May 27, 2026
A detailed investigation of sub-surface ice in the lunar South Polar Region using observations from the Chandrayaan-2 Dual Frequency Synthetic Aperture Radar (DFSAR) have been carried out by the scientists from Physical Research Laboratory (PRL), Ahmedabad.
The Dual Frequency Synthetic Aperture Radar (DFSAR) onboard Chandrayaan-2 orbiter is a microwave imaging instrument in L- and S-band frequencies and the first fully-polarimetric SAR to study the Moon. The study focuses on “doubly shadowed craters,” which are special craters located inside permanently shadowed regions (PSRs) of the Moon. Due to continuous shielding from sunlight and thermal radiation, these regions remain extremely cold (temperatures ~25K) and are considered favorable locations for preserving water-ice over long geological timescales.
Using advanced radar polarimetric analysis, scientists identified radar signatures consistent with the possible presence of subsurface ice beneath the floors of four doubly shadowed craters in the lunar South Polar Region. The study proposes a refined radar-based criterion for identifying subsurface ice, where Circular Polarization Ratio (CPR) values greater than 1 together with Degree of Polarization (DOP) values lower than 0.13 indicate volumetric scattering potentially associated with subsurface ice. DOP is a radar polarimetric parameter that measures how much of the reflected radar signal retains its original polarization state after interacting with the surface or subsurface material.This approach helps distinguish genuine ice signatures from radar signals produced by rough rocky terrain.
Among the investigated craters, one crater of 1.1 km diameter within Faustini crater shows particularly strong evidence of subsurface ice(Figure 1), supported by both radar observations (Figure 2)and distinctive lobate-rim morphological characteristics. A lobate-rim morphology refers to flow-like or lobed appearance, suggesting the impact may have penetrated subsurface ice, producing the observed lobate-rim crater.
Figure 1: Left panel: Shadowcam image mosaic of the Faustini crater (centered at 87.2 o S, 84.3 o E) permanently shadow region in the South Polar Region of the Moon, adapted from Williams et al. 2024 (Planet. Sci. J. 5, 209). Right panel:ShadowCam image of a 1.1 km diameter doubly shadowed crater (centered at 87.39 o S, 82.31 o E) characterized by lobate-rim morphology and a combination of high Circular Polarization Ratio (CPR > 1) and low Degree of Polarization (DOP < 0.13).
Figure 2: CPR map derived from Chandrayaan-2 DFSAR data for the ~1.1 km diameter doubly shadowed crater shown in Figure 1. F2 denotes the second crater (1100m dia) within Faustini crater.
These findings provide important new insights into the distribution of lunar polar volatiles and have significant implications for future lunar exploration missions, including identification of potential ice-bearing regions for future landing and in-situ resource utilization (ISRU) activities.
Reference: “Subsurface ice in doubly shadowed craters as revealed by Chandrayaan-2 dual frequency synthetic aperture radar” Sinha et al., Nature portfolio journal npj Space Exploration, https://www.nature.com/articles/s44453-026-00038-9