On 9 October 2009, the Lunar Crater Observation and Sensing Satellite (LCROSS) mission impacted a spent Centaur rocket into the permanently shadowed region (PSR) within Cabeus crater and detected water vapor and ice, as well as other volatiles, in the ejecta plume. The Lyman Alpha Mapping Project (LAMP), a far ultraviolet (FUV) imaging spectrograph on board the Lunar Reconnaissance Orbiter (LRO), observed this plume as FUV emissions from the fluorescence of sunlight by molecular hydrogen (H-2) and other constituents. Energetic charged particles, such as galactic cosmic rays (GCRs) and solar energetic particles (SEPs), can dissociate the molecules in water ice to form H-2. We examine how much H(2)can be formed by these types of particle radiation interacting with water ice sequestered in the regolith within PSRs, and we assess whether it can account for the H-2 observed by LAMP. To estimate H(2)formation, we use the GCR and SEP radiation dose rates measured by the LRO Cosmic Ray Telescope for the Effects of Radiation (CRaTER). The exposure time of the ice is calculated by considering meteoritic gardening and the penetration depth of the energetic particles. We find that GCRs and SEPs could convert at least 1-7% of the original water molecules into H-2. Therefore, given the amount of water detected by LCROSS, such particle radiationinduced dissociation of water ice could likely account for a significant percentage (10-100%) of the H(2)measured by LAMP.

A new model is presented on how chemically driven cryovolcanism might contribute to episodic outgassing at the icy moon Enceladus and potentially elsewhere including Europa and Kuiper Belt Objects. Exposed water ices can become oxidized from radiolytic chemical alteration of near-surface water ice by space environment irradiation. In contact with primordially abundant reductants such as NH3, CH4, and other hydrocarbons, the product oxidants can react exothermically to produce volatile gases driving cryovolcanism via gas-piston forces on any subsurface liquid reservoirs. Radiolytic oxidants such as H2O2 and O-2 can continuously accumulate deep in icy regoliths and be conveyed by rheological flows to subsurface chemical reaction zones over million-year time scales indicated by cratering ages for active regions of Enceladus and Europa. Surface blanketing with cryovolcanic plume ejecta would further accelerate regolith burial of radiolytic oxidants. Episodic heating from transient gravitational tides, radioisotope decay, impacts, or other geologic events might occasionally accelerate chemical reaction rates and ignite the exothermic release of cumulative radiolytic oxidant energy. The time history for the suggested Old Faithful model of radiolytic gas-driven cryovolcanism at Enceladus and elsewhere therefore consists of long periods of chemical energy accumulation punctuated by much briefer episodes of cryovolcanic activity. The most probable sequence for detection of activity in the current epoch is a long evolutionary phase of slow but continuous oxidant accumulation over billions of years followed by continuous but variable high activity over the past 10(7)-10(8) years. Detectable cryovolcanic activity could then later decline due to near-total oxidation of the theologically accessible ice crust and depletion the accessible reductant abundances, as may have already occurred for Europa in the more intense radiation environment of Jupiter's magnetosphere. Astrobiological potential of Enceladus could correspondingly be higher than at Europa due to a less extreme state of oxidation and greater residual abundance of organics. Published by Elsevier Ltd.