Energetic charged particle dose rates in water ice on the Moon

Abstract

Water ice in permanently shadowed regions on the Moon is exposed to galactic cosmic rays (GCRs) and solar energetic particles (SEPs). Because this radiation alters the chemistry of the ice, constraining the total radiation dose is important for understanding both the origin and evolution of the ice. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) onboard the Lunar Reconnaissance Orbiter (LRO) has measured the energetic charged particle dose rate for more than a solar cycle, providing the longest continuous dataset of radiation in the lunar environment. CRaTER’s unique design enables us to measure the dose rates behind three amounts of mass shielding and thus constrain the GCR and SEP dose rates as a function of depth in the regolith. In a further improvement on prior studies, we combine these dose rates with a model for how impact gardening affects the exposure time of the regolith. We can thus calculate the total dose received by water ice in gardened regolith and find that impact-gardened ice has received a dose of ∼0.1–1 eV molecule−1 over the past 1 Gyr. This dose is one to two orders of magnitude lower than the doses calculated in studies that do not incorporate the effects of gardening. Relatively undisturbed ice may have received a higher dose, but no more than ∼10 eV molecule−1 in the top centimeter. This result provides a valuable constraint for researchers studying radiation processing of lunar water ice.