Solar photons drive water off the Moon

Jun 17, 2014 by Jason Maderer
A lunar sample in a ultra-high vacuum system is hit with ultraviolet (157 nm) photons to simulate conditions in space.

(Phys.org) —Water is thought to be embedded in the moon's rocks or, if cold enough, "stuck" on their surfaces. It's predominantly found at the poles. But scientists probably won't find it intact on the sunlit side.

New research at the Georgia Institute of Technology indicates that ultraviolet photons emitted by the sun likely cause H2O molecules to either quickly desorb or break apart. The fragments of water may remain on the , but the presence of useful amounts of water on the sunward side is not likely.

The Georgia Tech team built an ultra-high vacuum system that simulates conditions in space, then performed the first-ever reported measurement of the water photodesorption 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 . 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.

Georgia Tech's 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," said Thomas Orlando, the Georgia Tech professor who led the study.

A fragment of the moon. New research at the Georgia Institute of Technology indicates that ultraviolet photons emitted by the sun likely cause H2O molecules to either quickly desorb or break apart.

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 Ph.D. 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," said Orlando, a professor in the School of Chemistry and Biochemistry and School of Physics. "It's similar on the . 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, according to Orlando, 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 and the O(3PJ) formation on a lunar impact melt breccia.

Explore further: Digging deep in search of water on the moon

More information: Photodissociation of water and O(3PJ) formation on a lunar impact melt breccia, Journal of Geophysical Research: Planets, onlinelibrary.wiley.com/doi/10… 013JE004598/abstract

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antialias_physorg
5 / 5 (3) Jun 17, 2014
but the presence of useful amounts of water on the sunward side is not likely

Erm...is there a non-sunward side of the moon?
Birger
5 / 5 (2) Jun 17, 2014
-The author has f*cked up by thinking of the expression "the dark side of the moon", actually just the non-visible (from Earth) side of the moon.

Those parts of the moon shielded from exposure to UV ( present in all sunlight) are the same parts of the moon where ice may be present: The deep permanently shaded areas in craters at the lunar poles.
dramamoose
5 / 5 (1) Jun 17, 2014
Seriously? A science news article which says 'the sunlit side'? They're clearly talking about the areas in perma-darkness because of craters. Seriously?
El_Nose
5 / 5 (1) Jun 18, 2014
lol i saw sunlit side and immediately went to the comments -- poor Jason Maderer