Water and Hydroxyl Sources Are Abundant on the Moon, New Map Analysis Finds

A new analysis of maps of the near and far sides of the Moon shows that there are multiple sources of water and hydroxyl in sunlit rocks and soils, including water-rich rocks carved out by meteor impacts at all latitudes.

“Future astronauts could find water even near the equator by exploiting these water-rich areas. Previously, it was thought that only the polar region, and particularly the deeply shadowed craters at the poles, was a place where water could be found in abundance,” said Roger Clark, senior scientist at the Planetary Science Institute. “Knowing where water is not only helps us understand the geological history of the Moon, but also where astronauts might find water in the future.”

Clark is the lead author of the paper “Global distribution of water and hydroxyl on the Moon as seen by the Moon Mineralogy Mapper (M3)” published in the Journal of Planetary Sciences.

Clark and his research team, which includes PSI scientists Neil C. Pearson, Thomas B. McCord, Deborah L. Domingue, Amanda R. Hendrix and Georgiana Kramer, studied data from the Mineralogy Mapper (M3) imaging spectrometer on the Chandrayaan-1 spacecraft, which orbited the Moon from 2008 to 2009, mapping water and hydroxyl on the Moon’s near and far sides in greater detail than ever before.

Locating water in the sunlit parts of the Moon is done using infrared spectroscopy to look for fingerprints of water and hydroxyl (a chemical functional group with one hydrogen and one oxygen atom) in the infrared spectrum of reflected sunlight. While a digital camera records three colors in the visible part of the spectrum, the M3 instrument recorded 85 colors from the visible spectrum to the infrared.

Just as we see different colors of different materials, the infrared spectrometer can see many colors (infrared) to better determine the composition, including water (H₂O) and hydroxyl (OH). Water can be directly recovered by heating rocks and soils. Water can also be formed by chemical reactions that release hydroxyl and combine four hydroxyls to create oxygen and water (4(OH) -> 2H₂O + O₂).

By studying the location and geological context, Clark and his team were able to show that water on the lunar surface is metastable, meaning that H₂O is slowly destroyed over millions of years, but the hydroxyl, OH, remains present. A cratering event that exposes water-rich subsurface rocks to the solar wind will degrade over time, destroying H₂O and creates a diffuse aura of hydroxyl, OH, but destruction is slow, taking thousands to millions of years.

Elsewhere on the lunar surface, a patina of hydroxyl appears, likely created by solar wind protons impacting the lunar surface, destroying silicate minerals where the protons combine with oxygen in the silicates to create hydroxyl, in a process called space weathering.

“Putting all the evidence together, we see a lunar surface with complex geology with significant subsurface water and a surface layer of hydroxyl. Both cratering and volcanic activity can bring water-rich materials to the surface, and both are observed in the lunar data,” Clark said.

The Moon is made up of two main types of rock: dark, basaltic rocks (lava like the one seen in Hawaii), and lighter, anorthosite rocks (the lunar highlands). Anorthosites contain a lot of water, basalts very little. Both types of rock also contain hydroxyls bound to different minerals.

This study sheds new light on previously unknown mysteries. When the sun shines on the lunar surface at different times of the day, the absorption strength of water and hydroxyl changes. This led to the calculation that a large amount of water and hydroxyl must be moving around the moon during a daily cycle.

However, this new study showed that the very stable mineral uptakes of water and hydroxyl have the same daily effect, but on minerals, such as pyroxene, an igneous silicate mineral common in lunar soils, they do not evaporate at lunar temperatures. The reason for this effect is rather due to a thin layer of enriched composition and/or a different soil particle size deep in the soil.

When the Sun is low in the lunar sky, more light passes through the top layer, enhancing infrared absorptions, than when the Sun is high in the sky. There may still be water moving, but to quantify how much, new studies will also need to quantify layering effects. Lunar rover tracks are darker in images from Apollo-era rovers, another indicator that the surface layer is thin and different.

Related to the Moon’s thin surface layer are puzzling features called lunar vortices, diffuse patterns in visible light in several areas of the Moon. Magnetic fields are thought to play a role in the formation of the vortices by deflecting the solar wind, which would also reduce hydroxyl production.

A previous study, led by PSI senior scientist Georgiana Kramer and co-authored by Clark, showed that lunar vortices are deficient in hydroxyl. The new study confirms this, but also shows an additional complexity in that the vortices are also low in water, but are sometimes richer in pyroxene.

This new study with global hydroxyl maps also shows previously unobserved areas that are similar to known vortices, but do not exhibit the diffuse patterns seen in visible light, and thus can only be seen in hydroxyl absorption. These new features may be old eroded vortices and include new types, including arcs and linear features.

By mapping the Moon in new ways like this, we can see that the lunar surface shows itself to be more complex than we imagined.