A MANTLE PLUME MODEL FOR THE EQUATORIAL HIGHLANDS OF VENUS

The Equatorial Highlands of Venus consist of four main structures, Atla, Beta, Ovda, and Thetis regiones. Each has a circular to oval-shaped planform and rises 4-5 km above the mean planetary radius. These highlands are associated with long-wavelength geoid highs, with amplitudes ranging from 35 m at Ovda to 120 m at Atla. They also contain topographic valleys, interpreted as extensional rift zones, and Beta is known to contain shield volcanoes. These characteristics are all consistent with the Equatorial Highlands being formed by mantle plumes. An alternative model, in which Ovda and Thetis are interpreted as spreading centers analogous to terrestrial mid-ocean ridges, fails to explain most of the observed geoid anomalies and topography in these regions. Some smaller highlands, such as Bell Regio, Eistla Regio, and the Hathor/Innini/Ushas region, may also be plume related, but most coronae are unlikely to be the direct result of plume activity. We have modeled plumes using a cylindrical, axisymmetric finite element code and a depth-dependent, Newtonian rheology. We compare our model results with profiles of geoid and topography across Atla, Beta, Ovda, and Thetis; our best model fits are for Beta and Atla. Assuming whole mantle convection and that Earth and Venus have similar mantle heat flows, Venus must lack an Earth-like low-viscosity zone in its upper mantle in order satisfy the observed geoid and topography for these features. This conclusion is consistent with the long-wavelength admittance spectrum of Venus and with the observed differences in the slopes of the geoid spectra for the two planets. One explanation for the different viscosity structures of the two planets could be that the mantle of Venus is drier than Earth's mantle.