The effect of a climatic thermal runaway on the tectonic regime of Venus

The origin of the observed differences between Earth and Venus remains a mystery. On Earth, surface deformation is focused at narrow plate margins resulting in plate tectonics (or a mobile-lid regime). On Venus, a global network of connected plate margins is absent, but the surface is young and has preserved evidence of at least regional crustal mobility. Therefore, the planet must be in a yet-to-be-defined regime distinct from plate tectonics, for example an episodic-lid regime. The array of Venus missions planned for the next decade provides us with an unprecedented chance to refine our knowledge of this tectonic regime, but to use the upcoming data, we need hypotheses to test and a physical framework in which to contextualize the data. To explain the discrepancy on the tectonic regime, a popular hypothesis is that Venus’ higher surface temperatures foster a stiffer lithosphere due enhanced grain growth. Thermally assisted grain growth is supposed to increase the lithospheric viscosity, since diffusion creep depends on grain size, and therefore subduction becomes less efficient. In a previous work [Manjón-Cabeza Córdoba, A., Rolf, T., and Arnould, M: Feasibility of the mobile-lid regime controlled by grain size evolution. EGU General Assembly 2022], we showed that high grain reduction can decrease the interval of yield stresses for which the episodic regime applies, but the results on grain growth were not too conclusive. Here, we present a new set of convection models in spherical annulus geometry using different surface temperatures to specifically address the differences between Earth and Venus. Our results suggest that the effect of the climate thermal runaway depends on the strength of the lithosphere. For yield stresses that yield Earth-like behaviors at lower surface temperatures, an increase in surface temperature does not result in the episodic regime, but rather a sluggish-dripping regime with relatively low plateness. We conclude that either Venus is not in an episodic-regime, or a different explanation must be put forward for the tectonic regime of Venus (e.g., lack of liquid water at the surface).