X-ray spectrometry for remote exploration of Mercury and the Moon

Abstract

Any future mission to Mercury or the Moon should include an X-ray spectrometer in order to address fundamental questions of planetary origin. In the case of Mercury, where no direct geochemical measurements exist, such measurements are crucial in the understanding of the origin of Mercury and the early solar system. For the Moon, where actual surface samples are available, more complete geochemical mapping is crucial in the assessment of the potential for future development of resources and permanent bases. Orbital X-ray detectors measure emission induced by the solar high energy flux incident on planetary surfaces. Characteristic lines generated for a number of elements are of sufficient intensity to allow measurement by orbital detectors. The X-ray instrumentation we propose for these experiments, which is being flown or developed for current space missions, would consist of an array of solid state Si PIN detectors in the case of a lunar mission, and a multi-wire divided chamber proportional counter in the case of a Mercury mission. In both cases, one pinhole size Si PIN detector would act as a solar monitor. The spin-stabilized orbiter proposed for launch in the first or second decade of the twenty first century as part of the ESA's Horizon 2000-plus plan is considered the nominal mission design for the target Mercury. In the case of the Moon, we consider a generic three-axis stabilized polar orbiter at 100 km. Model spectra have been generated and integration times and spatial resolutions estimated for a variety of rock types which could be found on Mercury or the Moon, including ANT Suite materials (anorthosite, anorthositic gabbro, and troctolite), a variety of basalts, and enstatite chondrites in the case of Mercury. Calculation indicate that Mg, Al, Si, Ca, Ti, Fe maps could be generated by nominal one to two year missions in polar orbit, and could determine the overall distribution of major rock types observed or proposed on either body early in such a mission.