New study reveals the origin and location of water on the Moon

The search for water on the Moon has captivated scientists for decades. Beyond its implications for understanding planetary science, lunar water could play a pivotal role in humanity’s ambitions to establish off-world colonies.

By examining the origins of water within the Earth-Moon system, researchers aim to uncover how and where it exists, offering critical guidance for future lunar missions and habitats.

Understanding the source of lunar water has posed a significant challenge. Early Apollo missions suggested the Moon was entirely dry, as the methods used to analyze the samples were unable to distinguish between terrestrial contamination and native lunar water. However, recent advancements, including high-precision isotopic analysis, have painted a far more nuanced picture.

Researchers now believe lunar water may originate from three primary sources: it could be native to the Moon, formed by solar wind interactions with lunar minerals, or deposited by icy comets that collided with its surface.

To determine these origins, scientists have turned to oxygen isotopic analysis. Isotopes of oxygen, which vary based on their neutron count, serve as chemical fingerprints that can trace a sample’s origin.

One groundbreaking study analyzed the triple oxygen isotopic composition of water in nine Apollo samples, which included basalts, breccias, and regolith.

Using a stepwise heating technique, scientists released water bound in these samples at progressively higher temperatures—loosely bound water at 50°C, moderately bound water at 150°C, and tightly bound water at 1,000°C. These steps allowed researchers to isolate different water types and determine their unique isotopic signatures.

The isotopic data from lunar samples revealed that much of the Moon’s water likely originated from two main sources: indigenous water from the Moon’s formation and cometary impacts.

These findings suggest that water formed in the lunar mantle during the Moon’s early history and was later supplemented by icy comets. These comets, such as Jupiter-family comets or those from the Kuiper Belt, are known to contain water with hydrogen isotopic compositions similar to terrestrial water.

Interestingly, researchers also measured oxygen isotopes from comets and compared them to lunar samples. Their data indicated that cometary impacts likely played a critical role in delivering water to the Moon, contributing to its overall water inventory.

The isotopic composition of this water often overlapped with enstatite chondrites, a type of meteorite believed to be the primary source of Earth’s water. This connection hints at a shared history of water delivery within the Earth-Moon system.

Maxwell Thiemens, a researcher involved in the study, noted, “The more complicated method of solar wind-derived water doesn’t appear to have been that productive. Much of the water has been there since the beginning, and more was added by these icy comet impacts.”

For years, scientists hypothesized that solar wind interactions with the Moon’s surface might produce significant amounts of water. Solar wind consists of high-energy hydrogen ions that can react with oxygen in lunar minerals to form water. This mechanism has been suggested as a potential source of diurnal water variability observed on the Moon’s surface.

However, recent isotopic analyses challenge this assumption. The study revealed that solar wind-derived water likely represents a minor contribution to the Moon’s overall water inventory. The distinct isotopic signatures of solar wind water, characterized by low deuterium-to-hydrogen (D/H) ratios, were not prominent in the analyzed samples. Instead, most water isotopes pointed to indigenous and cometary origins.

Mark Thiemens explained, “We’re using the most advanced scientific measurements, and it supports common sense ideas about lunar water—most of it has been there since the beginning.”

The implications of these findings extend far beyond scientific curiosity. For future lunar missions, knowing the origins and distribution of water is critical for determining where to establish sustainable habitats.

Permanently shadowed regions near the Moon’s poles, such as the Cabeus and Clavius craters, are believed to hold substantial water ice deposits. Recent estimates suggest these regions could contain up to 100 million metric tons of water ice, far exceeding earlier predictions.

The Artemis program, NASA’s ambitious campaign to return humans to the Moon, aims to leverage these water reserves to support life and fuel production. By extracting water from lunar soil and rock, astronauts could generate oxygen and hydrogen, essential components for breathing and rocket fuel.

“This kind of work hasn’t been done before, and we think it can provide NASA with valuable clues about where water is located on the Moon,” Thiemens added. “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.”

Lunar water research also offers insights into the broader history of water in the solar system. By analyzing isotopic compositions, scientists can trace how water was distributed among planets, comets, and other celestial bodies during the solar system’s early history.

For example, studies of comet 103P/Hartley 2 revealed that its water’s hydrogen isotopic composition closely matches that of Earth, suggesting comets may have been a significant source of terrestrial water.

Additionally, the study’s stepwise heating technique has potential applications for analyzing water on Mars. Preliminary measurements of Martian samples indicate the presence of significant water reservoirs, possibly even exceeding those on the Moon. If Artemis succeeds in establishing a lunar base, the knowledge gained from lunar water research could pave the way for human exploration of Mars.

Despite these advances, many questions remain unanswered. While scientists have identified potential sources of lunar water, extracting it in usable quantities poses a significant challenge.

Existing methods rely on heating lunar rocks to extremely high temperatures, which may not be practical on a large scale. Developing more efficient techniques for water extraction will be crucial for future lunar missions.

Moreover, understanding the long-term stability of lunar water in exposed regions remains an open question. The diurnal variability of water in sunlit areas suggests that surface water may be transient, evaporating during the day and redepositing at night. Future missions equipped with advanced spectroscopic instruments will be essential for monitoring these dynamic processes.

The ongoing study of lunar water exemplifies how scientific inquiry can directly inform space exploration. As humanity prepares to venture beyond Earth, the Moon serves as both a testing ground and a gateway to the wider solar system.

By unraveling the mysteries of lunar water, researchers are laying the groundwork for a future where humans can thrive off-planet.

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