The Moon is a rock in space without an atmosphere, and yet, it continues to fascinate us. So much so that we’re planning to send humans back to the Moon with the Artemis mission, and as we’ve discussed with recent Mars research, one area of interest is whether or not there is accessible water on the Moon.

Accessibility is the key.

When I was growing up, we thought the Moon was basically just that rock, floating in space, not much else to offer. Interesting, but we’d been there, and it didn’t really provide much in the way of resources we could use here. Then in 2008, the Chandrayaan-1 orbiter and its Moon Mineralogical Mapper instrument found water ice inside the polar craters of the Moon. The orbiter also had an impact probe that detected water molecules in the exosphere. That was kind of big news.

A year later, NASA’s Lunar Reconnaissance Orbiter launched, and it used its upper stage rocket to smack into a crater at the Moon’s south pole, and then flew an impact probe called LCROSS through the debris plume. Sensors on the probe detected about 155 kilograms of water. Numerous Moon missions since then have confirmed the findings that yes, there is water ice on the moon.

And last year, the latest big news was released where the SOFIA telescope looked at the Moon in infrared and found water ice in non-polar regions. Meaning there is water ice in places where there is also sunlight and not just in permanently shadowed areas. Again, big news, and that led to scientists determined to understand the mechanisms for how water ice could exist in areas that receive light on a body with no atmosphere to keep water molecules close.

It turns out that a lot of the models being used to understand water on the Moon were using an artificially flat surface, but photos taken during the Apollo missions on the Moon show that the lunar surface is strewn with boulders and covered in small craters. All of these features can cause their own shadows where frost can form as temperatures swing wildly from minus 210 degrees Celsius to 120 degrees Celsius. Add in the fact that the amount of water actually changes throughout the day, decreasing before noon and increasing after noon, and now it’s possible to explain all this water ice we keep finding.

Per the press release: As the Sun tracks through the lunar day, the surface frost that may accumulate in these cold, shaded areas is slowly exposed to sunlight and cycled into the Moon’s exosphere. The water molecules then refreeze onto the surface, reaccumulating as frost in other cold, shaded locations.

Or as co-author Björn Davidsson explains: Frost is far more mobile than trapped water. Therefore, this model provides a new mechanism that explains how water moves between the lunar surface and the thin lunar atmosphere.

And water on the Moon, accessible water, is great for lunar missions because it can be used for drinking and as a fuel source, two things space agencies need to keep crewed missions operating, especially if establishing a lunar base. Maybe someday.

More Information

NASA press release

Your Guide to Water on the Moon (Planetary Society)

“Implications of surface roughness in models of water desorption on the Moon,” Björn J R Davidsson and Sona Hosseini, 2021 August 2, Monthly Notices of the Royal Astronomical Society