Constraining the interior structure and thermal state of Venus

Kurzfassung

Often termed the twin sister of the Earth, Venus represents an alternative outcome of the evolutionary path taken by large terrestrial planets. Given its extreme surface conditions, lack of surface water, and the absence of plate tectonics, the present-day thermal state of its mantle is likely very different from the Earth. Venus also remains the most enigmatic of terrestrial worlds in terms of interior structure. Both its tidal Love number k2 and the moment of inertia factor, the main sources of information on the core size and interior structure, are known with a large uncertainty of about 10% [1, 2], and the magnitude of tidal dissipation, sensitive to the planet’s thermal state, has only been determined indirectly [e.g., 3]. Yet, the set of observables acquired by the Magellan and Pioneer Venus Orbiter missions can still be used to put constraints on the interior structure. In this study, we perform a Bayesian inversion of several observational and theoretical constraints (such as the tidal Love number, maximum elastic thickness, or absence of intrinsic magnetic field) to gain insight into the present-day interior structure and thermal state of Venus. This is done by combining the calculation of a global tidal deformation with a 1d parameterised model of mantle convection in the stagnant-lid regime [4,5]. The convection model is based on the thermal boundary layer theory and incorporates partial melting, crustal growth, and inner core crystallization. The elastic structure of the mantle for three selected mineralogical models is obtained from the software Perple_X, based on the minimisation of Gibbs free energy [6]. Finally, to find the tidal parameters, we calculate the deformation of a layered compressible viscoelastic sphere [7]. The mantle is described by the Andrade rheological model, which has proven essential for distinguishing between a fully solid and a fully or partially liquid Venusian core [8]. We vary a large set of rheological, structural, and thermodynamic parameters and predict a range of mantle temperatures consistent with previous stagnant-lid models, average mantle viscosities between 1020-1022 Pa.s, and a tidal quality factor of Q=50+74-24, corresponding to a phase lag of 1.12+1.06-0.67 degrees. Additionally, we discuss how the future measurements of the tidal deformation and the moment of inertia of Venus from EnVision [9] and VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy) [10] can improve our understanding of the planet's interior.