Lunar water is water that exists on the Moon in several forms: as ice buried in permanently shadowed craters, as individual water molecules (H₂O) scattered across the surface, and as hydroxyl (OH) groups chemically bound to minerals in the soil. It’s not liquid water. The Moon has no atmosphere to speak of, so any exposed liquid would instantly evaporate. Instead, lunar water persists as ice in ultra-cold regions or as molecules trapped within the rocky, dustite surface layer called regolith.
The Three Forms of Lunar Water
Water on the Moon doesn’t look like water on Earth. It takes three distinct forms depending on where it is and how it’s stored.
The most familiar form is water ice, found in craters near the north and south poles where sunlight never reaches the floor. These permanently shadowed regions stay cold enough to keep ice stable for billions of years. The LCROSS mission confirmed this directly in 2009 when it crashed a rocket stage into Cabeus crater near the south pole and detected about 5.6% water by mass in the debris plume.
The second form is molecular water, individual H₂O molecules sitting on or just below the surface. In 2020, NASA’s SOFIA airborne telescope confirmed water on a sunlit portion of the Moon at concentrations of 100 to 412 parts per million. That’s tiny. For perspective, the Sahara Desert contains 100 times more water than what SOFIA found in the lunar soil. Still, it proved that water molecules can survive even in areas exposed to direct sunlight.
The third form is hydroxyl (OH), a molecule that’s one hydrogen atom short of water. Hydroxyl groups bond chemically to minerals in the regolith and are harder to release, requiring temperatures of 180°C to over 300°C. Multiple orbital instruments have detected hydroxyl signatures across all lunar latitudes, not just at the poles.
Where the Water Is Concentrated
The poles hold the most water by far. Neutron detectors in orbit have measured reduced radiation coming from polar regions above 70° latitude in both hemispheres, a signal that hydrogen (and therefore likely water ice) is concentrated there. Data from the Lunar Prospector mission estimated that the upper meter of soil near both poles contains more than 430 million tons of ice in total.
Not every shadowed crater holds significant ice, though. Of the many permanently shadowed regions studied, only three large ones showed clear signs of substantial hydrogen deposits: Shoemaker and Cabeus craters in the south and Rozhdestvensky U crater in the north. The rest appear no more hydrogen-rich than the sunlit areas around them at the same latitude. This uneven distribution means the ice isn’t simply everywhere it’s cold enough to survive. Delivery and retention processes matter, too.
Away from the poles, water and hydroxyl exist in much smaller quantities. Orbital measurements show these molecules vary throughout the lunar day, with concentrations shifting between sunrise, noon, and midnight. The water appears to migrate across the surface, potentially cycling toward the poles over geological time.
How Water Gets to the Moon
The Moon has no rain, no rivers, no water cycle. So where does the water come from? Scientists point to two main sources.
The leading explanation is the solar wind. The Sun constantly streams charged particles, including protons, toward the Moon. When these protons slam into the regolith, they pick up electrons and become hydrogen atoms. Those hydrogen atoms then bond with oxygen already present in surface minerals like silica, forming hydroxyl and water molecules. This process has likely been happening for billions of years, slowly building up a thin layer of hydration across the entire lunar surface.
The second source is comets and micrometeorites. In 2016, researchers found that water is released from the Moon during meteor showers. When a piece of comet debris hits the surface, the impact generates enough heat and shock to breach the dry upper layer of soil and vaporize water from a hydrated layer underneath. Comets themselves are largely ice, so direct comet impacts over the Moon’s history could have delivered water that migrated to cold traps at the poles and stayed there.
Micrometeorite impacts may also contribute by generating the heat needed to trigger chemical reactions that produce water from hydrogen and oxygen already in the regolith. Most researchers think the solar wind is the dominant source for the thin, widespread surface hydration, while comet delivery likely accounts for a larger share of the concentrated polar ice.
Why Temperatures Matter So Much
Water ice is only stable on the Moon in places that stay extremely cold. Without an atmosphere to hold in heat or provide pressure, ice exposed to warmth simply turns to vapor and escapes into space.
Loosely bound surface water starts to escape at around 150 K (roughly -123°C). Chemically bound water holds on longer, requiring 180 to 400 K before it releases. Hydroxyl that forms through chemical reactions with minerals may need temperatures of 500 to 600 K to break free. Water locked deep inside mineral grains requires partial or complete melting of rock, at 700 to 1,100 K.
Permanently shadowed craters at the poles can dip below 40 K (-233°C), cold enough to trap ice indefinitely. Even at mid-latitudes, temperatures below the surface (deeper than about 10 centimeters) hold steady around 200 K, which is cold enough that chemically bound water could persist. This means there may be trace amounts of bound water hidden just below the surface across much of the Moon, even in regions that seem bone-dry on the surface.
Why Lunar Water Matters for Exploration
Lunar water is one of the most strategically important resources for future space exploration, and the reason comes down to weight. Launching water from Earth costs thousands of dollars per kilogram. If astronauts can extract water from lunar soil instead, it transforms what’s possible on the Moon and beyond.
The most straightforward use is life support. Water extracted from regolith could provide drinking water and be used to grow food in controlled environments. Heating lunar soil under vacuum conditions can release trapped water molecules and other volatile compounds, which can then be collected and purified.
The higher-value application is rocket fuel. Water splits into hydrogen and oxygen gas through electrolysis, where an electrical current breaks H₂O into its components. Liquid hydrogen and liquid oxygen are already the standard propellant combination for many rocket engines. Producing fuel on the Moon instead of hauling it from Earth would make lunar launches far cheaper and could turn the Moon into a refueling station for missions heading to Mars or deeper into the solar system.
NASA and other agencies are actively developing the technology to make this work. Engineers at Johnson Space Center have been building and testing systems that extract oxygen from icy regolith mixtures, with laboratory demonstrations planned using both low-temperature and high-temperature electrolysis methods. The high-temperature approach is particularly promising for lunar use because it doesn’t require the water to be ultra-pure before processing, a practical advantage when your raw material is dusty lunar soil.
The estimated 430 million tons of ice near the poles represents an enormous potential supply, but how much of it is actually accessible remains an open question. Ice mixed into soil at low concentrations is far harder to extract than solid ice deposits. Pinpointing where the densest, most accessible reserves are located is a central goal of upcoming lunar missions, including NASA’s VIPER rover and several international landers targeting the south pole.