Timothy E. Leftwich

Crustal analysis of Venus from Magellan satellite observations at Atalanta Planitia, Beta Regio, and Thetis Regio

We investigated an improved quantitative method for interpreting attributes of the Venusian crust from Magellan spacecraft imagery and spherical harmonic topographic and gravity models. We used spectral correlation theory to compare gravity effects of topography against free-air gravity anomalies for three tectonically disparate crustal regions. Adjusting terrain gravity effects for correlated free-air anomalies yields compensated effects that we used to estimate the Moho and crustal thickness variations. For Atalanta Planitia, the crust may be less than 10 km thick, but thickened peripherally at concentric ridge belts and tessera terrain. Crustal thickness may increase rapidly between the interior of this lowland plain and the peripheral ridge belts. For Beta Regio, the crustal thickness may exceed 40 km. Of the three regions considered, the crust of Beta Regio may be the most poorly compensated because its free-air anomalies are largest relative to corresponding terrain gravity effects. For Thetis Regio, crustal thicknesses may exceed 50 km. The thickened crust of Thetis Regio may also have been rifted and embayed by plains forming lavas. The lavas appear to embay and cover complexly ridged areas except where these units remained above the reach of the plains forming lavas. Crosscutting relationships, which are evident from Magellan imagery, together with our crustal thickness estimates, suggest that the complexly ridged terrain (or tesserae) may be the oldest crustal units within the three study regions. Hence these units may represent relict surfaces that predate emplacement of the plains.