NASA Science Highlight: Planetary Program Support
Research by S. E. Smrekar and C. Sotin, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif.
Venus is a planet with striking surface characteristics, including mantle plumes, which are hot, roughly cylindrical structures that rise from deep in the mantle of the planet. On Earth, such plumes produce the volcanism observed in places such as Hawaii and Iceland. On Venus, similar mantle plumes are thought to produce features such as Beta Regio and Atla Regio. Plumes are one of the few manifestations of mantle convection that can be readily observed, and the number of active mantle plumes on Venus can be inferred from the volcano-rich, broad topographic swells with gravity anomalies implying low density anomalies at depth. The interpretation of high emissivity anomalies in the VIRTIS data set as indicating recent volcanism corroborates the presence of an active plume. Scientists used a spherical 3D simulation of mantle convection with temperature dependent rheology to constrain the balance between internal heating (Hs), temperature difference across the mantle (DT), and mantle viscosity needed to produce a small number of hot plumes.
Scientists were able to simulate thermal convection in Venus's mantle, using cubic-sphere geometry to solve the equations describing the conservation of mass, momentum and energy in a 3D spherical geometry. These equations included the Boussinesq approximation and an infinite value of the Prandtl number. The fluid was characterized by several parameters including thermal diffusivity (K), thermal conductivity (k), thermal expansion (a), density (p), and viscosity (n), which is strongly temperature dependent. The use of the cubic sphere allowed them to solve the equations in one sixth of the total sphere with periodic boundary conditions on the vertical planes. The temperature was imposed at the surface (T0) and at the core mantle boundary (T1). Free-slip or no-slip conditions were applied on the horizontal planes. Further, they assumed a surface temperature of 735 degrees K, K = 1 x 10-6 m2/s, a = 3 x 10-5 K-1, k =1 W K-1 m-1, p = 3500 kg/m3, g = 8.87 m/s2, a core radius of 3120 km, and a planetary radius of 6052 km.
These results have important implications for understanding the mantle structure of Venus. Results of this research indicate and are consistent with a hotter mantle due to the absence of plate tectonics. Second, wet melting is predicted through out much of the upper mantle. Thus the upper mantle may be lacking in light elements and be more fully outgassed than the lower mantle. Volcanism may have gone through a transition from more wide-spread, wet melting in the upper mantle to more localized melting in mantle plumes carrying un-melted, volatile rich material from deep within the planet's interior.
Significance to Solar System Exploration:
Using 3D spherical simulations of mantle convection including internal heating, determining the number of plumes and the prediction of pressure release melting significantly adds to a better understanding of not only the interior structure of Venus, but also adds important new constraints on the interior evolution of the planet. The research also adds to a greater understanding of chemical components that exist within Venus, including light elements and appreciable water.
Last Updated: 21 January 2014