Elastic Thickness Estimates for Venus from Line of Sight Accelerations

ran 1974, Watts 1978). It is of great importance to estimate the thickness of the elastic layer on Venus. It is, however, A new method of obtaining the transfer function, or admittance, Z , between the Fourier transforms of topography and only straightforward to do so if its thickness is sufficiently gravity of a planet is proposed that uses the line of sight (LOS) large to affect the gravity field at those wavelengths that Doppler velocities directly. The expected LOS accelerations are are well determined from Magellan’s orbit. The surface first calculated from the spherical harmonic coefficients of the gravity field of wavelength l is reduced by a factor of topography, the latitude, longitude, and height of the spaceexp(22fh/l) at the orbital height h, or approximately craft, and the direction of Earth viewed from the planet. The exp(26h/l). Furthermore, the spectrum of the gravity field admittance can then be obtained using standard signal prois red and is therefore dominated by the long wavelength cessing methods, by comparing the LOS acceleration calculated components. These two effects combine to prevent reliable from the topography with the time derivative of the observed estimates of the surface gravity field of Venus at waveLOS Doppler velocities. This method is applied to the Magellan data from cycle 4 for Atla and shows that the short wavelength lengths shorter than about 400–500 km. On Earth the elasbehavior of the admittance is that expected from an elastic tic effects only begin to dominate over the convective gravlayer whose thickness Te is 30 6 5 km. The main contribution ity anomalies at these and shorter wavelengths. Sandwell to the short wavelength gravity field comes from the large and Schubert (1992) estimated the elastic thickness on volcanoes Ozza, Maat, and Sapas Montes. Comparison with Venus from the topography of coronae to be 15–40 km, admittance estimates from spherical harmonic gravity fields and Johnson and Sandwell (1994) obtained values of 12–34 and from local inversions shows that these methods produce km. These values are similar to those from stable regions estimates of Z (k) that do not fit those expected from a simple on Earth, and are therefore likely to be difficult to estimate flexural model. The Te values from Beta and Ulfrun of 27.5 reliably using the surface gravity field. Attempts to do so and 33 km are similar to that of Atla, whereas Dali (12 km) gives a smaller value. No reliable value can yet be estimated (Phillips 1994 and written communication 1996, Smrekar from Aphrodite, probably because the topography is poorly 1994) using the spherical harmonic description of the gravdetermined. These estimates of Te cover the same range as ity field, or using inverse methods, have produced a considthose from flexural modeling of topography associated with erable range of values, probably because of problems with coronae.  1997 Academic Press noise and with leakage of longer wavelength energy to shorter wavelengths. For instance Smrekar (1994) estimated the behavior of