VERITAS gravity investigations: measuring Venus’ rotational state, moment of inertia, Love numbers, and atmospheric tides

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The key scientific objective of the NASA/JPL Discovery-class mission VERITAS (Venus Emissivity, Radio science, INSAR, Topography And Spectroscopy) is understanding the links between the interior, surface, and atmospheric evolution.After a 6-months cruise and a 11-months aerobraking phases, VERITAS is planned to operate during four Venus cycles (4x243 Earth days) in a near circular polar orbit (180x255km in altitude at 85.4 deg. inclination) providing gravity science data thanks to the 2-way X/Ka band Doppler link and VISAR (Venus Interferometric Synthetic Aperture Radar) instrument.The radio science data and VISAR landmark features (tie points) will allow a precise determination of the rotational state of Venus: we show that the precession rate can be measured with an accuracy of 13’’/cy. From this result, the moment of inertia factor (MOIF) C/MR2, can be estimated with a 0.3% accuracy (10x improvement). Moreover, the expected accuracy of the tidal Love number measurement is 0.2%: this will allow to resolve the ambiguity of the core state (solid/liquid) and to distinguish between different interior models (core radius, mantle viscosity) [1].The atmosphere of Venus is subject to a time-dependent mass redistribution due to pressure and temperature variations induced by solar heating. This phenomenon is called “thermal tide" and it moves eastward along the Venus’ surface with a 117d period (i.e. about a Venus solar day).Thermal tides can be detected as a time-variable perturbation to the Venus gravity field due to 1) the moving atmospheric masses (direct effect) and to 2) the planet’s response to the variations of the surface loading (indirect effect, parametrized through the load Love numbers).We show that VERITAS radio science and VISAR data can also be used to measure the load Love numbers up to degree 4 with good accuracy (4% for degree 2). In particular, the degree 2 coefficient can provide independent, and complementary, information on the mantle viscosity and composition.Moreover, a simultaneous measurement of the degree 2 tidal (k2, h2) and loading (k2') Love numbers can be used to provide finer bounds on the mantle viscosity and possibly to constrain the mantle rheology.[1] G. Cascioli, S. Hensley, F. De Marchi, D. Breuer, D. Durante, P. Racioppa, L. Iess, E. Mazarico and S. E. Smrekar (2021) Planet. Sci. J. 2 22