There may be less water on the moon than we had hoped

There may be less water on the moon than we had hoped

By KR Callaway edited by Lee Billings

When Apollo 11 astronauts returned to Earth after achieving history’s first manned moon landing, they brought back nearly 50 pounds of moon dust and rocks. Scientists who first analyzed the material’s dried composition came to an important (and flawed) conclusion: The moon was bone dry.

Undeterred, in all the decades since, some scientists continued their search for lunar water, eventually finding traces of it in samples returned by other lunar missions. Hints of a potentially revolutionary breakthrough emerged in the 1990s, when a US spacecraft, Clementine, spied preliminary signs of water ice on the floors of craters called permanently shadowed regions (PSRs) around the moon’s south pole. The case for water in the moon’s PSR has grown over the years, but scientists are still struggling to figure out how much might be there. Now a new study has been published today in The progress of science suggests that the likely answer is “not much.”

By analyzing images of the moon’s darkest regions 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 by weight of the material—and that many may not have surface ice at all.

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“I think, based on what data we have now …, we’re pretty confident 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 this ice is — and thus how much future explorers can rely on it to produce drinking water, produce rocket fuel, or simply study its composition to figure out how it fits into the big picture of H₂Our origin and evolution on the Moon.

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

Whatever water ice exists in the lunar PSR was not necessarily deposited there directly by impacting asteroids and comets; rather, a process called “cold trapping” could have allowed ice to accumulate on dark, icy crater floors on the Moon via the scents of water vapor from impactors or solar wind flowing 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 pre-existing measurements of water ice abundance in Mercury’s PSRs to better calibrate their analysis of ShadowCam images of lunar PSRs.

Their result, the authors say, puts an upper limit on how much water ice exists on the surface inside the moon’s shadowiest craters. Ice signaled its presence via the scattering and reflectance of light, as seen by the ShadowCam. Because the instrument, which has a detection limit of about 20 to 30 percent ice by weight, did not pick up 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 with regard to how much ice can lurk unseen under layers of overlying thin ice material.

So the search will continue. Li and his colleagues say the natural next step is to build and use better instruments that can identify even small amounts of water ice in lunar soil. But others argue that 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 reported in this paper are wonderful in that they can provide broad regional surveys, but often what you’re looking for can only be addressed by in situ, ‘boots on the ground’ exploration activities,” says Kring. “The sooner we get robotic and human assets on the lunar surface to investigate this particular problem, the sooner we’ll have some definitive answers.”

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