Study of scattering characteristics of lunar equatorial region using Chandrayaan-1 Mini-SAR polarimetric data

Abstract

The miniaturized synthetic aperture radar Mini-SAR onboard Indian Chandrayaan-1 mission was the first ever lunar orbiting SAR that acquired several data strips covering a wide variety of geological units over lunar equatorial and low latitude regions, some of which were not studied earlier at radar wavelengths. The Mini-SAR observations, complemented by high resolution optical imagery and higher incidence angle radar datasets, were effectively used to create a catalog of SAR backscatter properties of various lunar geological features. The radar backscatter along with the parameters circular polarization ratio (CPR), relative phase (δ) and m (the degree of polarization)–chi (the Poincare ellipticity) decomposition technique were used to study the scattering mechanisms, surface/sub-surface roughness and regional topography of some of the craters and their ejecta fields. The study revealed that the Taylor and Descartes craters in the lunar highlands region were characterized by high backscatter and low CPR values, while the Maunder and Kopff craters in the Mare Orientale basin were characterized by relatively low backscatter and elevated CPR values. The fresh crater Jackson and its ejecta blankets on the lunar far-side showed very high backscatter along with elevated CPR values due to the presence of abundant wavelength-scale scatterers. The radar dark Pyroclastic Orientale ring deposits showed both low backscatter and low CPR values, as reported by previous radar observations of pyroclastic deposits. The elevated CPR values corresponding to the interior and exterior of Santos Dumont crater could be attributed to rough crater walls and regional topography, respectively. Finally, attempt was made to bring out the differences in the origin of two similar-sized craters Taylor and Kopff, with the help of SAR polarimetric parameters. Differences in the CPR values of Mare and highland craters were attributed to the sensitivity of CPR to the ilmenite content and presence of surface/sub-surface rocks with diameter of about one-tenth the radar wavelength and larger. This could act as a potential method to distinguish craters of different origin and (or) composition, where the information regarding relative age and regolith composition are not available.