For over half a century, the scientific community has been intrigued by the detection of hydroxyl (OH) and oxygen (O) molecules across the lunar surface. Dating back to the 1960s, it has finally been unraveled by NASA scientists after a recent study involving lab tests on Apollo 17 moon dust. The lab test has provided compelling evidence that the detected OH is not a result of Earthly contamination but formed directly on the Moon when the solar wind collides with the lunar surface.
This groundbreaking discovery, published in a March 17 paper in JGR Planets, has significant implications for lunar exploration and NASA’s planned Artemis manned mission to the Moon’s South Pole. The dark side of the Moon, believed to harbor a substantial amount of frozen water due to its permanently shadowed nature, is a focal point of scientific interest. Moon may be dry but not entirely devoid of water-related chemistry, say NASA scientists after the recent findings.
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In a series of lab experiments, researchers subjected Apollo samples LS 78421 and LS 73131 to energetic hydrogen ions, mimicking the solar wind that is constantly streaming from the Sun. The formation of a distinct spectral feature at three micrometers, a clear indication of hydroxyl, was observed. And, this feature emerged without any trace of terrestrial water, confirming that the process occurs in situ, right on the Moon.
The samples exhibited different responses, contingent on their mineral structure and composition. Even a control sample of crushed silica reacted differently, underscoring the Moon’s unique chemistry and its crucial role in interacting with solar particles.
“The exciting thing here is that with only lunar soil and a basic ingredient from the Sun, which is always spitting out hydrogen, there’s a possibility of creating water. That’s incredible to think about,” says Li Hsia Yeo, the lead author of the study and a scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Solar Wind and Lunar Chemistry
Solar wind, composed largely of protons, flows incessantly from the Sun. These protons, which are nuclei of hydrogen atoms that have lost their electrons, travel at speeds exceeding one million miles per hour, virtually bathing the entire solar system. This phenomenon is even observable on Earth in the form of auroral light shows in the sky.
Unlike Earth, which has a magnetic shield and an atmosphere to deflect these solar particles, the Moon lacks such protection. Computer models have demonstrated how protons smash into the Moon’s surface, which is dusty and full of rocky material called regolith. They collide with electrons and recombine to form hydrogen atoms. These atoms then migrate on the surface to bond with the abundant oxygen atoms present in minerals like silica, forming hydroxyl (OH) molecules, a component of water, and water (H2O) molecules.
To confirm this process, Yeo and her colleagues used dust from two different samples collected on the Moon by NASA’s Apollo 17 astronauts in 1972. They first baked the samples to remove any possible water they could have picked up during storage. Then, they used a tiny particle accelerator to bombard the dust with mock solar wind for several days — the equivalent of 80,000 years on the Moon, based on the high dose of the particles used.
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The team used a spectrometer to measure how much light the dust molecules reflected, which showed how the samples’ chemical makeup changed over time. They observed a drop in the light signal that bounced to their detector precisely at the point in the infrared region of the electromagnetic spectrum — near 3 microns — where water typically absorbs energy, leaving a telltale signature.
The new discovery confirms that the Sun’s solar wind can hit the Moon’s surface and create OH, even though the Moon has no atmosphere. And this reaction happens without help from Earth’s moisture. When the Moon heats up to 400 K (about 127°C) on the sunlit side, hydrogen inside the soil starts to move around or escape, indicating that this “Moon-made OH” might not last long at high temperatures.
The experiment also reveals that different types of moon rocks create slightly different kinds of OH signatures, which helps scientists learn more about the lunar soil’s composition for future mining operations. Scientists are confident that future missions will benefit from the study by understanding how water-like molecules form and move on the Moon, which is key to planning human exploration and even mining lunar resources.