Space scientists from the University of New Hampshire, in Durham, and NASA's Goddard Space Flight Center in Greenbelt, Md., have published their results online in the Journal of Geophysical Research Planets.
Discovering molecular hydrogen on the moon was a surprise result from NASA's Lunar Crater Observation Sensing Satellite (LCROSS) mission, which crash-landed the LCROSS satellite's Centaur rocket into the Cabeus crater in the permanently shadowed region of the moon. These regions have never been exposed to sunlight and have remained at temperatures near absolute zero for billions of years, preserving the pristine nature of the lunar soil, or regolith.
Instruments on board LCROSS trained on the resulting immense debris plume detected water vapor and water ice, the mission's hoped-for quarry, while LRO, already in orbit around the moon, saw molecular hydrogen.
"LRO's Lyman Alpha Mapping Project, or LAMP, detected the signature of molecular hydrogen, which was unexpected and unexplained," said Andrew Jordan, research scientist and lead author of the paper from the University of New Hampshire's Institute for the Study of Earth, Oceans, and Space.
"After the finding, there were a couple of ideas for how molecular hydrogen could be formed but none of them seemed to work for the conditions in the crater or with the rocket impact." Jordan said. "Our analysis shows that the galactic cosmic rays, which are charged particles energetic enough to penetrate below the lunar surface, can dissociate the water, H2O, into H2 through various potential pathways."
The analysis was based on data gathered by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument aboard the LRO spacecraft. CRaTER characterizes the global lunar radiation environment by measuring radiation dose rates from galactic cosmic rays and solar energetic particles.
"We used the CRaTER measurements to get a handle on how much molecular hydrogen has been formed from the water ice via charged particles," said Jordan. Jordan's computer model incorporated the CRaTER data and showed that these energetic particles can form between 10 and 100 percent of the H2 measured by LAMP.
The study notes that narrowing down that percent range requires particle accelerator experiments on water ice to more accurately gauge the number of chemical reactions that result per unit of energy deposited by cosmic rays and solar energetic particles.
"This result indicates the importance of radiation exposure to the volatile chemicals stored in lunar cold traps, which has implications for our understanding of the history of the solar system as well as its future exploration," said Timothy Stubbs, research scientist and co-author on the paper from NASA Goddard.
LRO is managed by Goddard.
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