ISOSTATIC COMPENSATION OF EQUATORIAL HIGHLANDS ON VENUS

Spherical harmonic models for Venus' global topography and gravity incorporating Magellan data are used to test isostatic compensation models in five 30 degrees x 30 degrees regions representative of the main classes of equatorial highlands. The power spectral density for the harmonic models obeys a power-law scaling with spectral slope beta similar to 2 (Brown noise) for the topography and beta similar to 3 (Kaula's law) for the geoid, similar to what is observed for Earth. The Venus topography spectrum has lower amplitudes than Earth's which reflects the dominant lowland topography on Venus. Observed degree geoid to topography ratios (GTRs) on Venus are significantly smaller than degree GTRs for uncompensated topography, indicative of substantial compensation. Assuming a global Airy compensation, most of the topography is compensated at depths greater than 100 km, suggesting a thick lithosphere on Venus. For each region considered we obtain a regional degree of compensation C from a linear regression of Bouguer anomaly versus Bouguer gravity data. Geoid anomaly (N) versus topography variation (h) data for each sample were compared, in the least-squares sense, to theoretical correlations for Pratt, Airy, and thermal thinning isostasy models yielding regional GTR, zero-elevation crustal thickness (H), and zero elevation thermal lithosphere thickness (y(L0)), respectively. We find the regional compensation to be substantial (C similar to 52-90%), and the h, N data correlations in the chosen areas can be explained by isostasy models applicable on the Earth and involving variations in crustal thickness (Airy) and/or lithospheric (thermal thinning) thickness. However, a thick crust and lithosphere (y(L0) similar to 300 km) must be assumed for Venus. Compensation mechanisms are distributed as follows: For the high plateaus of Ovda (GTR similar to 9 m/km) and Thetis (GTR similar to 11 m/km), mainly Airy isostasy with H similar to 50-60 km; for the Chasmata area sample (GTR similar to 15 m/km), Airy with perhaps a thermal component; finally, a very strong thermal component for the broad swells of Beta (GTR similar to 32 m/km) and Atla (GTR similar to 25 m/km), with the lithosphere thinned to similar to 100 km. Aphrodite Terra, which shows a marked eastward increase in Pratt and Airy model parameter values, could be a candidate for thermal isostasy decay from east to west. (C) 1994 Academic Press, Inc.