Scattering mechanism-based algorithm for improved mapping of water-ice deposits in the lunar polar regions

Due to the smaller axial tilt of the Moon, interiors of some of the polar craters on the lunar surface never get sunlight and are considered to be permanently shadowed regions. Several of such regions are known to contain water-ice deposits. These regions are expected to show elevated values of circular polarization ratio (CPR). Hence, the interiors of craters containing water-ice deposits are characterized by elevated CPR values as observed by the S-band synthetic aperture radar (Mini-SAR) on-board Chandrayaan-1 mission of ISRO. However, elevated CPR values were also observed from the interiors of some non-polar craters and also from young, fresh polar craters. Thus, elevated CPR values are not a unique signature of water-ice deposits. Therefore, additional information related to geological setting and roughness patterns should also be considered while identifying the regions containing water-ice deposits. For identifying a unique signature of water-ice deposits, analysis of radar scattering mechanism in elevated CPR regions was carried out. Areas of elevated CPR due to double-bounce and surface scattering conditions were segmented and polarimetric, backscattering properties of diffuse scatterers were analysed. Based on the signatures of diffuse scatterers and radar backscattering coefficient, a scattering mechanism-based algorithm was developed, which has the advantage in classifying regions showing elevated CPR due to surface and double-bounce scattering effects. The algorithm was then tested using Mini-SAR data and it was also found to be useful in eliminating regions of elevated CPR in fresh craters observed due to the double-bounce effect.