When you look at the Moon, you don’t see any water on its surface. That doesn’t mean there isn’t any. In fact, there’s a lot of “wetness” on the Moon, but it’s in places and forms we can’t see. Understanding where all those resources are is the subject of a study based on NASA’s Moon Mineralogy Mapper (M3) data taken from aboard the Chandrayaan-1 spacecraft.
The analysis performed by a team led by Planetary Science Institute senior scientist Roger Clark shows that there are many sources of water and a group of chemicals called “hydroxyls” (OH). Water lies hidden in ice deposits in shaded areas, and inside enriched rocks.
Image showing the distribution of surface ice (which could supply water) at the Moon’s south pole (left) and north pole (right), detected by NASA’s Moon Mineralogy Mapper instrument. Credits: NASA
Hydroxyls are interesting. They form as solar protons interact with electrons on the Moon’s surface. That creates hydrogen atoms which hook up with oxygen atoms found in silicates and other oxygen-bearing molecules in the lunar regolith. Together, the hydrogen and oxygen make hydroxyl molecules, which are a component of water. While it would take some work, mining those “raw materials” for water on the Moon could be a huge boost for future crewed missions, according to Clark.
“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, and in particular, the deeply shadowed craters at the poles were where water could be found in abundance,” said Clark. “Knowing where water is located not only helps to understand lunar geologic history but also where astronauts may find water in the future.”
Searching out sources of lunar water requires special instruments. This is where the Chandrayaan mission and NASA’s mineralogy mapper data came in handy. Clark and his team zeroed in on a set of data taken by the lander’s imaging spectrometer from 2008-2009. This infrared spectroscopy data contains the spectral fingerprints of both water and hydroxyl in sunlight reflected from the Moon’s surface. The M3 instrument dissected the light into 85 different visible and infrared “colors”. That’s how they were able to spot the distinctive hints of water and hydroxyls across much of the Moon.
The team also looked at the location and geologic contexts of water and hydroxyl distribution. They also had to take into account the “lifetime” of these resources on the Moon. Interestingly, water gets slowly destroyed over time. Hydroxyl, however, lasts much longer. So, for example, if a crater smacks into the lunar surface, the “wet” rocks it “digs up” will lose that content over time through the action of the solar wind. The result is a diffuse layer or “aura” of hydroxyls that remain behind. In other places, solar wind protons that collide with the surface contribute to a thin layer or “patina” of hydroxyls on the surface. The hydroxyls last much longer and exist on the Moon up to millions of years.
“Putting all the evidence together, we see a lunar surface with complex geology with significant water in the sub-surface and a surface layer of hydroxyl. Both cratering and volcanic activity bring water-rich materials to the surface, and both are observed in the lunar data,” Clark said.
Near-infrared image of the Moon’s surface by NASA’s Moon Mineralogy Mapper on the Indian Space Research Organization’s Chandrayaan-1 mission. The mapper helped identify water- and hydroxyl-rich areas on the lunar surface.
Image credit: ISRO/NASA/JPL-Caltech/Brown Univ./USGSLunar rocks may well help supply water to future visitors to the Moon. There are two kinds of rocks there. The dark mare rocks are mainly basaltic (like Hawaiian lava). The other type is the anorthosite rock. It exists in various places, including the lunar highlands. The anorthosites are relatively “wet” while the basalts remain very dry. The two rock types also contain hydroxyls bonded to different minerals.
The water-rich anorthosites should be a target for harvesting by lunar astronauts. To get a good supply, you have to heat the rocks and soils. The result of that process could be a long-lasting water supply. You could also get it by using methods to create chemical reactions that liberate hydroxyl and combine four hydroxyls to create oxygen and water.
Of course, a more immediate source lies at the poles. That’s where ice lies hidden inside shaded crater walls or under the surface, preserved for millions of years. That source is likely more easily harvested, but you still have to transport the water to other lunar regions. The downsides of getting water from rocks are the expense and the energy required to heat them for extraction. NASA and other agencies (such as the Chinese space agency) are looking at all the methods of producing supplies for upcoming missions. Studying the locations of ice deposits and hydroxyls is just one part of a larger “search for water” that will benefit future lunar bases.
Sources of Water and Hydroxyl are Widespread on the Moon
The Global Distribution of Water and Hydroxyl on the Moon as Seen by the Moon Mineralogy Mapper (M3)
