Dark Crater-related Deposits on Venus in Radar Cross-section – Emissivity Domain

Introduction: The population of Venus impact craters is rather small (about 1000). About 10% of all craters have distinctive associated dark diffuse features (DDF) like parabolas or halos. Such craters are thought to be the youngest on the planet [1,2]. The DDF are interpreted as surface deposits of loose material (dust) formed by the impact. DDF change their form with geological time and disappear due to eolian activity, which moves the dust and scatters it over large areas, and/or due to some weathering processes [3]. In [4] the morphological degradation stage of DDF was used to arrange the craters according to their relative age. The objective of this work is the study of DDF material and processes of its evolution with the radiophysical properties of the DDF. Approach and general observations: Data of Magellan radar experiment in active and passive modes were used. C1-MIDR mosaics were used as a source of SAR radar cross-section σ data and emissivity maps (GEDR data set) to get the emissivity values E. Spatial resolution of SAR data was decreased to make it comparable to GEDR maps. Data from the first cycle of Magellan survey were used. Surroundings of craters with diameters of 30 – 80 km were studied. There are 91 craters with DDF in the low-latitude zone. The set includes 21 “dark parabola” craters (DP), 39 “clear dark halo” craters (CDH), and 31 “faint dark halo” craters (FDH), according to classification from [4]. Three general observations were made. (1) The range of E values of DDF is narrower in comparison to the surroundings. The DDF material is more homogeneous in comparison to the diversity of the surrounding surface units. (2) The radar contrast (the difference in the radar crosssection) between the DDF and the surrounding surface is rather high for DP craters, lower for CH craters, and even lower for FH craters. (3) For 57 craters, areas with a gradual change of radar brightness and nearly constant emissivity were found. These areas have different size and shape, but they are always associated with DDF. An example of such a feature for crater Ban Zhao (17.2°N, 146.9°E) is presented in Fig. 1. Fig.1a shows SAR image (σ), Fig.1b emissivity E for the same region. σ-E diagram for the area is presented in Fig. 2. Crosses in Fig. 1 and Fig. 2 show how observed properties vary across the feature. Rather distinctive “emissivity” features were found for every DP craters (with 2 exceptions due to rough surface masked the emissivity changing), 20 CDH and 18 FDH craters.