The thermal state and interior structure of Venus

Often referred to as the Earth’s twin, Venus represents today one of the most extreme places in the Solar System, with a dense 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 = 0.295±0.066 (Konopliv & Yoder, 1996), 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 suggesting a core radius of 3500±500 km (Margot et al., 2021). The phase lag of the deformation, whose value is particularly sensitive to the thermal state of the interior, has not yet been measured but will be constrained by future missions.

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, Maia et al. (2023)  showed 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.

In this study, we use the mantle convection code GAIA (Hüttig et al., 2013) to compute the thermal evolution of Venus. We 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 are pressure- and temperature-dependent (Tosi et al., 2013), and we consider core cooling and radioactive decay as appropriate for thermal evolution modeling. We vary the core radius (3025–4000 km), mantle viscosity (10²⁰–10²² Pa s), and the viscosity increases with depth (up to three orders of magnitude). Based on the assumed core size and on the thermal state, temperature variations, and viscosity structure obtained from our models we calculate the tidal deformation, the MoIF, and evaluate the dynamic topography and geoid signatures.

We find that models with a core radius ≥4000km are incompatible with current estimates of the tidal Love number k₂. Our models also show a lower tidal quality factor for Venus compared to solid Earth, which suggests a hotter interior. The increase of viscosity with depth needs to be lower than two orders of magnitude to avoid a significant decrease of the spectral correlation and admittance, at odds with observations.

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 refine models of the interior evolution.