Journal Article

The Mantle Viscosity Structure of Venus

Wieczorek, M. A., Plesa, A.‐C., Maia, J. S.

2023

Overview

The long‐wavelength gravity and topography of Venus are dominated by mantle convective flows, and are hence sensitive to the planet's viscosity structure and mantle density anomalies. By modeling the dynamic gravity and topography signatures and by making use of a Bayesian inference approach, we investigate the viscosity structure of the Venusian mantle by constraining radial viscosity variations. We performed inversions under a wide range of model assumptions that consistently predicted the existence of a thin low‐viscosity zone in the uppermost mantle. The zone is about 235 km thick and has a viscosity reduction of 5–15 times with respect to the underlying mantle. Drawing a parallel with the Earth, the reduced viscosity could be a result of partial melting as suggested for the origin of the asthenosphere. These results support the interpretation that Venus is a geologically active world predominantly governed by ongoing magmatic processes. Plain Language Summary On Venus, convective up and downwellings in the mantle are intrinsically linked to density variations in the interior. The manner in which the planet is deformed by these mantle flows depends upon its viscosity, and this in turn affects the gravity and topography signals that were recorded by orbiting spacecraft. We tested a large range of model parameters that describe the mantle viscosity on Venus and retrieved the viscosity structure that is compatible with the observations. Our results indicate the presence of a low viscosity zone beneath the Venusian lithosphere in the uppermost mantle. In this region, the viscosity is one order of magnitude smaller than that of the underlying mantle. Starting at a depth of about 80 km and extending over about 235 km, this low viscosity zone could reflect the presence of partial melting and, similarly to the Earth, an asthenosphere on Venus. The presence of partial melt in the interior of Venus supports recent observations of a volcanically active planet and holds major implications for present‐day magmatic activity. Key Points The long‐wavelength gravity and topography of Venus are investigated in the spectral domain using a dynamic loading model Bayesian inference constraints on the mantle viscosity structure indicate the existence of a low viscosity zone in the upper mantle The low viscosity zone is potentially associated with partial melting beneath the lithosphere

• Publisher: John Wiley & Sons, Inc,American Geophysical Union,Wiley
• Subject: Anomalies / Asthenosphere / Bayesian analysis / Convective flow / Density / Earth mantle / Gravity / interior / Inversions / Lithosphere / mantle convection / mantle viscosity / Melting / Planetary mantles / Planets / Probability theory / Sciences of the Universe / Spacecraft / Statistical inference / Topography / Venus / Venus surface / Viscosity / Wavelength