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

<p><span lang="EN-US">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.</span></p> <p><span lang="EN-US">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.</span></p> <p><span lang="EN-US">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&#8217;&#8217;/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&#160;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].</span></p> <p><span lang="EN-US">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 &#8220;thermal tide" and it moves eastward along the Venus&#8217; surface with a 117d period (i.e. about a Venus solar day).</span></p> <p><span lang="EN-US">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&#8217;s response to the variations of the surface loading (indirect effect, parametrized through the load Love numbers).</span></p> <p><span lang="EN-US">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.</span></p> <p><span lang="EN-US">Moreover, a simultaneous measurement of the degree 2 tidal (k₂, h₂)<em>&#160;</em>and loading (k₂') Love numbers can be used to provide finer bounds on the mantle viscosity and possibly to constrain the mantle rheology.</span></p> <p><span lang="IT">[1] G. Cascioli,&#160;S. Hensley,&#160;F. De Marchi,&#160;D. Breuer,&#160;D. Durante,&#160;P. Racioppa,&#160;L. Iess,&#160;E. Mazarico&#160;and&#160;S. E. Smrekar (2021) <em>Planet. Sci. J.</em>&#160;<strong>2</strong>&#160;220</span></p>