On a cold January morning in 1848, James Marshall was busy building a lumber mill in northern California. to process timber destined for the nearby towns of Sacramento and San Francisco. But within the mill’s outflow, Marshall caught a glimpse of some sparkling pebbles. After close inspection, Marshall realized he had found gold in “them thar hills.” His chance discovery marked the opening of the famous California Gold Rush.
The California Gold Rush highlighted the abundance of a valued resource in a constrained area. Now, there is a new “gold rush” in the offing at the south pole of the Moon. It’s a race to find not gold or silver, but something much more precious to lunar visitors: water.
Water is key
The spacefaring nations of the world plan to congregate at the lunar south pole to make use of the precious natural resource. Many will come with the Artemis Accords as their guide to cooperation. Some may come for national prestige, or to “make a buck,” while others will come to add to humanity’s knowledge. But for all of them, water will be the key.
Water is handy: it can be separated into oxygen atoms for breathing and hydrogen atoms for fuel. You can even generate electricity as you put it back together. If you don’t split it in the first place, you can drink it. But under the harsh lunar vacuum, the only form in which water can last is either locked in the minerals of boulder and stone (where it’s hard to get to), or as rock-hard ice. In the heat of a lunar day, such as the Apollo astronauts endured, water doesn’t stand much of a chance.
But at the poles, it’s a different story.
In permanent shadow
Before the Space Age, water on the Moon seemed improbable. But as early as 1961, some researchers speculated that sheltered craters at the lunar poles could form cold traps. At temperatures of –175º C (–285º F), water vapor from comet and asteroid impacts would gather and remain in these so-called Permanently Shadowed Regions, or PSRs. While many moons have deep craters and chasms, sunlight makes its way to the floors of most, destroying any volatiles that might be stashed there. Earth’s Moon is an exception: although Earth’s spin tilts by some 23.5º, the Moon’s axis is nearly vertical (1.5º), which means that at the lunar poles, the Sun circles near to the horizon. This keeps the deepest crater floors in permanent shadow, just the kind of places that those 1961 researchers envisioned.
Traces of water ice in the vacuum of space came in 1991, not from the Moon, but rather from Mercury. Using the massive 70-meter antenna at Goldstone, a JPL/Caltech team beamed radar toward the planet’s surface. The radar bouncing back indicated ice at the poles of the tiny world, which had several deep craters whose floors were permanently shadowed.
A succession of more detailed measurements from lunar orbit found signs of water in significant amounts at both poles in the PSRs. But did these hollows conceal water ice in their gloom? More answers to the big water questions began to arrive at the hands of the Lunar Reconnaissance Orbiter (LRO). NASA’s sophisticated Moon mapper found hydrogen—a telltale sign of hidden water ice—leaking from many of the crater floors and rims. LRO was sensitive enough to see the shadowed regions of craters using the light from stars and ultraviolet skyglow. Near the south pole, the instrument detected water frost. Finally, the LCROSS mission fired a projectile into the rim of Cabeus Crater, finding water as it flew directly through the plume of debris.
Supply and demand
One consideration facing Artemis Moon mission planners is how much water the astronauts will need versus how much is available. To replenish life support systems, drilling for water may be adequate. But supporting a lunar base of operations with a dozen inhabitants may require what amounts to an extensive mining operation. The best solution is to find rich, concentrated water deposits, and they may well lie at the floors of some smaller craters in the region.
Aside from water in those PSRs, the southern “wilderness” offers another advantage: extended periods of solar illumination for power. Because of the Moon’s subdued tilt, the Sun remains visible at the poles almost year-round. 19th-century astronomers Wilhelm Beer, Johann von Madler, and Camille Flammarion proposed that some of the Moon’s high ground constituted “Peaks of Eternal Light.”
Because of the likely presence of ice and the high percentage of sunlit periods for solar power, the Shackleton area — including the Malapert Crater and massif — has become a finalist in the search for the next crewed lunar landings. The neighboring South Pole-Aitken basin and the environs near Shackleton Crater may yield the greatest scientific return from any lunar missions to date.
Shackleton Crater itself dives three times as far below the surrounding ground as does the Grand Canyon of the western U.S. Its interior may be too treacherous to explore, says lunar geologist Clive Neal. “Going for the big PSRs is not where you go first,” he says. “You go with smaller ones that we know exist. They’re not going to be like Shackleton Crater where you may have a good water potential but it’s unobtainium, because you have a [2.8-mile] 4½-kilometer downward trek at a 30º slope at [temperatures of] 40 to 60 K.”
Explorers may need to find their water in shallower, more accessible craters.
Water everywhere
The Moon hides water at other sites as well. In Luna’s midlatitudes, where Apollo astronauts explored, water comes in the form of a thin film of molecules, an interconnected microscopic web, coating the tiny grains of regolith. The solar wind interacts with oxides in the regolith to create water.
Chinese investigators studying lunar samples returned by their Chang’e 5 lander concluded that millions of tons of water may be spread across the Moon’s landscape within glass beads. The microscopic beads likely formed through the process of impacts, and offer another possible form of water storage on the Moon. Every 1.3 cubic yards (1 cubic meter) in the water-rich regolith of Cabeus Crater contains about 8 ounces (240 milliliters) of water. But water in the Chang’e, Apollo, and other samples is spread thin: to collect the same 8 ounces of water found at Cabeus would require anywhere from 650 to 6,500 times the amount of regolith at more northern sites.
The scientific bonanza of the lunar south pole is worth the practical challenges of working in those extreme conditions, but it will take water to pull off the extensive long-term exploration of Earth’s nearest neighbor.
