Lunar Water Reservoir Discovered in Glass Impact Beads

No humans have set foot on the Moon for more than 50 years, but the coming decade could see multiple crewed landings and the construction of an orbiting lunar station. Harvesting water from the Moon could make a permanent human presence easier to maintain, but where will we find it? A new study from the Chinese Chang’e rover team suggests there might be usable water everywhere on the Moon.

In the past, scientists believed the Moon was just a big ball of dust, but robotic missions eventually confirmed water ice exists in shadowy craters. Water also appears to evaporate from lunar soil when exposed to sunlight. New data from the Chang’e 5 mission reveals the details.

Chang’e 5, which landed on the Moon in 2020, was China's first lunar sample return mission. It sent back 3.7 pounds (1.7 kilograms) of lunar regolith, some collected from a meter below the surface. Throughout the samples, the researchers from the Chinese Academy of Sciences spotted glass beads resulting from meteorite impacts. The researchers analyzed the chemical composition and structure of the impact glass, finding they can be up to 0.2% water by mass. In absolute terms, the researchers calculate that lunar soil has between 300 billion and 270 trillion kilograms of water locked up in glass beads. If collected in one place, it could fill Lake Tahoe almost twice over at the high end of the estimate.

Credit: Prof. HU Sen/Chinese Academy of Sciences

But how does it get there? The lunar water cycle is nothing like Earth's, with water falling from the sky. Although, it's more similar than you think. The solar wind rains down on the Moon, bringing a shower of protons (hydrogen nuclei). These atoms can interact with lunar minerals to form water molecules. The impact glass acts like a sponge that absorbs traces of water from the solar wind, retaining it like a distributed reservoir. Thus, some water evaporates daily as the solar wind continuously replenishes the reservoir. The Moon has seen its fair share of impacts over the eons, so there's enough glass to keep the water cycle going.

The impact beads are tiny but relatively easy to separate from the rest of the lunar soil. Future missions could harvest water from this reservoir, meaning less mass to launch from Earth. It's also possible this mechanism is not exclusive to the Moon. Perhaps we'll find water locked up in impact beads on other worlds, just waiting to be harvested.