Elastic thickness estimates for Venus using line-of-sight accelerations from Magellan cycle 5

David N. Barnett, Francis Nimmo and Dan McKenzie


Abstract

The elastic thickness, Te, for various regions of Venus is estimated by comparing the observed line-of-sight (LOS) acceleration of the Magellan spacecraft with that predicted using a spherical harmonic representation of the topography, to degree and order 360. At long wavelengths (typically longer than about 500 km) the transfer function between the topography and gravity, or admittance, usually has a flat spectrum with a magnitude of between 20 and 50 mGal/km, which is most likely due to convective support. In particular, the topographic highs associated with Beta, Phoebe, Bell and Eistla are thought to be dynamically supported. At shorter wavelengths, the admittance increases, suggesting a component of flexural support. The elastic thicknesses are constrained by fitting theoretical admittance curves to the observed short wavelength values for the admittance. Results from Magellan cycle 5 show evidence of regional variations in elastic thickness between about 19 and 29 km, with a mean value of around 21-23 km, assuming a crustal thickness of 16 km and a density of 2670 kg/m3. The observed variations in admittance between different regions are unlikely to be due to differences in crustal thickness or density, and probably represent real variations in Te. The values obtained are similar to those from an identical analysis using cycle 4 data. Estimates of the elastic thickness of the Ovda and Alpha regions are unreliable, probably because the topography is not well-determined. No reliable estimates of elastic thickness could be made from cycle 6 data where the altitude of the spacecraft was higher than about 300 km, due to the reduction in short wavelength signal amplitude with altitude.

Key words: Venus, elastic thickness, admittance, gravity, line-of-sight acceleration.


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