Most of the Moon’s Water Is Locked in Its Deep Interior, Not at the Poles
Featured image: Geologist-astronaut Harrison Schmitt beside a split boulder at Apollo 17’s Station 6, Taurus-Littrow; credit: NASA
For decades after Apollo, the Moon was described as bone dry, an anhydrous world where water existed, if at all, only in trace amounts. That view began to crack in 2009, when NASA’s LCROSS mission detected water ice in permanently shaded polar craters. But the real story, planetary scientists now say, is hidden much deeper.
The vast majority of the Moon’s water is not sitting as accessible ice at the poles. It is chemically bound inside minerals, as hydroxide (OH), locked deep in the lunar interior. The polar ice deposits, while potentially valuable for future crews, represent only a small fraction of the Moon’s total water budget.
“It was always a bit weird that the Apollo samples appeared to be so dry,” Neil Bowles, a professor of planetary science at the University of Oxford, told Universe Today.
From Bone Dry to Hydroxide-Rich
The Apollo program returned more than 380 kilograms (840 pounds) of lunar rock and soil, and for 40 years, every analysis pointed to the same conclusion: the Moon was essentially waterless. The first cracks in that dogma appeared in 2008-2010, when high-precision instruments detected hydroxyl (OH) in Apollo samples, locked inside a mineral called apatite.
Apatite, a calcium phosphate mineral, is the only significant hydrous mineral phase found in lunar rocks. Its crystal structure, just a few hundred microns across, traps water molecules in a form that survives the Moon’s vacuum and temperature extremes. Follow-up studies confirmed that hundreds to thousands of parts per million of water (as hydroxyl) are present in lunar apatite from multiple sample types, suggesting that water may be ubiquitous within the lunar interior.
The source of that water remains debated. The leading hypothesis holds that water was accreted during the Moon’s formation roughly 4.5 billion years ago, after a Mars-sized impactor struck Earth and the Moon coalesced from the resulting debris disk. An alternative scenario suggests water was delivered later by carbonaceous chondrite asteroids, whose isotopic fingerprint matches lunar water samples.
The Polar Ice Question
The water ice in permanently shaded polar craters (PSRs) is the aspect that excites future lunar colonists and resource-mining entrepreneurs. LCROSS confirmed its presence in 2009, and subsequent missions have mapped its distribution. But the total quantity remains uncertain, and extracting it presents serious engineering challenges.
“That would tell us how it was brought there and would preserve a record of that delivery process in the solar system,” Bowles said of polar water extraction.
A Lost Opportunity: Lunar Trailblazer
NASA’s Lunar Trailblazer mission, launched in February 2025, was designed to answer the big questions about lunar water: its form, abundance, and distribution across the landscape. The spacecraft carried two instruments, including the Lunar Thermal Mapper (LTM) provided by the University of Oxford and funded by the UK Space Agency, with Bowles serving as instrument scientist.
The two-year mission failed shortly after launch due to a human-induced misconfiguration following spacecraft separation from the launch vehicle. The spacecraft was lost before it could begin its survey.
A spare LTM instrument sits in an Oxford basement lab. Bowles hopes it will fly on a future NASA mission called UCIS, the Ultra-Compact Imaging Spectrometer for the Moon.
Why It Matters
Understanding the Moon’s water budget is not just a scientific curiosity. It has direct implications for the Artemis program and any long-term human presence on the lunar surface. Water equals drinkable water, breathable oxygen, and rocket fuel. Knowing where the water is, how much exists, and in what chemical form it is stored will determine whether future crews can live off the land or must bring everything from Earth.
The answer, for now, is complicated. The Moon does have water, but most of it is locked inside rocks hundreds of kilometers below the surface, not sitting as ice waiting to be scooped up. The polar deposits remain the most accessible target, but they represent only the tip of a much larger, much deeper reservoir.
“We need all the evidence we can get to understand how you end up with the Moon as we see it today, but also how the Moon has influenced Earth,” Bowles said.