"Future astronauts may be able to find water even near the equator by exploiting these water-rich areas. Previously, it was thought that only the polar region, particularly the deeply shadowed craters, were where water could be found in abundance," explained Roger Clark, Senior Scientist at the Planetary Science Institute. He is the lead author of the study titled "The Global Distribution of Water and Hydroxyl on the Moon as Seen by the Moon Mineralogy Mapper (M3)," published in the 'Planetary Science Journal'. "Knowing where water is located not only helps to understand lunar geologic history, but also where astronauts may find water in the future," Clark added.

Clark's team, including PSI scientists Neil C. Pearson, Thomas B. McCord, Deborah L. Domingue, Amanda R. Hendrix, and Georgiana Kramer, examined data from the Moon Mineralogy Mapper (M3) spectrometer onboard the Chandrayaan-1 spacecraft, which orbited the Moon from 2008 to 2009. This data provided the most detailed mapping of water and hydroxyl on both the near and far sides of the Moon.

Using infrared spectroscopy, the researchers identified the signatures of water and hydroxyl in the reflected sunlight in the infrared spectrum. Unlike a standard digital camera, which captures three colors in the visible spectrum, the M3 instrument detected 85 different colors across both visible and infrared wavelengths. This advanced imaging allowed the team to analyze the composition of the Moon's surface, identifying water (H2O) and hydroxyl (OH) in the rocks and soils. Water can potentially be extracted by heating these materials, and hydroxyl might be converted into water through chemical reactions (4(OH) -> 2H2O + O2).

Their analysis revealed that water in the lunar surface is metastable, slowly breaking down over millions of years while hydroxyl remains. When meteor impacts expose subsurface water-rich materials to solar wind, H2O degrades, leaving a diffuse presence of hydroxyl that dissipates over time, in processes that can take thousands to millions of years. Additionally, solar wind protons contribute to the formation of hydroxyl through interactions with lunar minerals, a process called space weathering.

"By analyzing all the evidence, we see a lunar surface with complex geology, including significant amounts of subsurface water and a surface layer containing hydroxyl. Both volcanic and cratering activities have been shown to bring water-rich materials to the surface," Clark noted. The Moon's surface consists of two primary types of rocks: basaltic lava, such as that found in Hawaii, and lighter andesitic rocks. The andesites are notably water-rich, whereas the basalts have minimal water content.

This study also resolved a longstanding mystery regarding water's behavior on the lunar surface. Previously, researchers observed changes in water and hydroxyl absorption at different times of the day, suggesting that water might be moving around the Moon daily. However, the new findings indicate that this absorption effect is more likely due to a thin layer of enriched material near the surface, which alters infrared readings as sunlight shifts angles throughout the day. This thin surface layer, as evidenced by lunar rover tracks, is different from the material deeper into the soil.

The research also explores lunar swirls-enigmatic surface patterns seen in visible light. These patterns are thought to form due to magnetic fields diverting the solar wind, which reduces hydroxyl production. A prior study led by PSI Senior Scientist Georgiana Kramer, co-authored by Clark, found that lunar swirls are low in hydroxyl content. This new research confirms that swirls also have reduced water levels and occasionally higher pyroxene levels. Interestingly, global hydroxyl maps have uncovered swirl-like features previously undetected in visible light, such as arcs and linear formations, which may represent ancient, eroded swirls.

Initial data analysis for this research was conducted with support from the Moon Mineralogy Mapper science team, while the primary funding came from NASA's Solar System Exploration Research Virtual Institute's (SSERVI) Cooperative Agreement (80ARC017M0005).

Research Report:The Global Distribution of Water and Hydroxyl on the Moon as Seen by the Moon Mineralogy Mapper (M3)

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