The Venus gravity field from VERITAS

The Venus Emissivity, Radio Science, InSAR, Topography and Spectroscopy (VERITAS) mission will address important scientific questions about Venus’ evolution, structure, and past and on-going geological processes by collecting data from a gravity science experiment, and two instruments: an X-band interferometric SAR (VISAR), and a near-infrared imaging spectrometer (VEM). The gravity experiment will be enabled by an Integrated Deep Space Transponder that will be part of the communication subsystem: it can simultaneously lock to either an X- or Ka-band link and retransmit coherently to ground X- or Ka-band signals. This will allow to obtain a Doppler noise level of 18 μm/s at 10 s integration time for a Sun-Probe-Earth angle larger than 15°, well below the requirement of 33 μm/s. Additionally, the telecom system enables ranging at the level of 4 cm at 2 s integration time. The current resolution of Venus’ gravity field (Konopliv, et al. 1999), based on tracking data from Magellan and Pioneer Venus Orbiter, is uneven with a spatial resolution ranging between 475 km and 170 km. The VERITAS mission, given the lower noise level of the Doppler data and the near-polar near-circular low-altitude orbit chosen for Science Phase 2, will enable to generate a gravity map with a substantially higher and more uniform spatial resolution. Gravity science investigations provide important information on the interior structure of the planet, thus a higher accuracy will help to better constrain Venus’ interior from core to crust. In this study, the results of the Radio Science experiment numerical simulations will be presented, with a particular focus on the gravity field resolution achievable by VERITAS. The simulations are conducted using the orbit determination software MONTE (JPL). The dynamical model adopted is in agreement with Cascioli, et al. 2021, taking into account important characteristics of the Venus system as thermal tides and short-term sidereal period oscillations of the solid planet. The gravity field is reconstructed via spherical harmonics and the Stokes coefficients up to degree and order 220 are estimated, as well as additional parameters such as the precession rate of its rotation axis and the tidal response of the planet to the forcing exerted by the Sun.