Elastic Thickness Estimates for Venus from Line of Sight Accelerations

Abstract

A new method of obtaining the transfer function, or admittance,Zf, between the Fourier transforms of topography and gravity of a planet is proposed that uses the line of sight (LOS) Doppler velocities directly. The expected LOS accelerations are first calculated from the spherical harmonic coefficients of the topography, the latitude, longitude, and height of the spacecraft, and the direction of Earth viewed from the planet. The admittance can then be obtained using standard signal processing methods, by comparing the LOS acceleration calculated from the topography with the time derivative of the observed LOS Doppler velocities. This method is applied to the Magellan data from cycle 4 for Atla and shows that the short wavelength behavior of the admittance is that expected from an elastic layer whose thicknessTeis 30 ± 5 km. The main contribution to the short wavelength gravity field comes from the large volcanoes Ozza, Maat, and Sapas Montes. Comparison with admittance estimates from spherical harmonic gravity fields and from local inversions shows that these methods produce estimates ofZf(k) that do not fit those expected from a simple flexural model. TheTevalues from Beta and Ulfrun of 27.5 and 33 km are similar to that of Atla, whereas Dali (12 km) gives a smaller value. No reliable value can yet be estimated from Aphrodite, probably because the topography is poorly determined. These estimates ofTecover the same range as those from flexural modeling of topography associated with coronae.