How much water is on the Moon?
Earlier this month, NASA announced the unequivocal discovery of water ice in lunar polar regions in a PNAS article. This is exciting news, and confirms what was already tantalizing evidence from LCROSS and earlier measurements. However, the article does not speculate on the total amount of water implied by these new results, so Emerging Futures decided to make an estimate from known data.
The PNAS article found up to 30% ice by mass mixed with regolith in permanently shadowed regions of the lunar north and south poles. In total, about 3.5% of these "cold traps" were found to contain ice in the first few millimeters of regolith, which was as deep as the instruments could penetrate. Let us assume that this depth is 3 mm. Furthermore, prior work has estimated the area of polar cold traps on the Moon at 50,430 km^2. Putting this all together, we came up with an estimated 1,588,500 m^3 of lunar regolith containing ice. Assuming an average regolith density of 1,660 kg/m^3, we estimate the total mass of ice at ~2.6 million tonnes.
While certainly a lot of ice—enough, for instance, to sustain 400,000 people for a year without any recycling—it is far less than some previous estimates that placed the total water resource at ~2.9 billion tonnes (e.g., Crawford 2015), more than 1,000 times larger. However, 3 mm represents a minimum depth of water; it could be far larger. Therefore, it is important to get rovers and excavators on the surface as soon as possible, to determine the average depth of water ice in these regions. If the depth were closer to 3 m, there would be as much water as implied by LCROSS and estimated by Crawford.
Analysis by SpaceVault and Emerging Futures using this larger estimate concluded that a sustainable lunar settlement of 50,000 people is possible, with plenty of water available for export as propellant. If the ice depth extended for tens of meters, the global supply would be that much greater. Making prospecting missions a reality is of paramount importance in planning economically viable and sustainable utilization of lunar water.