There might be less water on the moon than we’d hoped
New satellite data come up dry as the search for lunar ice continues
NASA’s ShadowCam photographed some of the moon’s darkest regions, including the permanently shaded regions at the bottom of craters.
JAXA/NHK/ZUMAPRESS.com/Alamy
When Apollo 11 astronauts made their historic return to Earth, they brought back nearly 50 pounds of lunar dust and rocks. Initial examinations of these materials led researchers to the flawed belief that the moon was completely dry.
Despite this, the quest for lunar water continued. Over the years, scientists found evidence of water in samples from other moon missions. A potential breakthrough came in the 1990s when the U.S. spacecraft Clementine detected possible signs of water ice in permanently shadowed regions (PSRs) near the lunar south pole. Although the evidence for water in these regions has increased, the exact amount remains uncertain. A recent study, published in Science Advances, indicates that the likely amount may be limited.
Using images from ShadowCam, a NASA instrument on the Korea Pathfinder Lunar Orbiter, researchers determined that water constitutes less than 20 to 30 percent of the material by weight in most of the moon’s darkest craters, with many potentially lacking surface ice entirely.
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“I think, based on what data we have now…, we are pretty sure there is ice on the surface,” notes Shuai Li, the study’s lead author and a planetary geologist at the University of Hawaii at Manoa. The crucial question is how abundant this ice is and whether future explorers can depend on it for drinking water, rocket fuel production, or research into its origins and evolution on the moon.
This issue has not significantly impacted Chinese and American plans to establish a lunar base but could be key in understanding water’s history throughout the solar system. According to David Kring, leader of the Center for Lunar Science & Exploration, who did not participate in the study, most of the moon’s water likely came from asteroid and comet impacts around four billion years ago. Mapping the water’s presence across the lunar surface could reveal the characteristics and frequency of these water-rich bodies that once existed in the solar system.
The water ice in lunar PSRs might not have been deposited directly by impacting asteroids and comets. Instead, a process known as “cold trapping” could have allowed ice to form on cold, shadowy crater floors from vapor that drifted in from elsewhere, either from impacts or solar wind. Similar processes occur on other celestial bodies like Mercury and the dwarf planet Ceres. In their study, researchers used existing data on water ice abundance in Mercury’s PSRs to refine their analysis of ShadowCam images of lunar PSRs.
The study’s results set an upper limit on the surface water ice within the moon’s shadowy craters. Ice presence was indicated by light scattering and reflection detected by ShadowCam. Given the instrument’s detection threshold of about 20 to 30 percent ice by weight, the research team is confident that most PSRs either lack ice or have minimal surface concentrations. However, the study leaves some uncertainty about the amount of ice that might exist below the surface.
The search for lunar ice will continue. Li and his team plan to develop better instruments to detect even tiny amounts of water ice in lunar soil. Yet, some experts argue that direct exploration of the moon’s dark and cold PSRs might be the best way to solve the mystery.
“Orbital measurements like those in the current paper are excellent for broad surveys, but often what you’re looking for requires in situ, ‘boots on the ground’ exploration,” explains Kring. “The sooner we deploy robotic and human resources on the lunar surface to address this issue, the sooner we’ll have definitive answers.”
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