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When Apollo 11 astronauts returned to Earth after accomplishing history’s first-ever crewed moon landing, they brought back nearly 50 pounds of moon dust and rocks. Researchers who initially analyzed the material’s parched composition came to an important (and flawed) conclusion: the moon was bone dry.

Undeterred, in all the decades since, some scientists kept up the search for lunar water, ultimately finding traces of it in samples returned by other moon missions. Hints of a potentially revolutionary breakthrough emerged in the 1990s, when a U.S. spacecraft, Clementine, spied tentative signs of water ice at the floors of craters called permanently shadowed regions (PSRs) around the lunar south pole. The case for water in lunar PSRs has grown across the years, but scientists are still struggling to pin down just how much might be there. Now, a new study published today in Science Advances suggests the likely answer is “not much.”

Analyzing images of the moon’s darkest areas from ShadowCam, a NASA instrument on the Korea Pathfinder Lunar Orbiter, the study’s authors determined that, in most of the moon’s darkest craters, water makes up less than about 20 to 30 percent of the material by weight—and that many may have no surface ice at all.

“I think, based on what data we have now…, we are pretty sure there is ice on the surface,” says Shuai Li, lead author of the study and a planetary geologist at the University of Hawaii at Manoa. The multibillion-dollar question remains just how abundant that ice is—and thus how much future explorers might rely on it for producing potable water, manufacturing rocket fuel, or merely studying its composition to better determine how it fits into the bigger picture of H₂O’s origins and evolution on the moon.

This latter matter has scarcely influenced competing Chinese and American efforts to build a moon base but could prove crucial for efforts to learn more about water’s history throughout the entire solar system. The bulk of the moon’s water was likely delivered via asteroid and comet impacts about four billion years ago, says David Kring, leader of the Center for Lunar Science & Exploration, who was not involved in the study. So tracking that water’s abundance and distribution across the lunar surface could constrain the nature and number of the water-rich projectiles that are thought to have populated the inner solar system at that time.

Whatever water ice exists in lunar PSRs wasn’t necessarily deposited there directly by infalling asteroids and comets; rather a process called “cold trapping” could have allowed ice to accumulate on dark, frigid crater floors on the moon via whiffs of impactor- or solar-wind-derived water vapor that wafted in from elsewhere. Similar processes are at play on other celestial bodies, such as Mercury and the dwarf planet Ceres. And for their new study, the researchers used preexisting measurements of water ice abundance within Mercury’s PSRs to better calibrate their analysis of ShadowCam images of lunar PSRs.

Their result, the authors say, sets an upper limit on just how much water ice exists at the surface inside the moon’s most shadowy craters. Ice signaled its presence via the scattering and reflectance of light, as seen by ShadowCam. Because the instrument, which has a detection limit of about 20 to 30 percent ice by weight, didn’t pick up on these telltale signs in most PSRs, the research team is confident that most of these regions either lack ice or have lower concentrations of it—at least on the surface. The results are somewhat ambiguous as to how much ice may lurk unseen beneath layers of overlying ice-sparse material.

So the search will continue. Li and his colleagues say the natural next step is to build and use better instruments that could identify even minuscule amounts of water ice in lunar soil. But others argue direct exploration of the treacherously dark and cold depths of lunar PSRs will offer the best chance of solving this mystery.

“Orbital measurements like those that are reported in the current paper are fabulous in that they can provide broad regional surveys, but oftentimes what you’re looking for can only be addressed by in situ, ‘boots on the ground’ exploration activities,” Kring says. “The sooner that we get robotic and human assets on the lunar surface to investigate this particular issue, the sooner we’ll have some definitive answers.”

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