Astronomers map out the Moon's best watering holes

The Moon seems like a pretty barren place, but studies in 2009 revealed traces of water in the lunar soil. Now, scientists from Brown University have used data gathered from NASA’s Moon Mineralogy Mapper instrument to construct a detailed map of where water can be found on the Moon – and it turns out it’s more widespread than previously thought.

In 2009, NASA announced that water molecules had been detected in the regions around the lunar poles. That moisture is mostly thought to have been left on the surface by solar winds, where protons from the Sun creates hydroxyl (a molecule related to water made of one hydrogen and one oxygen atom), but other studies have speculated that it could have been deposited by asteroid impacts. Wherever it came from, the Brown University team created a map using data from the Moon Mineralogy Mapper, to show that water can be found across the entire Moon.

“The signature of water is present nearly everywhere on the lunar surface, not limited to the polar regions as previously reported,” says Shuai Li, lead author of the study. “The amount of water increases toward the poles and does not show significant difference among distinct compositional terrains.”

The newly-minted map reveals a few things about the Moon’s moisture. The water is fairly neatly distributed, with the majority lying at the poles and less found around the equator, which is consistent with the solar wind origin story. But there’s an exception: lunar volcanic deposits near the equator tend to hold a higher than average amount of water, which suggests that this moisture is bubbling up from the Moon’s mantle. That idea was discussed in a paper a few months ago, authored by some of the researchers on this new study.

At its peak concentration around the poles, water can be found at around 500 to 750 parts per million – by no means does that make the Moon a lush oasis, but it is wetter than we’d previously given it credit for. Intriguingly, close to the equator water levels can fluctuate wildly on a daily basis, with the Moon having some 200 parts per million more water overnight than it does at noon.

“We don’t know exactly what the mechanism is for this fluctuation, but it tells us that the process of water formation in the lunar soil is active and happening today,” says Ralph Milliken, co-author of the study. “This raises the possibility that water may re-accumulate after extraction, but we need to better understand the physics of why and how this happens to understand the timescale over which water may be renewed.”

There are a few limitations to the data: the Moon Mineralogy Mapper can only take readings of the first few millimeters of soil, so there could be even more water deeper under the surface. The instrument can also only operate on the daylight sections of the Moon, so water or ice levels in areas in permanent darkness can’t be measured using this technique.

The map could prove a useful resource for future lunar explorers, with places showing the highest concentrations of water being the best targets for missions, if we can develop ways to efficiently extract the stuff.

“This is a roadmap to where water exists on the surface of the Moon,” says Milliken. “Now that we have these quantitative maps showing where the water is and in what amounts, we can start thinking about whether or not it could be worthwhile to extract, either as drinking water for astronauts or to produce fuel.”

The research was published in the journal Science Advances.

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