Identification of lunar rock types and search for polar ice by gamma ray spectroscopy

Abstract

The ability to map the surface composition of the Moon from orbit by means of gamma ray spectroscopy has been studied with attention to the possible use of small spinstabilized spacecraft. Two types of detectors were considered, a sodium iodide scintillator and a cooled germanium solid‐state device. The latter has superior sensitivity for all determinable elements, the difference ranging from factors of 3 to 11. A design for accommodating the preferred germanium detector gamma ray spectrometer (GeGRS) on either a small or large spacecraft has been devised. Detection sensitivity has been applied to typical rock compositions to determine measurement times and the ability to discriminate among representative lunar rock types. For sets comprising nine highland and 16 mare types, the most useful elements are found to be Mg, Al, K, Ti, Fe, U, and Th. Compositional distinctions combined with the sensitivity of the GeGRS instrument should allow each major rock type to be recognized in accumulation periods of 2 hours or less. The presence of two or more rock types in comparable abundance will require longer periods. Permanently shadowed polar areas of the Moon may be repositories of water ice. The expected instrument response to the resulting gamma ray and neutron fluxes has been analyzed. A neutron mode added to the spectrometer will be more sensitive to the possible presence of ice than the gamma ray mode, although without the application of compositional corrections based on analysis of the gamma ray data, most of the neutron mode sensitivity will be lost. With a pair of selected neutron absorbers, and a model which provides that 2.5% of the area above 75° latitude is occupied by trapping sites, the most sensitive of the configurations examined will provide a one year mission detection limit of 0.056% H2O by weight for each polar region. Given the postulated abundance of trapping sites, a determination of whether water is present in adequate abundance for resource utilization can be made within a week. These results are applicable to alternative spacecraft by simply adjusting the duty cycle factor.