ELASTIC THICKNESS ESTIMATES FOR VENUS FROM LINE-OF-SIGHT ACCELERATIONS

A new method of obtaining the transfer function, or admittance, Z(f), between the Fourier transforms of topography and gravity of a planet i s proposed that uses the line of sight (LOS) Doppler velocities direct ly. The expected LOS accelerations are first calculated from the spher ical 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 ve locities. This method is applied to the Magellan data from cycle 4 for Atla and shows that the short wavelength behavior of the admittance i s that expected from an elastic layer whose thickness T-e is 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 of Z(f)(k) that do not fit those expected from a simple flexural model. The T-e values from Beta and Ulfrun of 27.5 and 33 km are similar to that of A tla, whereas Dali (12 km) gives a smaller value. No reliable value can yet be estimated from Aphrodite, probably because the topography is p oorly determined. These estimates of T-e cover the same range as those from flexural modeling of topography associated with coronae. (C) 199 7 Academic Press.