Tomokatsu Morota

The global distribution of pure anorthosite on the Moon

It has been thought that the lunar highland crust was formed by the crystallization and floatation of plagioclase from a global magma ocean, although the actual generation mechanisms are still debated. The composition of the lunar highland crust is therefore important for understanding the formation of such a magma ocean and the subsequent evolution of the Moon. The Multiband Imager on the Selenological and Engineering Explorer (SELENE) has a high spatial resolution of optimized spectral coverage, which should allow a clear view of the composition of the lunar crust. Here we report the global distribution of rocks of high plagioclase abundance (approaching 100 vol.%), using an unambiguous plagioclase absorption band recorded by the SELENE Multiband Imager. If the upper crust indeed consists of nearly 100 vol.% plagioclase, this is significantly higher than previous estimates of 82–92 vol.% (refs 2, 6, 7), providing a valuable constraint on models of lunar magma ocean evolution.

Long-Lived Volcanism on the Lunar Farside Revealed by SELENE Terrain Camera

We determined model ages of mare deposits on the farside of the Moon on the basis of the crater frequency distributions in 10-meter-resolution images obtained by the Terrain Camera on SELENE (Selenological and Engineering Explorer) (Kaguya). Most mare volcanism that formed mare deposits on the lunar farside ceased at ∼3.0 billion years ago, suggesting that mare volcanism on the Moon was markedly reduced globally during this period. However, several mare deposits at various locations on the lunar farside also show a much younger age, clustering at ∼2.5 billion years ago. These young ages indicate that mare volcanism on the lunar farside lasted longer than was previously considered and may have occurred episodically.

Performance and scientific objectives of the SELENE (KAGUYA) Multiband Imager

The Multiband Imager (MI) is one of the 14 instruments for the Japanese SELENE (KAGUYA) mission. Goal of the SELENE (KAGUYA) mission is to understand origin and evolution of the Moon by obtaining global element and mineral compositions, topological structure, gravity field of the whole Moon, and electromagnetic and particle environment of the Moon. MI is designed to be a high-resolution multiband imaging camera with a spatial resolution in visible bands of 20 m and a spatial resolution in near-infrared bands of 62 m from the 100 km SELENE (KAGUYA) orbit altitude. The MI flight model has been manufactured and integrated. MTF, viewing vector, over-all sensibility, sensor linearity and electrical noise level (S/N estimation test) were measured, and the results indicate that the MI will provide sufficient MTF and low-noise data, just as estimated in the MI design phase. Operation and data analyses plans have been established, and related tools and algorithms have been developed and checked. One of MI scientific objectives is to investigate small but scientifically very important areas such as crater central peaks and crater walls and to investigate magnesian anorthosites.

Lack of Exposed Ice Inside Lunar South Pole Shackleton Crater

The inside of Shackleton Crater at the lunar south pole is permanently shadowed; it has been inferred to hold water-ice deposits. The Terrain Camera (TC), a 10-meter-resolution stereo camera onboard the Selenological and Engineering Explorer (SELENE) spacecraft, succeeded in imaging the inside of the crater, which was faintly lit by sunlight scattered from the upper inner wall near the rim. The estimated temperature of the crater floor, based on the crater shape model derived from the TC data, is less than ∼90 kelvin, cold enough to hold water-ice. However, at the TCs spatial resolution, the derived albedo indicates that exposed relatively pure water-ice deposits are not on the crater floor. Water-ice may be disseminated and mixed with soil over a small percentage of the area or may not exist at all.

Asymmetrical distribution of rayed craters on the Moon

The synchronous rotation of the satellite ought to cause a spatial variation in the cratering rate over its surface. The crater density is expected to be maximum at the apex of the orbital motion and decrease with the increase of the angular distance from the apex. The ratio of the density at the apex (maximum) to that of the antapex (minimum) depends on the average encounter velocity of impactors to the satellite. Although the Moon is also in a state of the synchronous rotation, it has been supposed that the asymmetry in the crater density on the Moon can be hardly observed. We report here a spatial variation in the density of rayed craters on the Moon, which may be associated with the synchronous rotation. Since the lifetime of a ray is relatively short (<0.8 billion years), the results provide information on recent impacts. Rayed craters are identified on Clementine 750-nm mosaic images. We investigate craters in a lower latitude zone from 42°N to 42°S. To avoid an effect of material difference on the ray preservation, we analyze craters on the highland from 70°E to 290°E in east longitude. A total of 222 rayed craters larger than 5 km in diameter are identified in the study area of about 1.4×107 km2. The average density of rayed craters on the leading side is substantially higher than that on the trailing side. The crater density decreases as a sinusoidal function of the angular distance from the apex. The observed ratio of the density at the apex to that at the antapex is about 1.5. The ratio suggests that recent craters on the Moon are formed mainly by near-Earth asteroids rather than comets with higher encounter velocities.

Mare volcanism in the lunar farside Moscoviense region: Implication for lateral variation in magma production of the Moon

[1] Accurate estimates of the duration and volume of extrusive volcanism of the Moon are essential for understanding the lunar thermal evolution. Here, using new high-resolution images obtained by the SELENE Terrain Camera, we determined the thicknesses and ages of basalts in Mare Moscoviense, one of the most prominent mare deposits on the farside. Mare volcanism in Mare Moscoviense was active for at least ∼1.5 Ga following the formation of the Moscoviense basin. Mare basalts are estimated to be at least 600 m thick, corresponding to a total volume of 9,500-16,000 km 3 . The long duration and large volume of extrusive volcanism are plausibly attributed to the thinner crust of the Moscoviense basin relative to those of other farside basins. From a comparison with mare volume within a same-sized nearside basin, we concluded that a magma production in the farside mantle was 3―10 times less than that of the nearside.

Geologic structure generated by large‐impact basin formation observed at the South Pole‐Aitken basin on the Moon

The South Pole-Aitken (SPA) basin is the largest clearly recognized basin on the lunar surface. Determining the composition and structure of the SPA basin interior provides critical constraints on the deep crustal and/or mantle composition of the Moon and improves our understanding of large-basin-forming impact processes. Here we present a new mineralogical map of the SPA basin interior, based on high-spatial-resolution reflectance spectra using the SELENE (Kaguya) multiband imager, which is combined with topographic data in order to interpret the geologic context. The derived mineralogical map suggests extensive distribution of ejected low-Ca pyroxene-dominant mantle material with the presence of purest anorthosite crustal materials surrounding a possible melt pool of 0.26 to 0.33 of the basin diameter near the basin center, which is significantly smaller than that suggested by the crater-scaling law. The absence of clear evidence of lower crustal material is consistent with recent impact simulation results.

Lunar Holes and Lava Tubes as Resources for Lunar Science and Exploration

The Moon is the nearest celestial body to the Earth. As such, it has long been investigated to understand its formation and evolution, as a paradigm for better understanding the terrestrial planets, as well as all airless bodies in our solar system (e.g., Vesta, Phobos). The Moon’s proximity to the Earth—more than one hundred times closer than any planet — makes it a convenient target for exploration by spacecraft. Since the dawn of the space age in the previous century, we have explored the Moon with several spacecraft and even succeeded in sending astronauts there. One of the lessons of those explorations that hinders any future lunar expeditions is the severe conditions on the lunar surface. The lack of an atmosphere (10-12 torr) means that cosmic/galactic/solar rays, as well as the many micrometeorites directly striking the surface; in addition, surface temperatures vary widely, over a day-night range of more than 300 K.

Variation of the lunar highland surface roughness at baseline 0.15–100 km and the relationship to relative age

We report the surface roughness analysis of the lunar highlands for the baseline range 0.15–100 km. We use the Median Differential Slope αm to investigate the scale dependency of the roughness and derive the global αm distribution from SELENE Laser Altimeter and Terrain Camera data. While αm(l) versus baseline l (km) plots vary among different highland types, all highlands commonly show a peak at 3–30 km. The Pre-Nectarian surface shows a relatively large αm(20–30 km). Our analysis is supported by the simulation of synthetic surface cratering models and crater statistics. In our simulation, a peak of αm(30 km) is successfully reproduced. The actual crater density shows good correlation with an empirical roughness indicator. However, a large part of the Nectarian surface shows a peak at 6–9 km baseline. This peak may be caused by secondary craters and ejecta deposit textures from the Nectarian system basins.

Global occurrence trend of high‐Ca pyroxene on lunar highlands and its implications

We present details of the global distribution of high-Ca pyroxene (HCP)-rich sites in the lunar highlands based on the global data set of hyperspectral reflectance obtained by the SELENE Spectral Profiler. Most HCP-rich sites in the lunar highlands are found at fresh impact craters. In each crater, most of the detection points are distributed on the ejecta, rim, and floor of the impact craters rather than the central peaks, while the central peaks are dominated by purest anorthosite (PAN). This indicates that HCP-rich materials originate from relatively shallower regions of the lunar crust than PAN. In addition, while all ray craters with sizes larger than ∼40 km possess HCP-rich materials, small fresh craters with sizes less than ∼6–10 km do not, indicating that the uppermost mixing layers in the lunar crust are not dominated by HCP. Based on these results, we propose that in the upper lunar crust, a HCP-rich zone overlying the PAN layer exists below the uppermost mixing layer. This HCP-rich zone may originate from interstitial melt during the formation of the flotation anorthositic cumulate, while an impact ejecta origin, impact melt origin, and/or magmatic intrusion into the upper lunar crust may also account for the occurrence of HCP-rich sites in the highlands.