Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p51b0927y&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #P51B-0927
Physics
5430 Interiors (8147), 5475 Tectonics (8149), 5724 Interiors (8147), 6024 Interiors (8147), 8149 Planetary Tectonics (5475)
Scientific paper
A series of numerical simulations of thermal convection of Boussinesq fluid with infinite Prandtl number, with realistic Rayleigh number 107, and with the strongly temperature- and depth- dependent viscosity in a three-dimensional spherical shell is carried out to study the mantle convection of single-plate terrestrial planets like Venus or Mars without an Earth-like plate tectonics. Basic equations governing the mantle convection are solved by a second-order finite difference discretization. A kind of the overset, or Chimera grid system, "Yin-Yang grid" (Kageyama and Sato, 2004), is used. The Yin-Yang grid is suitable to solve the mantle convection problems because it automatically avoids the pole problems, i.e., the coordinate singularity and grid convergence that are inevitable in the usual latitude-longitude grid (Yoshida and Kageyama, 2004). The constant viscosity convection with the rigid boundary condition, assuming that it is the base of an immobile lithosphere of terrestrial planets, on the top surface shows that the convection has long-wavelength structures; the spherical harmonic degree-one becomes dominant throughout the convecting layer. In contrast, the models only with strongly temperature-dependent viscosity (the viscosity contrast across the shell is 105 or over) and the stress-free condition on the top surface show that the convection under spontaneously generated stagnant-lid has short-wavelength structures; the degree 6--10 is dominant throughout the depth. Numerous, cylindrical upwelling plumes are developed because of the secondary downwelling plumes arising from the bottom of lid. This convection pattern is inconsistent with that inferred from the geoid observation of the Venus or Mars. The effects of the stratified viscosity at the upper/lower mantle (the viscosity contrast is varied from 30 to 300) are considered. It is found that the combination of the strongly temperature- and depth-dependent viscosity causes long-wavelength structures of convection in which the degree is dominant at 2--4 or lower. The geoid anomaly calculated by the simulated convections shows a long-wavelength structure, which is compared with observations. The degree-one convection like the Martian mantle is realized in the wide range of viscosity contrast from 30 to 100. Our results suggest that the viscosity stratification is indispensable to understand the mantle convection of the terrestrial planets when strongly temperature dependent viscosity is taking into account.
Kageyama Akira
Yoshida Makiko
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