plausible explanation for radio scintillations [2 - 3]. The radio scintillations analysis provides therefore valuable information on the intensity and the global distribution of gravity waves. The radio occultation experiment on EnVision is supported with a dual-frequency downlink consisting of an X-band (8.4 GHz) and a Ka-band (32 GHz) signal. As the Ka-band radio signal is sensitive to both, the gaseous and liquid part of H2SO4, the usage of both signals allows to determine simultaneously the gaseous and liquid H2SO4 content in the Venus atmosphere. Furthermore, both signals can be used to analyze the frequency dependent radio scintillations which occur between about 55 and 70 km altitude. At the same time, the lower scintillation region occurring below about 48 km can be analyzed. We present the latitudinal distribution of H2SO4(g) between 40 and 55 km altitude and that of SO2 near the cloud base derived from the VEX 3.6 cm radio signals between the years 2006 and 2014. On the basis of these VEX observation and model calculations we show expected EnVision X-band and Ka-band absorptivity profiles at different latitudinal regions. We also present power spectra of amplitude scintillations observed by VEX in the altitude region between about 55 and 70 km altitude and discuss expected scintillation effects for the EnVision spacecraft.

 

References:

[1] Oschlisnok, J., B. Häusler, M. Pätzold, S. Tellmann, M. K. Bird, K. Peter, T. P. Andert, Sulfuric acid vapor and sulfur dioxide in the atmosphere of Venus as observed by the Venus Express radio science experiment VeRa, Icarus, 362, 114405, 2021.

[2] Hinson, D. P. and J. M. Jenkins, Magellan radio occultation measurements of atmospheric waves on venus, Icarus, 114, (2), 310327, 1995.

[3] Gubenko, V. N., I. A. Kirillovich, D. V. Gubenko, V. E. Andreev, T. V. Gubenko, Activity of Small-Scale Internal Waves in the Northern Polar Atmosphere of Venus by Radio Occultation Measurements of Signal Intensity (Λ = 32 cm) from Venera-15 and -16 Satellites, Solar System Research, 55, 1, 1-10, 2021.