ABSTRACT
Numerous studies show that major Venus highlands display anomalously high radar reflectivity and low radar emissivity relative to the planetary average. This is thought to be the result of the formation of minerals having high dielectric constants via weathering reactions occurring between the surface and the deep atmosphere in these elevated terrains, where temperatures are lower. These reactions are a function of rock composition, atmospheric composition, and degree of weathering, or age. Here, we examine the Magellan radar emissivity, altimetry and backscatter data for all mapped tesserae and mountain belts on Venus. We characterize and classify each contiguous highland according to its pattern of the variation of radar emissivity with increasing altitude. The highlands can be assigned to 7 distinct patterns of emissivity that correspond to at least 2 discrete types of mineralogy based on the altitude (and temperature) of the emissivity changes from the global average (excursions). The majority of the emissivity changes occur at altitudes above 6053 km (temperature below 726 K). The emissivity signature of the major tesserae of Aphrodite Terra, Beta Regio and Phoebe Regio are consistent with the presence of ferroelectric minerals in their rocks (Curie temperatures of ~700-720 K). Fortuna tesserae and the mountains belts (Maxwell, Freyja, Akna and Danu montes) in Ishtar Terra are consistent with the presence of semiconductor minerals. Some tesserae in Ishtar Terra (Clotho, Itzpapatotl and Jyestha tesserae) lie at altitudes over 6055 but lack the emissivity excursions seen in Fortuna tesserae and the mountains at same altitudes and thus may represent a third type of tessera composition. Finally, the spatial distribution of radar emissivity classes correlates to different geologic settings which may reflect differences in the mantle dynamics. Alternatively, this variability could be ascribed to changes in the atmospheric conditions.
