Analyzing local variations in Venus tesserae: Measurements of backscatter coefficient

Abstract:

Tesserae only cover 7% of Venus, but these materials represent the oldest rock record on the surface and may preserve evidence of different earlier climate conditions. Despite the importance of the tesserae for understanding Venus prior to ~500 Ma, its surface properties are not well constrained. The range in radar brightness, or backscatter coefficient, across the tesserae has not been quantified in detail, and could provide important information about the distribution of crater ejecta or locally-derived regolith, as well as inherent differences in the original tessera materials.

Magellan SAR data are used to calculate the backscatter coefficient of individual slopes within tesserae. The polarization of the Magellan SAR (HH) is sensitive to roughness variations at the few-cm scale and to slopes on the scale of tens to hundreds of meters. The range in backscatter is compared with nearby geologic landforms (e.g. impact craters, volcanoes) to assess whether there is a correlation between backscatter coefficient and tessera geologic settings/processes. The backscatter coefficient varies across the 21 tesserae measured, with values from -28 dB up to 13 dB at the highest elevations. Correlations between backscatter coefficient and the expected location of crater ejecta exist, where ejecta-mantled regions have lower backscatter values. For example, the fine-grained ejecta from Stuart crater smooths the surface of eastern Alpha Regio. Our analysis indicates that crater ejecta is preserved in the rougher tessera longer than on adjacent low-lying plains (>35±15 Ma). Lower scattering values also occur in Tellus Tessera, but are not connected with patterns in the adjacent plains. While no correlations between tessera backscatter and extended deposits of volcanoes have been identified, there is evidence from northern Nedoyla Tesserae that differences in tesserae morphology and/or local elevation differences are associated with variations in backscatter coefficient. These radar brightness variations may be related to the inherent tesserae properties, rather than to materials emplaced as airfall by craters or volcanoes. The results of this study provide tantalizing information about the geologic history of Venus preserved in the tesserae, a story which can only be enhanced with new mission data.