The spent Centaur rocket of the Lunar Crater Observation and Sensing Satellite (LCROSS) mission impacted into the permanently shadowed region (PSR) of Cabeus crater on 9 October 2009. The resulting impact plume contained various volatiles, including water vapor and ice, as detected by the shepherding spacecraft . Other volatiles, in particular molecular hydrogen (H2), were detected by the Lyman Alpha Mapping Project (LAMP), a far ultraviolet spectrograph on the Lunar Reconnaissance Orbiter (LRO) [2,3].
We examine whether energetic charged particles--galactic cosmic rays (GCRs) and solar energetic particles (SEPs)--can create the observed H2 via the radiolysis of water ice . We create a simple model incorporating four components [Jordan et al., submitted to J. Geophys. Res., 2013]. First, we estimate the historical GCR and SEP radiation dose using data from LRO/CRaTER (Cosmic Ray Telescope for the Effects of Radiation) [5,6] and the OMNI data set . Second, we use the National Institute of Standard’s Stopping-Power and Range Tables for Protons (PSTAR) to find the typical penetration depth of GCRs and SEPs . This depth determines how much regolith receives the radiation dose at any given time. Third, we estimate how meteoritic gardening mixes the regolith and the water ice it contains, thus enabling the incident radiation to dissociate otherwise inaccessible ice . Finally, we estimate the G-value of H2 due to GCRs and SEPs [10-12]. These four components enable us to estimate the creation of H2 from H2O. We find that, over 1000 Myr, GCRs and SEPs can convert at least 1-7% of the original H2O molecules into H2. We also consider how much H2O has been destroyed by being converted into other byproducts, as this also affects the ratio of H2 to H2O molecules. Energetic particle radiolysis of the water ice in PSRs likely accounts for a significant percentage (10-100%) of the H2 measured by LAMP.
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