Venus' present-day interior dynamics and seismicity from geodynamical modelling

INTRODUCTION

Despite its similarities to Earth in mass, radius and distance to the Sun, Venus stands out by its high surface temperatures, dense CO2-dominated atmosphere, and the lack of clear evidence of large-scale plate tectonics. Its young surface, covered by more than 85,000 volcanic features (Hahn & Byrne, 2023), indicates that volcanism has played and possibly still plays a major role on Venus. Additionally, Venus possesses a variety of tectonic features (e.g., Ghail et al. 2024,  for a review). Venus’ young surface and the large number and the diversity of volcanic and tectonic features observed support the growing evidence for a geologically active planet with a high potential for seismic activity.

Studying Venus’ geodynamics is crucial to understand the level of geological activity of our sister planet. Detecting Venusquakes would be a particularly efficient way to study the composition and structure of the planet’s interior, as demonstrated by the InSight mission on Mars. With the upcoming launch of NASA’s VERITAS and ESA’s EnVision in the 2030s—the first missions to investigate Venus’ interior since NASA’s Magellan mission concluded in 1994—the planet is a key topic to study. Even if neither mission possesses dedicated equipment for seismic studies, surface deformation studies with Repeat Pass Interferometry planned for VERITAS will pave the way for future missions. Several proposals are being developed to study the planet's seismicity, either through ground sensors, pressure sensors on balloons, or orbital imagers (Garcia et al., 2024). The study of the impact of geodynamical regimes on present-day surface velocities and seismicity of Venus can thus help us anticipate those future missions and link the data gained to possible geodynamic regimes for the planet.

METHOD

In this study, we aim to provide an estimation of the present-day seismic activity of Venus through the exploration of the different possible geodynamic regimes of the planet. To achieve this goal, we are using the GAIA geodynamical code (Hüttig et al, 2013) with a spherical annulus geometry (Fleury et al., 2024). Our model includes partial melting, a pseudo-plastic rheology, and pressure- and temperature-dependent viscosity, thermal expansivity and conductivity. We vary the fraction of extracted melt between 0% (i.e. fully intrusive cases) to 100% (i.e. fully extrusive cases). In our models, we use a pseudo-plastic rheology to allow for plastic deformation and surface mobilisation when the convective stresses exceed an imposed yield stress, which we vary at the surface from 5 MPa to 200 MPa.

To determine the nature of the geodynamic regime for each simulation, we extract the surface velocities and use several criteria (Tackley, 2000; Lourenço et al, 2020), namely: (1) the mobility to distinguish between mobile-lid, episodic-lid and stagnant-lid regimes, (2) the plateness to detect the presence of plates, and (3) the quiescent plateness to distinguish the plutonic squishy lid regime from the mobile lid regime.

For all our models that lie in different geodynamic regimes, we compute the volcanic flux in km³/year and compare our results to literature values for present-day Venus, in order to determine which combinations of yield stress and magmatic style best represent our current understanding of volcanic activity on Venus.

We compute the mechanical lithosphere thickness based on the present-day thermal state of the lithosphere obtained in our geodynamic models. The values that we obtained are compared to literature estimates that indicate a thin mechanical thickness at least locally, compatible with high lithospheric thermal gradients.

Using the 873 K isotherm, we calculate the seismogenic layer thickness, the layer in which Venusquakes could nucleate. Moreover, we compute the present-day seismic velocities based on the mantle temperature values at present day and lithostatic pressure from our geodynamic models, as well as the present-day seismicity, providing a moment-frequency relation. To calculate the moment-frequency-relation, we use the total seismic moment budget, which considers the contribution of both stresses from planetary contraction and convective stresses, similar to Plesa et al. (2018).

RESULTS

               Our parameter space presents four geodynamic regimes: mobile-lid (yield stress < 10 MPa), stagnant-lid (> 20 MPa), plutonic squishy-lid (10–20 MPa, highly intrusive), and episodic-lid (10–20 MPa, intermediate extrusive magmatism efficiency). Characteristics from the episodic lid regime simulations, such as the dimensions of the subduction zones and the initiation of subduction by the plume-lithosphere interaction, are consistent with the work of Davaille et al. (2017), who suggest that Quetzalpetlatl and Artemis coronae could be potential sites of plume-induced subduction.

Comparisons with literature values (see van Zelst, 2022 for an overview) indicate that low to intermediate extrusive-intrusive ratios (from 0.1 to 0.6) best reproduce the expected order of magnitude for Venus' present-day volcanic rates. As the present-day surface of Venus does not show signs of large-scale surface mobilization, we also exclude simulations with very low yield stresses (< 10 MPa).

Average profiles of present-day shear and compressional wave velocities are similar among simulations with different geodynamic styles or assuming different mantle compositions. However, substantial spatial variations in seismic velocities are expected for models where surface mobilization occurs, at least locally, at present day.

Our results will provide critical constraints to anticipate future missions designed to study the seismicity of Venus, helping to distinguish, based on the number of earthquakes, the geodynamic regime that predominantly defines the planet.

REFERENCES

Davaille et al. (2017). Nat. Geosci. https://doi.org/10.1038/ngeo2928

Fleury et al. (2024). G3. https://doi.org/10.1029/2023GC011114

Garcia et al. (2024). ESS. https://doi.org/10.1029/2024ea003670

Ghail et al. (2024). Space Sci Rev. https://doi.org/10.1007/s11214-024-01065-2

Hüttig et al. (2013). PEPI. https://doi.org/10.1016/j.pepi.2013.04.002

Hahn & Byrne (2023). JGR: Planets. https://doi.org/10.1029/2023JE007753

Lourenço et al. (2020). G3. https://doi.org/10.1029/2019GC008756

Plesa et al. (2018). GRL. https://doi.org/10.1002/2017GL076124

Tackley (2000). G3. https://doi.org/10.1029/2000GC000036

van Zelst, I. (2022). JGR: Planets. https://doi.org/10.1029/2022JE007448