Naito et al., (2017) describe the distribution of iron on the Moon as obtained by the Kaguya high energy resolution gamma-ray spectrometer (KGRS). The authors achieved for the first time the identification of iron based on the fast neutron flux obtained by the KGRS. The iron distribution obtained by KGRS is compared to that of the Lunar Prospector Gamma-Ray Spectrometer (LP GRS), showing that the FeO distributions observed by KGRS and LP GRS, in general, are in good agreement. Furthermore, we compare the iron content data obtained by KGRS and LP GRS to spectral reflectance measurements of the Clementine, Kaguya and Chandrayaan-1 spacecraft as well as those inferred from returned samples.
The authors found differences in FeO concentration and distribution in areas of moderate abundance (6-15 wt%) of the South Pole-Aitken basin, Mare Orientale, and around the crater Tycho crater. It implies that high concentrations of FeO at Mare Ingenii in the South Pole-Aitken basin and Mare Orientale are due to the presence of mare basalts, whereas the enriched FeO content in the central depression of the South Pole-Aitken basin and the Tycho crater indicates the presence of mafic materials such as impact melt breccia.
We have conducted a spectral analysis of iron gamma-rays obtained by the KGRS instrument. Gamma-ray spectra with high energy resolution and sawtooth peak gamma-ray spectra observed by the HPGe detector enabled an elemental analysis of high accuracy. The count rates of iron gamma-rays and fast neutrons were obtained to construct a global FeO map. With the fast 475 neutron count rates obtained by KGRS, the iron abundance was for the first time obtained without data acquired by another neutron spectrometer. The comparison of the KGRS and LP GRS [6] FeO maps, in general showed a good agreement, especially in the area of > 6 wt% FeO despite the different calibration methods. However, some differences between the KGRS and LP GRS 480 maps are apparent in high-FeO areas within the SPA, Mare Orientale and the crater Tycho.
It is considered that the inelastic scattering gamma-rays (56Fe(n,n ) at 846 keV) analyzed by KGRS were more useful than the neutron capture gamma-rays (56Fe(n, ) at 7.36–7.38 MeV) measured by LP GRS to construct the iron map. Variations over the lunar surface of the fast neutron flux yielding 485 inelastic scattering gamma-rays are smaller than those of the thermal neutron flux inducing neutron capture gamma-rays. The effect of the thermal neutron variations might be relevant in low-FeO areas of the LP GRS map. A low variation of the fast neutron flux allows for the construction of an accurate FeO map. Considering previous studies based on near-infrared reflectance spectra 490 and gamma-ray measurements, the origin of the high FeO content in the above regions has been discussed.
The FeO abundance at Mare Ingenii (12 wt%) in the SPA is consistent with that of mare basalt (> 13 wt% [41]). This is a plausible result since the smoothing of the KGRS map due to its relatively low spatial resolution decreases the apparent FeO concentration. Although the FeO 495 abundance at the central depression is 14 wt%, it is interpreted as impact melt dominated by high-Ca pyroxene. The highest FeO concentration at the central depression is considered to be increased by mare material in the Apollo basin because of the limited spatial resolution. The distributions of Th and CaO in these regions are consistent with this origin. 500 A high FeO content of presumably basaltic origin has been observed at Mare Orientale. A very low FeO abundance below the detection limit is also observed in the KGRS map in the area occupied by the Orientale basin rings. This result supports the fact that the materials in this region are dominated by anorthsite having a low FeO content. This interpretation is also supported by the distribution of CaO. However, the characteristic features of the Orientale impact ejecta as proposed by the Chang’E-2 gamma-ray spectrometer measurements are not present in our FeO map at the locations found by the Chang’E-2 team.
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