Seismic Velocities in the Interior of Venus: Estimates from Global Geodynamic Models

Similar to the Earth in size, mass and potentially also the composition, Venus is often referred to as our sister planet. However, today’s Venus represents one of the most extreme places in the Solar System. It possesses a dense CO2 atmosphere with a surface pressure 90 times higher than the Earth and surface temperatures able to melt lead. Its young surface is dominated by volcanic features at all spatial scales (Hahn & Byrne, 2023), and recent reanalysis of Magellan radar data suggests that Venus might be volcanically active today (Herrick & Hensley, 2023; Sulcanese et al., 2024).

Venus also possesses a large variety of tectonic features at its surface, ranging from rift zones of thousands of kilometers in length (Foster and Nimmo, 1996), to wide-spread distribution of wrinkle ridges (Billoti and Suppe, 1999), and a substantial number of round formations consisting of a ring wall and radial cracks and fractures in the interior (the so-called coronae), some of which have been associated with regional subduction processes (Davaille et al., 2017). These tectonic structures and the growing evidence for a geologically active Venus at present day point toward a potentially seismically active planet.

In the absence of seismic measurements, the seismicity of Venus is poorly constrained and estimates only stem from theoretical models. A recent study (van Zelst et al., 2024) that scaled the Earth seismicity to Venus, by using Earth-analogs for different tectonic settings on Venus, suggests that our sister planet might experience several thousands of quakes with magnitude 4 or higher per Earth year. The seismogenic layer thickness, the layer in which quakes can nucleate, is closely linked with the thermal state of the lithosphere. The latter has been derived using the elastic lithosphere thickness obtained from local flexural analyses, geodynamics models, and temperature variations associated with density anomalies retrieved from geophysical inversions of gravity and topography data. The seismogenic thickness estimates show a range of values from about 4 to 40 km (Maia et al., 2024), indicating that quakes can occur on Venus.

In this study, we combine global scale thermal evolution models in a 3D spherical geometry with thermodynamic calculations, and seismic wave propagation codes to investigate seismic body and surface waveforms on Venus. We use the geodynamical convection code GAIA-v2 (Hüttig et al., 2013) to compute the full thermal evolution of Venus. The conservation equations of mass, linear momentum, and thermal energy are solved numerically to obtain the spatial distribution of the temperature field in the interior of Venus at present day. Our models use a pressure- and temperature-dependent viscosity following Arrhenius law, and include pressure- and temperature-dependent thermal expansivity and conductivity adopting the parametrizations described in Tosi et al., (2013). We consider partial melting and the effects of magmatic intrusions that can considerably affect the thermal state of the lithosphere (Herrera et al., 2024), leading to the so-called plutonic squishy lid geodynamic regime (Lourenço et al., 2020), in which regional scale surface mobilization and lithospheric foundering can occur. 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. As the interior structure of Venus is poorly constrained, we investigate models with a core radius between 3025 km and 4000 km (Margot et al., 2021).

We use the present-day thermal state of the interior as obtained from our geodynamic models to calculate the distribution of seismic velocities in the interior of Venus. For each of our geodynamic models, we test various compositions for Venus’ mantle that have been proposed in the literature, including an iron-poor and iron-rich composition as tested in Dumoulin et al. (2017), and also include a mantle composition with an iron content similar to Earth's mantle that is able to reproduce the Venera 14 basalts (Jennings et al. 2024).  We calculate the seismic velocities at each location in the 3D model using the thermodynamic formulation and thermodynamic database of Stixrude and Lithgow-Bertelloni (2024) with the Perple_X software (Connolly, 2009).

In a next step, the full 3D seismic wavespeeds are included into a seismic wave propagation code to calculate synthetic waveforms for geodynamic models in a plutonic squishy lid regime that show local-scale resurfacing, as well as models with episodic and continuous large-scale surface mobilization. Such calculations will inform us about the expected seismic waveforms on Venus for models that are compatible with currently available observational constraints on the Venusian interior.

 

References:

Billoti, F. & Suppe, J. (1999). Icarus.

Connolly, J. A. D. (2009). G3.

Davaille, A., Smrekar, S. E., & Tomlinson, S. (2017). Nat. Geosci.

Dumoulin, C., Tobie, G., Verhoeven, O., Rosenblatt, P., & Rambaux, N. (2017). JGR: Planets.

Foster, A. & Nimmo, F. (1996). EPSL

Hahn, R. M., & Byrne, P. K. (2023). JGR: Planets.

Herrick, R. H. & Hensley, S. (2023). Science.

Herrera, C., Plesa A.-C., Maia, J., Jennings, L., & Klemme, S. (2024). EPSC 2024.

Hüttig, C., Tosi, N., & Moore, W. B. (2013). PEPI.

Jennings, L., Collinet, M., Plesa, A.-C., Herrera, C., Maia, J., & Klemme, S. (2024). EPSC 2024.

Stixrude, L., & Lithgow-Bertelloni, C. (2024). GJI.

Lourenço, D. L., Rozel, A. B., Ballmer, M. D., & Tackley, P. J. (2020). G3.

Maia, J., Plesa, A.-C., van Zelst, I., Brissaud, Q., De Toffoli, B., Garcia, R., Ghail, R., Gülcher, A., Horleston, A., Kawamura, T., Klaasen, S., Lefèvre, M., Lognonné, P., Näsholm, S. P., Panning, M., Smolinski, K., Solberg, C., & Stähler, S. (2024). EPSC 2024.

Margot, J. L., Campbell, D. B., Giorgini, J. D., Jao, J. S., Snedeker, L. G., Ghigo, F. D., & Bonsall, A. (2021). Nat. Astron.

Sulcanese D., Mitri, G., & Mastrogiuseppe, M. (2024). Nat. Astron.

Tosi, N., Yuen, D. A., de Koker, N., & Wentzcovitch, R. M. (2013). PEPI.

van Zelst, I., Maia, J., Plesa, A.-C., Ghail, R., & Spühler, M. (2024). JGR: Planets.