The Chandrayaan-2 spacecraft, launched on July 22, 2019, carries the Dual-Frequency Synthetic Aperture Radar, or DFSAR, an instrument that operates in both L-band and S-band frequencies. What makes the L-band particularly valuable is its penetration capability: longer radio wavelengths can probe several meters beneath the dusty lunar surface, retrieving subsurface parameters that earlier S-band missions such as Chandrayaan-1’s Mini-SAR could not reach. In fully polarimetric mode, the radar transmits and receives signals in multiple polarization combinations, allowing researchers to distinguish how electromagnetic waves bounce off surfaces, scatter within volumes of rubble, or reflect in double-bounce geometries from rocks and angled structures.

The team focused on four craters: two polar craters, Hermite-A and Peary, which sit within permanently shadowed regions where temperatures remain below 120 Kelvin, and two non-polar comparison craters, Gardner and Korolev X. Permanently shadowed regions are natural cold traps. Because the Moon’s axial tilt is only about 1.5 degrees, the floors of high-latitude polar craters have not seen sunlight for billions of years, making them ideal repositories for water molecules delivered by comets, asteroids, and solar wind interactions over geological timescales.

The analytical workflow was rigorous. Level-1A single-look-complex DFSAR data from the ISRO Science Data Archive were processed using the MIDAS software developed at ISRO’s Space Applications Centre. The researchers generated a 2×2 scattering matrix capturing the HH, HV, VH, and VV polarization channels, applied a multilooking factor of 38 to improve signal-to-noise ratio, and then used a 5×5 Refined Lee filter to suppress speckle noise while preserving fine surface details. From the resulting coherency matrix, they deployed two complementary decomposition techniques: the eigenvalue-eigenvector-based H/A/alpha decomposition and the model-based Yamaguchi four-component decomposition, which separates scattering into surface, double-bounce, volume, and helix contributions.

The results were striking. Within the Hermite-A crater, volume scattering dominated the interior, contributing approximately 74 percent of the total scattering power, while surface scattering accounted for 21 percent and double-bounce just 5 percent. Outside the crater, the picture reversed entirely: surface scattering dominated at roughly 85 percent. The H/A/alpha analysis reinforced this contrast, showing high entropy values concentrated along crater interior walls, indicative of multiple or random scattering mechanisms, while the smooth exterior terrain exhibited low entropy consistent with simple surface reflection.

Complementing the decompositions, the team computed the Circular Polarization Ratio, or CPR, a diagnostic metric defined as the ratio of same-sense to opposite-sense circularly polarized backscatter. Inside Hermite-A, CPR values ranged from 0.250 to 1.950, with a mean of 1.221, suggesting rough textures and a high probability of water ice deposits. The crater exterior showed a lower mean of 0.594. Similar patterns emerged at Peary crater, where interior CPR values spanned 0.246 to 1.930 compared with 0.064 to 0.962 outside. These elevated CPR values arise from the coherent backscatter opposition effect, a phenomenon in which radio waves traversing low-loss dielectric media such as water ice undergo constructive interference along reciprocal scattering paths, producing anomalously bright, highly polarized returns.

Crucially, the non-polar control craters behaved differently. Gardner crater’s interior was dominated by surface scattering, indicating a relatively smooth floor, while Korolev X showed predominantly surface scattering at its base with only minor volume-scattering regions. Although localized CPR values exceeding unity appeared at both non-polar sites, attributable to rough rocky terrain rather than ice, the pattern of dominant volume scattering within crater walls remained unique to the permanently shadowed polar craters. This distinction supports the idea that the polar anomalies reflect subsurface volatiles rather than mere topographic roughness, aligning with earlier Mini-RF studies that identified so-called anomalous craters whose radar properties differ from typical lunar craters.

To validate the radar interpretation against ground truth, the researchers turned to an unprecedented natural experiment: the Chandrayaan-3 landing site. Using high-resolution 75-megahertz L-band compact polarimetric DFSAR imagery captured both before and after the Vikram lander’s touchdown, they applied the improved S-Omega decomposition. Before landing, the site’s smooth fine-grained regolith produced 61.8 percent surface scattering, 11.0 percent double-bounce, and 27.1 percent volume scattering. After landing, the radar response shifted dramatically toward double-bounce and volume scattering, reflecting the lander’s structure and the ejecta-blanketed, disturbed terrain. CPR jumped from 0.5 pre-landing to approximately 2 post-landing, with a maximum of 1.99 plus-or-minus 0.50, demonstrating exactly how rough, blocky debris alters polarimetric signatures.

The authors are careful to emphasize that high CPR values and volume scattering alone cannot uniquely confirm water ice; rough surfaces, blocky ejecta, and fresh impacts can mimic the same signatures. However, when integrated with permanently shadowed conditions, persistently frigid temperatures, and prior neutron spectrometer evidence of hydrogen-rich regions, the combined polarimetric framework provides a substantially more reliable means of flagging candidate ice deposits. This matters enormously for exploration strategy: water ice at the lunar poles represents a potential resource for drinking water, oxygen, and rocket propellant, making accurate identification of ice-bearing craters essential for planning future crewed and robotic missions.

Looking ahead, the team proposes integrating additional radar-derived parameters, including degree of polarization, dielectric constant, and surface roughness, with complementary datasets such as Diviner thermal measurements, LOLA topography, and neutron spectrometer observations. Such multi-sensor fusion should further reduce ambiguities inherent to radar-only detection. For now, the study stands as a demonstration that Chandrayaan-2’s fully polarimetric L-band radar, combined with disciplined polarimetric decomposition and ground-truth validation from Chandrayaan-3, has given planetary scientists a powerful new toolkit for reading the Moon’s frozen archives, one shadowed crater at a time.

Subject of Research: Detection of potential lunar water ice using polarimetric decomposition of Chandrayaan-2 L-band DFSAR radar data in permanently shadowed polar craters

Article Title: Polarimetric decomposition-based analysis of Chandrayaan-2 L-band DFSAR data for potential water ice detection in lunar polar regions

Article References: Polarimetric decomposition-based analysis of Chandrayaan-2 L-band DFSAR data for potential water ice detection in lunar polar regions. (n.d.). https://doi.org/10.1007/s10686-026-10077-5

Image Credits: AI Generated

DOI: 10.1007/s10686-026-10077-5

Keywords: Chandrayaan-2, DFSAR, water ice, lunar polar regions, permanently shadowed regions, polarimetric SAR, circular polarization ratio, Hermite-A crater, Peary crater, Yamaguchi decomposition, Chandrayaan-3, lunar regolith