Indian Scientists Detect Possible Subsurface Ice In 'Doubly Shadowed Craters' On Moon Using Chandrayaan-2 Radar Data

 

Scientists from Ahmedabad-based Physical Research Laboratory (PRL) have detected signs of potential subsurface ice deposits on the Moon, buried beneath the floors of four "doubly shadowed craters" in the lunar South Polar Region.

The findings were made using observations from the ISRO's Dual Frequency Synthetic Aperture Radar (DFSAR) onboard the Chandrayaan-2 orbiter. The equipment is a microwave imaging instrument in L- and S-band frequencies and the first fully-polarimetric SAR to study the Moon.

 

The doubly shadowed craters are special craters located inside permanently shadowed regions (PSRs) of the Moon. Since these regions are continuously shielded from sunlight and thermal radiation, they remain extremely cold (around 25K) and are considered ideal sites for preserving water ice over long geological timescales.

The study, published in the Nature portfolio journal njp Space Exploration, proposes a refined radar-based criterion for identifying subsurface ice. It combines Circular Polarisation Ratio (CPR) and Degree of Polarisation (DOP) to better separate true ice signals from those caused by rough, rocky terrain.

It suggests that CPR values greater than 1 and DOP values lower than 0.13 indicate volumetric scattering, which is potentially an indication of subsurface ice.

 

CPR is a radar measurement used in planetary remote sensing to assess how rough the surface and subsurface are on bodies like the Moon, asteroids, and Mercury. In contrast, DOP is a radar parameter that shows how much of the reflected signal keeps its original polarisation after bouncing off surface or underground materials.

The PRL researchers noted that, among the craters studied, a 1.1-kilometre-wide crater inside Faustini showed particularly strong signs of subsurface ice. They explained that this conclusion was supported both by radar data and by the crater’s distinctive lobate-rim shape.

 

Notably, a lobate-rim morphology refers to a flow-like or lobed appearance, which suggests that the impact may have reached subsurface ice and created the observed features.

Earlier this month, scientists from the PRL discovered that the Moon's surface is not just a uniform pile of dust, but a distinct two-layer "cake-like" structure with the top being only a few centimetres. For the study, they looked at the site where the Vikram lander of Chandrayaan-3 executed a brief "hop" in September 2023 and ended up acting as a blower, stripping away the top three centimetres of loose dust and exposing the older, more tightly packed lunar material underneath.

 

The Physical Research Laboratory is a national institute in India that studies space and related sciences. It is mainly supported by the Department of Space, Government of India.