Lunar water

Did You Know?

• Between 4 and 3.5 billion years ago, the Moon may have had sufficient atmosphere and liquid water on its surface.
• Isotope analysis of lunar samples suggests some lunar water originated from Earth, possibly from the Giant Impact event.
• Warm, pressurized regions in the Moon's interior might still contain liquid water, with subsurface ice layers potentially blocking diffusion and allowing subterranean lakes.

History of observations

Leonardo da Vinci, in his Codex Leicester, proposed that the Moon's luminosity came from sunlight reflecting off water on its surface, with waves scattering the light. In the 1830s, Wilhelm Beer and Johann Heinrich Mädler concluded from their Mappa Selenographica and Der Mond that the Moon lacks surface water and any appreciable atmosphere. The possibility of ice in permanently shadowed polar craters was first raised in 1961 by Caltech researchers Kenneth Watson, Bruce C. Murray, and Harrison Brown.

Earth-based radar later identified shadowed areas poleward of 87.5 degrees latitude, covering about 1030 km² at the north pole and 2550 km² at the south pole, with computer simulations suggesting up to 14,000 km² could be permanently shadowed. Trace water in Apollo lunar rock samples was initially dismissed as contamination, but a 2008 study of volcanic glass beads revealed trapped water molecules. The first direct detection of water vapor near the Moon came from the Apollo 14 ALSEP Suprathermal Ion Detector Experiment on March 7, 1971, which observed bursts of water vapor ions at the landing site.

Possible water cycle

Lunar water composition is not fully understood and is inferred remotely. The surface, shaped by meteoritic impacts, likely contains minerals such as epsomite, blödite, gypsum/bassanite, and jarosite that could harbor hydroxyls. Water may exist as a brine—water with dissolved salts and other volatiles—potentially forming from minerals delivered by carbonaceous chondrites and CI/CM chondrites. Micrometeorites and interplanetary dust particles add volatiles like H2O, CO, and possibly CO2 at a rate of about one impact per square meter per day.

The LCROSS mission confirmed volatiles including water, hydrogen, carbon monoxide, hydrogen sulfide, ammonia, sulfur dioxide, ethylene, carbon dioxide, methanol, mercury, and methane. Confirmed hydroxyl-bearing materials include glasses, apatite, and novograblenovite. Analysis of the Chang'e 5 return sample identified an unknown mineral, ULM-1, from a sunlit area, in which water makes up over 40% of the sample by mass. Water may originate from Earth, from comets and other bodies striking the Moon, or from in situ production where solar wind hydrogen ions combine with oxygen in lunar minerals to form water or hydroxyl groups trapped in crystal lattices.

Surface hydroxyl groups (X–OH) can further convert into water molecules adsorbed on mineral surfaces, as represented by reactions such as 2 X–OH → X=O + X + H2O or 2 X–OH → X–O–X + H2O.

Uses

Large quantities of lunar water would make habitation more cost-effective by avoiding the prohibitive expense of transporting water from Earth. Water ice could be mined for drinking and plant growth, and split into hydrogen and oxygen using solar or nuclear power to provide breathable air and rocket fuel. Hydrogen from the ice could also extract oxygen from lunar soil oxides. Analysis of lunar ice would yield scientific data on the Moon's impact history and the abundance of comets and asteroids in the early Inner Solar System.

Ownership

The discovery of usable lunar water raises legal questions about ownership and exploitation rights. The United Nations Outer Space Treaty does not prevent resource exploitation but bars nations from claiming the Moon itself, and is generally interpreted as prohibiting national ownership of lunar resources; most legal experts believe the issue will be settled through precedents of national or private activity. The Moon Treaty, which requires an international regime for resource exploitation, has been ratified only by a few nations without independent spaceflight capabilities. Luxembourg and the United States have granted their citizens the right to mine and own space resources, including those of the Moon, as affirmed by a US executive order on April 6, 2020.