Discovering Hidden Water Reservoirs on Moon

By analyzing lunar rocks, researchers uncovered that most of the moon’s water is either native or comes from comet impacts, debunking the idea that solar winds played a major role. Their research, featured in a special issue of Proceedings of the National Academy of Sciences (PNAS) titled “Water on the Moon and Mars,” could serve as a roadmap for lunar exploration. The cover of this issue even highlights Artemis I, the first step in NASA’s ambitious lunar initiative.

As humanity looks toward a future beyond Earth — on the Moon, Mars, and beyond — one of the biggest challenges is securing the essentials for survival: oxygen, food, and water. Scientists know water exists on the Moon, but pinpointing its exact location is crucial. Is it hidden in craters, trapped in permanently shadowed regions, or concentrated at the poles? Identifying these water sources is key to supporting long-term lunar habitation — something that, until now, has belonged to the realm of science fiction.

Researchers from the University of California San Diego are working to change that by helping guide future space missions, including NASA’s Artemis program, which aims to explore and eventually establish a human presence on the Moon.

Among the scientists contributing to this research is the father-son team of Mark Thiemens, a Distinguished Professor of Chemistry and Biochemistry at UC San Diego, and Maxwell Thiemens, a research fellow at Vrije Universiteit Brussel and an alumnus of Scripps Institution of Oceanography. Their work brings us closer to understanding the Moon’s water sources, a critical step in turning lunar exploration into a long-term reality.

In 1967, Nobel laureate Harold Urey and James Arnold — both faculty members in UC San Diego’s Department of Chemistry — were among the first to receive Apollo 11 lunar samples. Urey was one of the first scientists to theorize that there was water on the moon, particularly in the permanently shadowed regions of the moon’s poles.

On Earth, human civilizations often bubble up near bodies of water and it would be no different in space. On the moon, it’s important to know the origin of the water sources because it will give astronauts guidance on where it would be most prudent to set up bases and habitats.

To learn about the origin of water on the moon, Morgan Nunn Martinez (who was a UC San Diego graduate student at the time) extracted very small amounts from lunar rocks collected from the 1969 Apollo 9 mission. It may sound implausible to get water from a rock, but it is possible through “thermal release,” a process where lunar samples were heated to 50, 150, and 1,000 degrees Celsius (122, 302, and 1,832 degrees Fahrenheit respectively). As it turns out, these rocks were surprisingly “wet.”

The lowest temperatures released lightly bound water molecules — those molecules that are attached to other molecules (in this case, lunar rock) through a weak attraction. At 1,000 degrees Celsius, tightly bound water molecules, which are more deeply embedded in the rock, were released.

Through this process, gas water molecules are collected, then purified so that only the oxygen remains. The team then measured the composition of three different oxygen isotopes.

Isotopes are atoms of the same element that have varying numbers of neutrons, which changes their mass — the more neutrons, the heavier the atom. These measurements are particularly useful in determining a substance’s origin and age.

Think of it like space forensics. In the way humans have unique fingerprints, astronomical objects, like comets and the sun, have unique signatures. Scientists are able to look at the oxygen isotope measurements and determine the origin of the water.

Their data revealed that most of the lunar water likely originated from the moon itself or from comet impacts. Contrary to popular belief, solar winds did not significantly contribute to the moon’s water stores.

“What’s nice about this research is that we’re using the most advanced scientific measurements and it supports common sense ideas about lunar water — much of it has been there since the beginning and more was added by these icy comet impacts,” stated Maxwell Thiemens. “The more complicated method of solar wind-derived water doesn’t appear to have been that productive.”

Although not a main thrust of the paper, the researchers also measured samples from Mars. If NASA’s Artemis program is able to successfully colonize humans on the moon, it would bode well for the ultimate mission of inhabiting Mars.

“This kind of work hasn’t been done before and we think it can provide NASA with some valuable clues about where water is located on the moon,” stated Mark Thiemens. “The real goal of Artemis is to get to Mars. Our research shows that likely there is at least as much water on Mars as on the moon, if not more.”

Of course, locating the water is only the first step. Being able to extract it from lunar rocks and soil in quantities large enough to sustain life will require further technological advancements and discovery.

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