EnVision Gravity investigation

The Radio Science Experiment (RSE) onboard EnVision comprises a Radio-Occulation and a Gravity experiment. The scientific objectives of the Gravity experiment onboard EnVision are a thorough determination of Venus’ gravitational anomalies at high spatial resolution, tidal potential Love number k₂ and its phase lag, and pole orientation. Accurate measurements of the pole precession, which constrains the moment of inertia of the planet, and gravitational tides preserve key information on the size and state of the core, and the mean viscosity of the mantle. The gravity investigation will use the Doppler shift of the carrier of the telemetry radio-link (X-up and X/Ka-down) between the spacecraft and tracking stations on Earth (ESTRACK network). The Doppler tracking will be performed daily during at least 3.5 hours in the nominal mission that cover 6 entire Venus cycles (4 Earth’s years).

The Doppler measurements will be used to precisely determine the spacecraft trajectory from which the gravity field, Love number and moment of inertia solutions are derived. We present numerical simulations that show the radio tracking data analysis for the precise orbit determination (POD) of the spacecraft including the gravity field inversion. Our results show a significant enhancement in spatial resolution (up to 140 km) in areas mainly located in the northern hemisphere. A joint analysis of this high-resolved gravity map and topography will improve the determination of the spatial variation of the crust and lithosphere properties, in turn providing a better understanding of the thermal evolution of the planet.  A significant improvement is also obtained for the Love number k₂ whose expected accuracy is 0.003. Our analysis shows tidal phase lag and moment of inertia accuracies of 0.3° and 1.5%, respectively. The combination of these geophysical measurements is well-suited to enable tighter constraints on the state and size of the core and the mantle viscosity.

Further numerical simulations are carried out to detect the time-variable gravity signature expected from the atmosphere (thermal tides), especially through the estimation of the load Love number. This additional parameter also depends on the internal structure of the planet, leading to complementary information on the core state. We have also taken into account more realistic perturbations of the forces that drive the spacecraft motion, like the drag undergone by the spacecraft at low-altitude part of the orbit, and realistic perturbations of the a priori knowledge of the gravity field itself. These simulations are performed using the GINS software developed by the space French agency CNES.