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Radar Topography of Dome Volcanoes on Venus and Titan

Introduction: In October 2004, SAR imaging from Cassini’s TA encounter uncovered features suggestive of cryovolcanic constructs on Titan, including flows, sinuous channels, and the 180 km structure Ganesa Macula [1]. Ganesa Macula is a radar-dark, circular feature with bright flanks, sinuous channels, and a bright region in the center that may be a central depression. In many ways, Ganesa Macula resembles the domes seen on Venus. Some 145 ‘pancake’ domes (steep-sided flat-topped circular-plan-view domes of probable volcanic origin) were identified in a survey covering 95% of of Venus’ surface [2]. While altimetry was available from Magellan all over the Venusian surface [3], Cassini altimetry coverage is very sparse. Thus, we rely on SAR imaging alone to obtain topography on Titan. In this work, we adjust simulated SAR images to match the TA data in order to recover the shape and height of Ganesa Macula. With both altimetric and SAR data available for several domes, Venus provides an opportunity to test our assumptions and techniques. Further, because the intrinsic spatial resolution of SAR is much higher than altimetry, finer-scale topography can be resolved than was used in prior work to characterize the Venusian features. This is a particularly important benefit on the flanks of the domes, where typically only one or two altimeter footprints define the shape. Methods: An individual SAR image can reveal topography by radarclinometry, or shape-from-shading [4, 5]. In this work, we use a forward-modeling approach. We assumed a certain topographic profile, calculated the radar backscatter for each local incidence angle on the feature, and looked for a model that most closely resembles the observed SAR image. These models also include a correction for ‘layover’ distortion, whereby an elevated feature appears tilted towards the subspacecraft point. The radar backscatter observed by SAR varies as a function of the transmitting and receiving geometry, as well as the roughness and composition of the reflecting surface. Scattering models describe the radar backscatter function, σ, for different local incidence angles. The Muhleman model [6] was used to describe the average backscatter of the Venusian surface during the Magellan mission,