Geochemical tools for lunar exploration

The Lunar Prospector orbiter carried a gamma-ray/neutron spectrometer (GRS) that made precise measurements of the concentration and distribution of thorium (Lawrence et al., 1998) and hydrogen (Feldman et al., 2001). Subsequent spectral deconvolutions (Prettyman et al., 2006) have produced analyses of iron, titanium, potassium, magnesium, aluminum, calcium, and silicon. The principles of these analytical techniques are explained in Box 13.1. 

The Clementine orbiter obtained high-resolution multispectral reflectance data. In comparing spectra with soil compositions at 39 locations sampled by Apollo astronauts, Blewett et al. (1997) determined a correlation between specific spectral features and chemical composition. This enabled Lucey et al. (1998) to develop an algorithm to estimate accurate FeO and Ti02 abundances from Clementine spectra. 

Global mapping of FeO, Th, and Ti02 concentrations, especially, has allowed the abundance and distribution of different compositional regions of the lunar crust to be determined, and has provided new insights into heterogeneities in the mantle (Jolliff et al., 2000 Giguere et al., 2000). 

Gamma-ray spectrometers use scintillator detectors. These spectrometers sense y-rays from all directions, and hence have large footprints (commonly hundreds of kilometers diameter) with sizes determined by orbital elevation above the surface. The y-rays come from depths of less than a meter in the target material. 

In some cases, thermal neutrons can also be used to measure the absolute abundances of other elements. Transforming the neutron spectrum into elemental abundances can be quite involved. For example, to determine the titanium abundances in lunar spectra, Elphic et at. (2002) first had to obtain FeO estimates from Clementine spectral reflectances and Th abundances from gamma-ray data, and then estimate the abundances of the rare earth elements gadolinium and samarium from their correlations with thorium. They then estimated the absorption of neutrons by major elements using the FeO data and further absorption effects by gadolinium and samarium, which have particularly large neutron cross-sections. After making these corrections, the residual neutron absorptions were inferred to be due to titanium alone. 

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