Analysis and interpretation of high‐resolution topography and gravity of Ishtar Terra, Venus
Authors: Arkani‐Hamed Jafar 
Published: February 2004

The apparent depth of compensation (ADC) of the topography of Venus, determined on a global scale using the Airy isostasy model, shows that the topography specified by the spherical harmonics of degree lower than 10 is compensated at depths greater than 150 km, that of degree higher than about 35 is compensated at a constant depth of about 35 km, and the compensation depths of the harmonics of degree 10–35 shoal as the harmonic degree increases. Based on these characteristics of the globally determined ADC, three different maps of the topography and gravitational potential of the Greater Ishtar Terra are derived using the harmonics of degree 1–75 (LW), 10–75 (IW), and 30–75 (SW), respectively. The LW maps of the topography and potential do not correlate well, but the IW and SW maps show good correlation, emphasizing that the long‐wavelength components of the gravitational potential over the Ishtar Terra have contributions from sources broader than the Terra. The energy of the gravitational potential of the topography is about an order of magnitude greater than that of the observed potential, indicating that the topography is strongly compensated. The ADC of the topography is estimated by both spectral and space domain analysis. In the spectral domain, three independent procedures, the energy spectrum, the statistical method, and the wavelength‐dependent method, are employed. The first two yield a mean ADC value for the entire spectrum, whereas the last one calculates the ADC for different wavelengths. The ADC decreases as the longer‐wavelength components are excluded. The mean ADC values determined for the LW, IW, and SW maps by the energy spectrum method are 115, 65, and 40 km, respectively. Those calculated through the statistical method are 85, 55, and 34 km, respectively. These values reflect the upper and lower limits, because the energy spectrum method overestimates and the statistical method underestimates the ADC. The wavelength‐dependent method shows that the ADCs of the wavelengths 500–1350 km are similar for all the maps but those of the longer wavelengths significantly differ among the maps. The mean ADC values also differ from the ADC of local topographic features. For example, the ADC of Maxwell Montes is about 55 km in both the LW and IW maps. In the space domain, the ADC is determined for Pratt and Airy isostasy models through fitting a linear and a quadratic function to the geoid height versus topography data, respectively. The ADC values determined show good agreement with the above‐mentioned values. In addition, the relationship between the high‐resolution topography and the line‐of‐sight acceleration residuals of the western Ishtar Terra specified by wavelengths of 300–500 km suggests that the high‐resolution topography is compensated at a depth of about 25 km. We also determine the lateral density perturbations inside a surface layer so that together with the topography of the Greater Ishtar Terra they give rise to the observed gravitational potential over the Terra. Three layer thicknesses of 50, 100, and 200 km are examined. The resulting density perturbations are too large to be interpreted in terms of lateral temperature variations alone. This suggests lateral variations in the rock type; the crust beneath the mountain belts may contain considerable amounts of low‐density material.