春山 純一

Processes of water (OH and H2O) migration on the Moon remain unclear, prompting active research. Understanding lunar water migration requires investigation of the trapping and diffusion properties of water at various latitudes and local times. This study analyzed visible to near-infrared spectral data obtained by the Spectral Profiler (SP) onboard SELENE for shadowed regions at various local times and latitudes, not limited to the polar permanently shadowed regions. We assessed SP data for shadowed regions in 60 areas, each spanning a 10° × 10° latitude–longitude grid. Of the 1,061,907 analyzed shadowed-region data, 41,385 at various latitudes exhibited significant absorption in the 1.25 and 1.5 µm bands, indicating water ice particles. Data with the two absorption features suggest the presence of a water ice frost layer covering the lunar surface or suspended water ice particles above the lunar surface, at various latitude shadowed regions. Our spectral simulations have quantified the ice particles as being 0.1–1 µm in diameter, with a column density of 10–4–10–3 kg/m2. The spectral parameters for band absorption at the 1.5 µm band show symmetry between morning and evening sides, which is potentially attributed to the absence of variations in ice grain size and quantity. The 1.5 µm band absorption shows an increasing trend toward terminator regions, indicating variation in the water ice distribution and likely reflecting temperature conditions for water retention. The latitudinal trend of ice grain size and quantity remains uncertain because of the observed noise levels. Observations of water ice particles in shadowed regions at various latitudes and local times can provide new constraints on trapping and diffusion processes of lunar water migration.

Vertical holes of several tens of meters in diameter and depth have been discovered on the Moon, potentially serving as skylights into subsurface volcanic caverns resembling lava tubes. Because of their scientific importance and potential for use as future lunar bases, these lunar holes and caverns are expected to be targets of intensive exploration in the near future. Using numerical simulations, this study investigates the light environment within a directly and/or indirectly sunlit subsurface cavern accessed through a skylight hole. We specifically analyze the Mare Tranquillitatis Hole (MTH), one of the largest lunar vertical holes, situated near the Moon's nearside meridian. The floor and walls of the hole, and the walls and ceiling of its associated subsurface cavern are lit by reflected sunlight from other parts of the hole and cavern, such as the floors, depending on the solar elevation angle. Furthermore, this paper presents estimation results for camera imaging for a case in which solar elevation angle is 45° in the morning, which is appropriate timing for exploration. Assuming reflectance of 0.1 for the lunar hole and cavern surfaces, we estimate the incident energy onto each pixel of a camera as it descends to the hole's floor. We specifically simulate conditions for a camera with a 12-bit dynamic range, similar to the wide-angle optical navigation camera (ONC-W) onboard the Hayabusa 2 spacecraft. The results suggest that a camera with a fixed gain would struggle to capture both directly and indirectly sunlit areas simultaneously, without saturating bright areas and ensuring minimum incident energy resolution (say, a 10 digital number) for darker areas. To overcome this challenge, adjusting the camera gain based on the hole's and cavern's illumination conditions is necessary. Graphical abstract: (Figure presented.)

We studied the lunar mantle composition based on the characteristics of low-Ca pyroxene (LCP)-rich and olivine-rich rocks exposed on the lunar surface. Using data mining with SELENE (Kaguya) hyperspectral data, we found 531 sites with spectra dominated by an ultramafic LCP end-member mineral, most of which are located in the Imbrium basin and the South Pole-Aitken (SPA) basin. Stratigraphic analysis of each site revealed that LCP-rich rock bodies are exposed on fresh geological features that are less affected by space-weathering, such as steep slopes at peaks, and crater walls and ejecta deposits at smaller craters. We also found that, in the SPA and Imbrium basins, LCP-rich bodies are more numerous and more widely distributed than olivine-rich rocks, suggesting that LCP-rich materials deep in the mantle were excavated during the formation of these huge basins. However, olivine-rich rocks were abundant, and no LCP-rich rocks were found in the Moscoviense, Crisium, and Humboldtianum basins, which are known to have almost-zero crustal thicknesses, indicative of mantle excavation. Thus, our results suggest that the composition of rocks derived from the lunar mantle varies with the impact basin. Such a difference might indicate a layered structure of mantle composition, with the olivine-rich upper mantle overlying the LCP-rich mantle, a horizontal heterogeneity in the mantle composition, a regional heterogeneity in early lunar basaltic magmatism, or an impact melt origin. The layered structure or horizontal heterogeneity might have resulted from a mantle overturn caused by gravitational instability in the early stages of the lunar magma ocean.