The present-day interior of Venus as predicted by global geodynamical models and constrained by observations

Abstract

Although similar to the Earth in size and mass, Venus represents today one of the most extreme places in the Solar System having a dense CO2 atmosphere and a young surface dominated by volcanic features at all spatial scales (Hahn & Byrne, 2023). While the present-day interior structure and geodynamic regime is still debated, models agree that magmatism played a major role during the entire thermal history (Rolf et al., 2022).Limited constraints for the deep interior of Venus are available from measurements of the tidal Love number k2, which is sensitive to the size and state of the core, and moment of inertia factor (MoIF), which describes the distribution of mass in the interior. While the tidal Love number k2 = 0.295±0.066 has been determined from Magellan and Pioneer Venus Orbiter tracking data (Konopliv & Yoder, 1996), the phase lag of the deformation, whose value is particularly sensitive to the thermal state of the interior, has not yet been measured. A rough estimate of the core size of 3500 km with large (>500 km) uncertainties comes from the MoIF that was determined from Earth-based radar observations (Margot et al., 2021). The large uncertainties on the data available for the Venus interior make it thus difficult to constrain the size and state of the core and the composition and viscosity of the mantle (Dumoulin et al., 2017).Early studies that used Pioneer Venus and later Magellan data showed that Venus has a higher correlation of gravity and topography for long wavelengths and a globally large apparent depth of compensation (Sjogren et al., 1980). Recently, the study by Maia et al., (2023) used the long wavelength spectrum of the gravity and topography acquired by Magellan to constrain the interior viscosity of Venus. The dynamic geoid and topography estimates for Venus confirm that a viscosity jump at 700 km depth (corresponding to ringwoodite-bridgmanite phase transition) is inconsistent with the observations, while a 250-km-thick low-viscosity layer at the base of the lithosphere is favored by the data (Maia et al. 2023).In this study, we use the mantle convection code GAIA (Hüttig et al., 2013) to compute the full thermal evolution of Venus. GAIA solves numerically the conservation equations of mass, linear momentum, and thermal energy to obtain the spatial and temporal distribution of the temperature field in the interior of a planetary body. Our models use a pressure- and temperature-dependent viscosity, and allow for surface mobilization. Our models are compatible with the so-called plutonic squishy lid regime (Lourenco et al., 2020), in which magmatic intrusions can considerably affect the thermal state of the lithosphere (Herrera et al., this meeting). The thermal expansivity and conductivity in our models are pressure- and temperature-dependent and use the parametrizations described in Tosi et al., (2013). Furthermore, we consider the effects of core cooling and radioactive decay as appropriate for thermal evolution modeling. In our models we vary the size of the core and the viscosity of the mantle. For the viscosity we test reference values of 1e20, 1e21, and 1e22 Pa s, and vary its increase with depth over several orders of magnitude. We investigate models with a core radius between 3025 km and 4000 km. Based on the thermal state and temperature variations, viscosity structure, and core size from our models we calculate the tidal deformation, the MoIF, and evaluate the dynamic topography and geoid signatures.Our models indicate that intrusive melt can lead to local surface mobilization (Fig. 1a). The increase of viscosity with depth should be less than two orders of magnitude, since larger values would significantly decrease the spectral correlation and admittance obtained in our models, at odds with observations (Fig. 1b). Models with a core radius >4000 km are incompatible with current estimates of k2, but all models are compatible with current MoIF values (Fig. 1c). Furthermore, we obtain a lower tidal quality factor for Venus compared to the Earth, which suggests a hotter interior.  Future measurements of the NASA VERITAS (Smrekar et al., 2022) and ESA EnVision (Straume-Lindner et al., 2022) missions will provide unprecedented information to address the interior structure and thermal history of Venus, and will help to refine models of the interior evolution.