A recent study establishes how particle size, particle shape, phase angle, and ice abundance influence VNIR spectral signatures, providing a refined framework for accurately detecting and quantifying lunar water ice in polar regions.
Recently, researchers from the University of Hawaii investigated the Moon’s permanently shaded regions, examining how visible and near-infrared (VNIR) spectroscopy can locate and quantify water ice in these regions, as well as how factors such as particle size, particle shape, phase angle, and ice abundance alter VNIR spectral signatures (1). Published in the journal Icarus, the findings ultimately offer more information that can potentially help improve the accuracy of orbital and surface-based searches for lunar water (1).
Water ice on the Moon has long been considered an essential resource for sustaining future human activity, supporting everything from drinking water and radiation shielding to hydrogen-oxygen rocket propellant (1). Scientists first began to suspect the presence of water ice on the Moon’s permanently shaded regions after NASA’s Clementine mission in 1994 sent back data to suggest this (2). Four years later, the Lunar Prospector Mission found large concentrations of hydrogen in areas on the Moon not exposed to sunlight (2). Hence, both these results indicated water ice on the Moon.
As research into the Moon’s permanently shaded regions increased, scientists encountered roadblocks confirming and quantifying surface ice across the lunar poles. Variability in particle properties and viewing geometry can distort spectral readings, which has complicated interpretation for scientists relying on orbiters or landers (1).
In their study, the researchers simulated realistic lunar conditions to measure the spectral responses of pure water ice and mixtures of water ice and highland regolith simulant (HRS). The Icarus study confronts this issue directly through laboratory measurements designed to simulate realistic lunar conditions. They tested particle sizes ranging from 0 to 250 micrometers, particle shapes classified as angular versus spherical, phase angles from 0° to 105°, and ice abundances ranging from 0 to 50 weight percent (1). This systematic approach allowed the authors to isolate which variables most strongly influence spectral signals relevant to ice detection.
The findings revealed that coarser ice particles consistently generated stronger NIR absorptions and lower visible-light reflectance, largely because of longer optical pathlengths that increase photon absorption (1). The study also demonstrated that spherical particles show lower visible reflectance than angular particles, which was a result of longer photon travel paths (1). These patterns imply that interpreting VNIR data from the lunar poles requires close consideration of particle geometry, rather than assuming uniform grain behavior (1).
The researchers also found that phase angle plays an unexpectedly key role in visible-light reflectance. Because water ice strongly forward-scatters light, reflectance in the visible range increases significantly at high phase angles beyond 90° (1). This finding indicates that orbits or rover viewing conditions favoring high phase angles may be optimal for ice detection. This finding could help mission planners adjust observation strategies (1).
One of the most critical discoveries centers on detection sensitivity. The NIR absorption bands at 1.25, 1.5, and 2.0 micrometers deepen rapidly even at very low ice concentrations, between 0 and 5 weight percent (1). This provides strong evidence that ice can be detected even when it comprises a small fraction of the regolith, provided the correct spectral bands are analyzed. Complementing this, the study establishes a linear relationship between visible reflectance and ice abundance from 0 to 50 weight percent, providing a potential pathway for quantifying ice rather than simply identifying its presence (1).
Overall, the findings deliver an improved scientific framework for interpreting existing and future VNIR measurements from missions such as Lunar Reconnaissance Orbiter, Lunar Trailblazer, Artemis landers, and international lunar programs (1).
