UV light ‘burns’ water off the moon’s sunny side

Water on the moon could be embedded in rocks or, if cold enough, “stuck” on the rocks’ surfaces, but chances of finding water on the moon’s sunlit side are pretty slim, new research suggests.

Ultraviolet photons emitted by the sun likely cause H2O molecules, which are predominately found at the poles, to either quickly desorb or break apart. The fragments of water may remain on the lunar surface, but the presence of useful amounts of water on the sunward side is not likely.

“Overall, the moon will lose water efficiently when the solar photons are hitting it,” says Thomas Orlando, professor in the School of Chemistry and Biochemistry at Georgia Tech.

“The water also desorbs thermally. When they photodesorb or thermally desorb, the velocities are too low for the water to escape so it will bounce around until it gets trapped in the permanently shadowed regions at the poles or break apart in transit.”

Researchers built an ultra-high vacuum system that simulates conditions in space, then performed the first-ever reported measurement of the water photodesorption cross section from an actual lunar sample. The machine zapped a small piece of the moon with ultraviolet (157 nm) photons to create excited states and watched what happened to the water molecules. They either came off with a cross section of ~ 6 x 10−19 cm2  or broke apart with a cross section of  ~ 5  x 10−19 cm2.

According to the team’s measurements, approximately one in every 1,000 molecules leave the lunar surface simply due to absorption of UV light.

The cross section values can now be used by scientists attempting to find water throughout the solar system and beyond.

“The cross section is an important number planetary scientists, astrochemists, and the astrophysics community need for models regarding the fate of water on comets, moons, asteroids, other airless bodies, and interstellar grains,” Orlando says.

Fixed solar flux

The number is relatively large, which establishes that solar UV photons are likely removing water from the moon’s surface. This research, which was carried out primarily by former Georgia Tech PhD student Alice DeSimone, indicates the cross sections increase even more with decreasing water coverage. That’s why it’s not likely that water remains intact as H2O on the sunny side of the moon. Orlando compares it to sitting outside on a summer day.

“If a lot of sunlight is hitting me, the probability of me getting sunburned is pretty high,” he says. “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.”

The result is the release of molecules such as H2O, H2, and OH as well as the atomic fragments H and O.

The research is published in two companion articles in the Journal of Geophysical Research: Planets. The first discusses the water photodesorption. The second paper details the photodissociation of water and the  O(3PJ) formation on a lunar impact melt breccia.

NASA supported the work.

Source: Georgia Tech