Mapping the Topography of Maxwell Montes Using Ground-based Radar Inter- Ferometry

Introduction. One of the major unsolved puzzles about the surface of Venus is the composition and formation process of the radar-bright, low emissivity material that covers much of the high altitude terrain. Maxwell Montes, centered at a latitude of 65◦ N, is the tallest structure on Venus with elevations 12 km above the mean planetary radius (MPR) [1]. In the case of Maxwell Montes, the emissivity decreases for elevations greater than about ∼5 km above the mean radius and shifts back towards normal, plains-type values at elevations greater than ∼9 km [2]. The onset of the high Fresnel reflectivity at high altitudes suggests that a temperature and/or pressure dependent process is responsible for the changes; for example, alteration of a mineral phase or condensation of atmospheric constituents [3, 4]. Although the general trend in emissivity vs. height is known for Maxwell [5, 6, 7], it is not entirely clear whether the onset of the low emissivity occurs at a fixed altitude across the entire mountain, and whether the transition back to normal emissivities also occurs at a specific altitude. The Magellan altimeter data have a spatial resolution of about 10 by 30 km at the latitude of Maxwell, and although the nominal height resolution is ∼20 m, the digital elevation model (DEM) produced by the altimeter has unrealistic values for very steep and very rough areas of the mountains. Figure 1 shows the Magellan DEM for Maxwell, overlaid onto a Magellan Synthetic Aperture Radar (SAR) image for comparison. The low resolution and problems with measuring altitudes across steep slopes make it difficult to study local changes in the emissivity with altitude. For example, the transition to plains like emissivities at very high altitudes on Maxwell does not appear to follow a constant topographic level [6]. This might be due to post-emplacement tectonic activity or to temporal variability in the amount of condensate available, but it could also be due to errors in the altimetry [6]. Radar interferometry can be used to measure topography, and ground-based radar has been used previously to measure topography of lunar craters [8]. We use similar interferometric observations to derive a topographic map of Maxwell Montes with a higher spatial resolution than is available from Magellan. Observations. We used the Arecibo Observatory and Green Bank Telescopes to obtain interferometric observations of Venus. The Arecibo radar was used to transmit a circularly polarized 12.6 cm wave, and we received the echo at both telescopes. The delayDoppler images were mappped to Mercator projection and registered. We then computed the complex product of images from each telescope to produce maps of the 0 11.5