A new analysis of scans of the deepest lunar craters has revealed significantly fewer ice deposits than previously believed.
Early estimates from the University of Hawaii at Mānoa researchers who made the initial discovery of surface-exposed water ice on the moon in 2018 projected that as much as 30% of the surface area within these permanently shaded regions (PSRs) consists of ice deposits.
The new estimates made by researchers from that same university put the upper limit closer to 20%, significantly reducing the amount of frozen H2O available to future colonists hoping to use local materials to power lunar bases, produce their water, or even make the moon their permanent home.
The Search for Hidden Ice in the Deepest Lunar Craters
Several missions have explored the deepest and darkest parts of the lunar surface, including efforts to study the enigmatic south pole, examine mysterious swirling structures on the moon’s surface, or look beneath it. However, Shuai Li, an assistant researcher at the Hawaii Institute of Geophysics and Planetology (HIGP) in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST), was the first to find direct evidence for its presence in the PSRs of lunar craters.
That discovery was possible thanks to the “ShadowCam,” an instrument aboard the Korea Aerospace Research Institute’s Korea Pathfinder Lunar Orbiter (KPLO). Launched August 4, 2022, on a one-year orbital mission, the KPLO carried a suite of instruments, including ShadowCam. An ultra-light-sensitive imager, ShadowCam, was specifically designed to use the trace amounts of indirect sunlight reflected off opposing walls within the lunar craters to measure the reflectivity of the hidden lunar surface.
Since water ice is generally more reflective than lunar rock, these readings provided an estimated water ice content ranging between 5% and 30%. Curious if those estimates could be improved with the latest techniques, Jordan Ando, a planetary sciences graduate student in Li’s laboratory, decided to take a fresh look at the ShadowCam scans.
“Ice is generally brighter, that is, reflects more light, than rocks,” Ando explained. “We analyzed high-quality images from this sensitive camera to look really closely into these permanently shaded areas and investigate whether water ice in these regions leads to widespread brightening of the surface.”
Unfortunately, those new scans showed little increase in reflectivity in the deepest lunar craters, dashing hopes that more than 30% of the hidden lunar surface could be made of water ice. Instead, Ando’s newly published analysis reduced that upper limit by a third, with the new estimate at less than 20%.
Cosmic Ray Mission Could Help Unravel the Mystery
No missions designed to directly investigate the volume of water ice deposits on the moon’s surface are on the schedule of the world’s space agencies. However, several missions to build bases, railroads, and eventually colonies on the moon are already in various planning stages.
One mission proposed by another group of UH Mānoa researchers, with HIGP and the Department of Physics and Astronomy, proposed an entirely new approach to searching the moon for ice. According to Emily S. Costello, lead author of the study outlining the team’s novel approach and postdoctoral researcher at HIGP, naturally occurring cosmic rays are uniquely equipped to look in places with minimal or no light.
“These ultra-high-energy cosmic rays strike the lunar surface and penetrate to the layers below,” Costello explained. “The rays emit radar waves that bounce off buried ice and rock layers, which we can use to infer what’s below the surface.”
Using advanced computer simulations, the research team modeled how radar waves travel through the lunar soil and how the data from those travels could reveal the presence of various materials, including water and ice.
According to Christian Tai Udovicic, a study co-author who presented the findings at the recent Lunar and Planetary Science Conference in Houston, Texas, the team’s simulations captured the interest of scientists who may have never considered the approach.
“Since it relies on high-energy physics that only a few scientists in the world are experts in, even planetary scientists who are studying ways to find lunar water ice are often surprised when they hear about this technique,” Udovicic said.
Hitching a Ride to the Moon
Efforts to move the idea from theory to application are underway, as a team of researchers from HIGP and the Physics Department is constructing a specialized radar instrument. If the instrument’s tests, planned for 2026, show it can detect the signals of water ice using cosmic rays, the team says they will “look for opportunities” to send it to the moon. With a little luck and planning, the team says their approach may “detect large deposits of buried water ice on the Moon for the first time.”
“More and more, Hawai‘i is becoming a hub for space exploration, and specifically the exploration of the Moon,” said Costello. “These projects, led by UH Mānoa scientists, represent up-and-coming opportunities for students and professionals in Hawai‘i to lead and participate in the budding space industry.”
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.