The Venusian Crustal Plateaus Are Inconsistent with Felsic (Silica-Rich) Rheology

False color three-dimensional perspective view of the tessera and crustal plateau of Alpha Regio. Credit: NASA/JPL.

The current surface conditions of Venus are oppressive, with a surface pressure of approximately 90 bar (90 times Earth’s surface pressure) and temperatures approaching 735 K (hot enough to melt lead). A long-standing venusian mystery centers on whether the planet ever had liquid water oceans. While most of the venusian surface is covered in vast basaltic plains, thought to have been emplaced in the last ~300-1,000 Myr (million years), there are enigmatic highlands called crustal plateaus that are dominated by heavily deformed terrain (tessera). These plateaus are considered older than the plains and reflect an early epoch of venusian evolution. The compositions of these units are unknown. However, if they are felsic (silica-rich), they may provide evidence for liquid water in Venus’ past, indicating a more Earth-like ancient Venus.

The thick venusian atmosphere makes direct measurements of Venus’ surface difficult. However, a recent study by Francis Nimmo (University of California, Santa Cruz) and Stephen Mackwell (Rice University) uses models of viscous relaxation (crustal flow) to examine the composition and heat flux of the crustal plateaus. Plateaus of different compositions and heat flow will deform differently, with more mafic (silica-poor) basaltic rocks being deformation-resistant and felsic rocks being more malleable. Comparing the morphology of the plateaus from venusian topography data to models of relaxation allows for a determination of composition and heat flow from the planet’s interior. The results of this study indicate that silica-rich crustal plateaus would relax far too quickly for all plausible heat flow values to explain the observed plateau topography. The morphologies are more consistent with a basaltic composition. This suggests that the crustal plateaus and tessera may not be silica-rich and cannot constrain the presence of liquid water oceans. The upcoming Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) mission will allow for further determination of a tessera (Alpha Regio) composition. READ MORE