The Effects of Dynamic Topography and Thermal Isostasy on the Topogra- Phy and Geoid of Venus

Introduction: The Venusian geoid, gravity field, and topography have been used in a variety of applications and are the primary sources of information about the internal structure of the planet. We focus on the relationship between the geoid and topography to determine both the support mechanism for the topography and lithospheric density structure. Magnitude of Dynamic Topography: The long-wavelength topography on Venus can be supported by various types of isostasy including thermal isostasy due to the thermal thinning of the lithosphere as well as dynamic support due to the convective motions within the mantle. It has been suggested; however, that in the stagnant lid regime of temperature-dependent viscosity convection the dynamic support might be small and the topography and geoid anomalies are largely caused by the lithospheric thickness variation [1, 2, 3]. Here we analyze the relative magnitudes of the dynamic topography and thermal isostasy and show that the dynamic topography is indeed negligible in the well developed stagnant lid convection regime (at large viscosity contrasts). Thermal Isostasy and Topography: With the dynamic component of topography being negligibly small both the topography and geoid can be calculated in a variety of simple ways. Here we focus on two methods for calculating the topography and geoid and compare the results with previous work. Calculation of variations in lithospheric thickness. Simple thermal isostasy equations relating the topography and geoid (e.g. [4]) can be used in conjunction with a spherical harmonic representation of the Venu-sian geoid and topography (namely MGNP180U, [5] and shtjv360.a02, [6]; respectively) to calculate both a global average lithospheric thickness and variations in the lithospheric thickness. This calculation shows that the observed geoid can be explained by a combination of simple thermal isostasy and limited regions of a thickened crust supported by Airy isostasy. These regions of thickened crust include Ishtar Terra, Ovda Regio, Thetis Regio, and portions of the Beta, Atla, and Themis region. This result is consistent with previous work by Kucinskas and Turcotte [4] in a study of five equatorial highlands comparing the actual variation in the geoid and topography to models based on Airy, Pratt, and thermal isostasy. They suggested that simple thermal thinning of the lithosphere is the primary support mechanism for the volcanic rises but cannot com