A puzzling case: ShadowCam reveals scarce ice in Moon’s darkest regions

A new study led by researchers at the University of Hawaiʻi (UH) at Mānoa reveals that surface water ice in the Moon’s permanently shaded regions (PSRs) is less abundant than previously anticipated. Published recently in Science Advances, 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 publication 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.

In this latest effort, the team utilized NASA’s ShadowCam, an ultra-sensitive camera aboard the Korea Pathfinder Lunar Orbiter (KPLO, also known as Danuri). ShadowCam is specifically designed to image the Moon’s darkest corners by capturing sunlight reflected off nearby crater walls. 

The 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.

The 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,” explained 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." He added, "However, the forward-scattering signal was a true and exciting surprise because it required stereo observations that were only possible during the extended mission."

The search will continue

Many of the detected ice exposures are located near relatively young impact craters, suggesting that even more water may be buried just beneath the surface, protected from the harsh space environment. 

At the time when Li’s recently published work was prepared, stereo observations, which allow examining the unique forward-scattering feature of water ice, were only available at six locations. The team expects to see many more similar optical signatures of water ice as more stereo observations will be conducted during the extended mission until early 2028 when the mission will run out of battery in a lunar eclipse and crash onto the lunar surface.

Further, the study concludes that while ShadowCam has advanced our understanding, future missions, with more sensitive detectors capable of mapping ice at concentrations below 1%, will be necessary to fully map the Moon’s elusive water resources. With that future capability, researchers may still find widespread lower-abundance water ice.