Improving Venus' static and time-variable gravity field with the EnVision Radio Science Experiment

The interior of Venus is the lesser-known among the terrestrial planets. To probe its interior, dedicated investigations are required to enable the determination of its rotation variations and its gravity field. Our current knowledge is then limited since both rotation and gravity are poorly known to significantly constrain the models of the interior structure, and consequently, the models of the thermal evolution of the planet. The new missions selected by NASA (VERITAS) and ESA (EnVision) offer the opportunity to improve the determination of the gravity field of Venus. Here, we perform numerical simulations of the EnVision radio science experiment to assess the improvement in the spatial resolution and accuracy of the static and time-variable gravity of Venus. This gravity field solution is derived from the precise reconstruction of the trajectory of the spacecraft around the planet. Our simulations account for realistic modeling of all the forces acting on the spacecraft motion given the a prioriknowledge of the gravity field itself to degree and order 180 and of the non-gravitational forces, including the atmospheric drag. An important effect that is accounted for in our numerical simulations is the thermal tides that generates a significant gravity effect associated with the atmospheric mass redistribution. A thorough method is needed to adjust the fine time-variable gravity variations related to the Venus atmospheric dynamics, including the thermal tides. A contribution to these variations consists in the viscoelastic response of the solid planet to the atmospheric pressure loading. Given the accuracy of the current gravity solution, this response has not been detected yet, and its observation is expected to provide constraints on the properties of the interior structure, such as the state and the size of the core, in addition to the improved determination of the solar tidal contribution to the gravitational potential (i.e., Love number k2).