A new study led by researchers at the University of Hawaiʻi at Mānoa reveals that surface water ice in the Moon's permanently shaded regions (PSRs) is less abundant than previously thought. The research provides the most detailed look yet into the lunar PSRs where sunlight cannot reach directly, suggesting that while ice may exist, it is likely present in low concentrations or small, isolated pockets.
This study builds on nearly a decade of breakthroughs by the team, led by Shuai Li, an associate researcher at the Hawaiʻi Institute of Geophysics and Planetology in the UH Mānoa School of Ocean and Earth Science and Technology . Li previously led the 2018 discovery of the first direct evidence of surface ice using data from India's Chandrayaan-1 mission.
Less water on the Moon means future lunar explorers may face tighter constraints for sourcing drinking water and fuel, making planning and resource management even more critical.
Reflected sunlight, crater walls
In this latest effort, the team utilized NASA's ShadowCam, an ultra-sensitive camera aboard the Korea Pathfinder Lunar Orbiter. ShadowCam is specifically designed to image the Moon's darkest corners by capturing sunlight reflected off nearby crater walls.
Researchers found no evidence of "widespread" water ice at high concentrations (above 20% to 30% by weight). This discovery highlights a puzzling disparity between the Moon and other airless bodies like Mercury and Ceres, which host substantial, nearly pure ice deposits in their poles although the Moon's poles are even colder.
While the delivery of water via impacts may be similar across the Moon and Mercury, Li suggests Mercury's much hotter surface may facilitate substantially more water formation from solar wind than the Moon. Alternatively, the Moon's unique environment—including space weathering from solar wind, volcanic degassing and mixing of rock layers from impact—may destroy or bury surface ice more effectively.
Science of light scattering
This study was made possible during ShadowCam's extended mission, which allowed the team to capture images from multiple angles to analyze how light scatters off the lunar surface. This is the first time researchers used scattering properties of water ice to search for it on the Moon. Rocks and dust on the lunar surface sends more light back toward the direction from which it came, while water ice scatters light forward.
"Water ice doesn't just make the surface brighter," said Li. "The way it scatters light is a fingerprint. By using stereo observations to look at these shadowed craters from different perspectives, we were able to detect this distinctive forward-scattering behavior for the first time."
In the high-resolution images, the team identified a few small areas, roughly 20 to 50 meters in size, that exhibit both high reflectance and unique forward-scattering properties. These optical signatures are consistent with ice concentrations greater than 10%.
Li said, "I thought we'd find more bright, ice-rich areas, so the small number we found was a bit surprising. However, the forward-scattering signal was a true and exciting surprise because it required stereo observations that were only possible during the extended mission."
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