A Multi-Wavelength Grain-by-Grain Survey of Lunar Soils in Search of Rare Materials

Abstract:

The Moon is unique among terrestrial planets for its lack of an atmosphere and global tectonic or volcanic processes. These factors and its position in the inner solar system mean that it is a potential repository of meteoritic material from all of the terrestrial planets. The National Research Council’s 2007 report on the Scientific Context for the Exploration of the Moon highlighted this unique possibility and defined the search for rare materials including those from the early Earth as a key goal for future lunar exploration. Armstrong et al. (2002) estimated that Earth material could be present at the 7 ppm level in surface lunar regolith and emphasized that since a single gram of lunar fines contains over 10 million particles, the search for terran material in lunar soils should begin with the current stock of lunar samples. Joy et al. (2012) demonstrated that mineral and lithologic relics of impactors can survive and be recognized in lunar samples, and recent work by Burchell et al. (2014) suggests that fossil fragments from Earth could survive the extreme shocks associated with transport to the Moon. Following the concept laid out by Armstrong et al. (2002), we are conducting a survey of lunar soil samples using microscopic hyperspectral imaging spectroscopy across visible, near-infrared, and thermal infrared wavelengths to conduct a search for rare particles, including those that could be sourced from the early Earth. Our system currently consists of three microscopic imaging spectrometers with ~30 micron spatial resolution, permitting resolved imaging of individual grains. Fields of view of at least 1 cm and scan rates near 1 mm/sec permit rapid processing of relatively large quantities of sample. Existing spectrometers cover the 0.5 to 2.5 micron region, permitting detection and characterization of the common iron-bearing lunar minerals olivine and pyroxene, and the 8-14 micron region, which permits detection of other, rarer minerals of interest such as apatite and zircons. We are developing the capability to measure the 3 micron region to search for hydrated minerals. Our system also incorporates a micromanipulator which will be used for sorting of soils and isolation of grains of interest. We will report on the status of the system and progress towards identifying and isolating rare grains in lunar soils.