Radio science investigations by VeRa onboard the Venus Express spacecraft

Abstract

The Venus Express Radio Science Experiment (VeRa) uses radio signals at wavelengths of 3.6 and 13 cm (“X”- and “S”-band, respectively) to investigate the Venus surface, neutral atmosphere, ionosphere, and gravity field, as well as the interplanetary medium. An ultrastable oscillator (USO) provides a high quality onboard reference frequency source; instrumentation on Earth is used to record amplitude, phase, propagation time, and polarization of the received signals. Simultaneous, coherent measurements at the two wavelengths allow separation of dispersive media effects from classical Doppler shift. VeRa science objectives include the following: (1) Determination of neutral atmospheric structure from the cloud deck (approximately 40 km altitude) to 100 km altitude from vertical profiles of neutral mass density, temperature, and pressure as a function of local time and season. Within the atmospheric structure, search for, and if detected, study of the vertical structure of localized buoyancy waves, and the presence and properties of planetary waves. (2) Study of the H 2SO 4 vapor absorbing layer in the atmosphere by variations in signal intensity and application of this information to tracing atmospheric motions. Scintillation effects caused by radio wave diffraction within the atmosphere can also provide information on small-scale atmospheric turbulence. (3) Investigation of ionospheric structure from approximately 80 km to the ionopause (<600 km), allowing study of the interaction between solar wind plasma and the Venus atmosphere. (4) Observation of forward-scattered surface echoes obliquely reflected from selected high-elevation targets with anomalous radar properties (such as Maxwell Montes). More generally, such bistatic radar measurements provide information on the roughness and density of the surface material on scales of centimeters to meters. (5) Detection of gravity anomalies, thereby providing insight into the properties of the Venus crust and lithosphere. (6) Measurement of the Doppler shift, propagation time, and frequency fluctuations along the interplanetary ray path, especially during periods of superior conjunction, thus enabling investigation of dynamical processes in the solar corona.