Solar Photons Drive Water from the Sunlit Side of Earth's Moon

Scientists have discovered something new about the moon. While water is thought to be embedded in the moon's rocks or even "stuck" on their surfaces, researchers have found that solar photons may actually drive water off of the moon, which means that researchers are unlikely to find water on the moon's sunlit side.

In order to better understand how the sun might impact water on the moon, the researchers built an ultra-high vacuum system that simulates the conditions in space. Then, they performed the first-ever reported measurement of a water photodesorption cross section from a lunar sample. The machine zapped the small piece of the moon with ultraviolet (157 nm) photons in order to create excited states. Then, the scientists watched what happened to the water molecules.

So what happened? It turned out that the water molecules either came off with a cross section or broke apart with a cross section. In all, approximately one in every 1,000 molecules leaves the lunar surface simply due to the absorption of UV light. In fact, it's likely that the sun causes water molecules to either quickly desorb or break apart, which means that it's unlikely scientists will find water on the lit surface of the moon.

"If a lot of sunlight is hitting me, the probability of me getting sunburned is pretty high," said Thomas Orlando, one of the researchers, in a news release. "It's similar on the moon. There's a fixed solar flux of energetic photons that hit the sunlit surface, and there's a pretty good probability they remove water or damage the molecules."

This cross section is important for planetary scientists. It doesn't just reveal new information about the moon, it also reveals the fate of water on comets, asteroids and other airless bodies and interstellar grains. This could pave the way for future research about other celestial bodies.

The findings are published in the Journal of Geophysical Research: Planets.