Radio Sounding of the Venusian Atmosphere and Ionosphere with EnVision

EnVision has been selected in the M5 call of ESAs Cosmic Vision program as the next European led mission to Venus. It is dedicated to unravel some of the numerous open questions about Venus' past, current state and future and will help to understand why Venus and Earth evolved so differently. The Radio Science Experiment (RSE) on EnVision will perform extensive studies of the gravitational field but also Radio Occultations to sense the Venus atmosphere and ionosphere at a high vertical resolution of only a few hundred metres. These radio occultations provide electron density profiles in the ionosphere and atmospheric density, temperature and pressure profiles in the upper troposphere and mesosphere (~40 90 km). Additionally, they allow to study the H2SO4 absorption in the Venus cloud layer. The first radio occultation experiment at Venus was conducted during the Mariner 5 flyby in 1967, followed by Mariner 10, several Venera missions, Magellan, and the Pioneer Venus Orbiter, and Akatsuki. The most extensive radio occultation study of the Venus atmosphere so far was carried out by the VeRa experiment on Venus Express. EnVision will use two coherent frequencies (X- and Ka-band) to separate dispersive and nondispersive effects. This allows to distinguish between ionospheric wave structures and other noise induced effects in the ionosphere. The use of Ka-band, which has never been used to sense the Venus atmosphere before, allows to study the H2SO4 absorption in the Venus cloud layer due to its high sensitivity to sulfuric acid absorption. Ka-band is also sensitive to liquid H2SO4 which provides the opportunity (in combination with X-band) to distinguish between gaseous and liquid H2SO4 absorption features on Venus for the very first time. The short orbital period of EnVision in combination with its very small orbital inclination allows to cover all latitudes, longitudes, local times and solar zenith angles on Venus. Especially short-term variations caused by atmospheric waves can be identified to study traveling or stationary small scale atmospheric structures.