A team from the Physical Research Laboratory (PRL) in Ahmedabad has discovered that water exists in the form of ice crystals beneath the Moon's surface in areas that never receive direct or indirect sunlight or heat. This finding is based on strong radar evidence from data collected by ISRO's Chandrayaan-2 mission.
Radar Signatures from Doubly Shadowed Craters
The radar signatures from 'doubly shadowed' craters in the South Pole region provided new evidence on how water—essential for future human plans to use the Moon for space exploration—is retained for millions of years despite the absence of an atmosphere and extreme temperature fluctuations.
Over the past two decades, multiple missions from ISRO, NASA, and other space agencies have provided ample evidence that ice exists at the lunar poles, particularly inside permanently shadowed regions where temperatures remain extremely low. However, the exact form, depth, and spatial distribution of this ice, especially whether it exists within the subsurface mixed within the regolith, has remained one of the major unresolved questions in lunar science.
Study Published in Space Exploration Journal
The study, titled 'Subsurface ice in doubly shadowed craters as revealed by Chandrayaan-2 dual frequency synthetic aperture radar,' was authored by Rishitosh K Sinha, Rajiv R Bharti, Kinsuk Acharyya, Sanjay K Mishra, Neeraj Srivastava, and Anil Bhardwaj. It was recently published in the Springer Nature NPJ journal 'Space Exploration.'
Sinha, the lead author, explained that deep-impact craters are primarily located in the permanently shadowed region (PSR) of the Moon, which does not receive direct sunlight. "Moreover, its raised rim additionally blocks the scattered light and thermal emission from the nearby illuminated surfaces. Thermal models suggest that the interior temperature reaches around 25 kelvin (approx. -248°C), which is lower than even some of the outer planets of the solar system," he said.
Investigation of Nine Doubly Shadowed Craters
The team focused on these regions for their search for water ice on the Moon. They used full-polarimetric L- and S-band observations from Chandrayaan-2's Dual-frequency Synthetic Aperture Radar (DFSAR) to investigate nine doubly shadowed craters located within three larger craters: Faustini, Haworth, and Shoemaker.
The radar scan of these areas showed potential signatures for the presence of ice beneath the surface of four of the nine craters. Simplifying the method of discovery, the team members explained that they employed circular polarisation ratio (CPR), noting that radar waves undergoing volumetric scattering within ice-rich material can produce CPR values greater than one.
However, rough rocky terrains can also generate elevated CPR values. "To reduce ambiguity from rough rocky terrains, another radar diagnostic known as degree of polarisation (DOP) was used in the study. The regions showing CPR values above 1 and DOP values below 0.13 most likely pointed to the presence of subsurface ice," said Bharti. "The radar signatures helped identify regions that are likely to host buried ice deposits."
One Crater Shows Excavated Ice Deposit
One of the craters inside Faustini crater particularly interested the researchers as it showed the possibility that the impact may have occurred on a surface that had subsurface ice in the past, and the impact may have excavated into this potentially buried ice deposit.
Chandrayaan missions by ISRO have been instrumental in providing evidence of water on the Moon in the form of ice. Several other studies and probes by other countries have also substantiated the belief that the Moon contains water and that conditions such as permanently shadowed regions with very low temperatures could hold ice stably for millions of years.
Implications for Future Missions
Anil Bhardwaj, director of PRL and a co-author of this study, said that the latest study adds to the knowledge of potential locations where future missions could look for water ice. The researchers caution that not all craters meet the desired values but instead point to more potential sites compared to others while searching for the scarce water on the Moon.
It is notable that both Chandrayaan II and III targeted the South Pole Region of the Moon because of its scientific importance and potential volatile deposits.
