Chikatoshi Honda

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.

Size-frequency statistics of boulders on global surface of asteroid 25143 Itokawa

The surface of asteroid 25143 Itokawa is covered with numerous boulders although gravity is very small compared with that of other asteroids previously observed from spacecraft. Here we report the size-frequency statistics of boulders on the entire surface of Itokawa based on high-resolution images (1 pixel ≈0.4 m) obtained by the Hayabusa spacecraft. There are 373 boulders larger than 5 m in mean horizontal dimension on the entire surface—0.393 km2—and the number density is nearly 103/km2. The cumulative boulder size distribution on the entire surface has a power-index of −3.1 ± 0.1. For the east and west sides and the head and body portions of Itokawa, the power-index of the size distributions and the number densities of boulders of these areas are thought to be similar from the statistical point of view. A global mapping of boulders shows that there is no apparent correlation in the locations of boulders and craters. The ratio of the total volume of the boulders to the total excavated volume of the craters on Itokawa is ≈25% when only craters larger than 50 m in mean diameter are considered, and this ratio is extremely larger than that on Eros and the Moon, respectively. The origin of boulders on the surface of Itokawa was examined quantitatively by calculating the number of boulders and the size of the largest boulder using a model based on impact cratering experiments. The result indicated that the boulders on the surface of Itokawa cannot solely be the product of craters. Our results suggest that the boulders originated from the disruption of the larger parent body of Itokawa, as has been described in previous papers (Fujiwata et al., Science, 312, 1330–1334, 2006; Saito et al., Science, 312, 1341–1344, 2006).

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.

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.

Preflight and In-Flight Calibration of the Spectral Profiler on Board SELENE (Kaguya)

The Spectral Profiler (SP) is a visible-near infrared spectrometer on board the Japanese Selenological and Engineering Explorer, which was launched in 2007 and observed the Moon until June 2009. The SP consists of two gratings and three linear-array detectors: VIS (0.5-1.0 μm ), NIR 1 (0.9-1.7 μm), and NIR 2 (1.7-2.6 μm). In this paper, we characterize the radiometric and spectral properties of VIS and NIR 1 using in-flight observational data as well as preflight data derived in laboratory experiments using a calibrated integrating sphere. We also proposed new methods for radiometric calibration, specifically methods for nonlinearity correction, wavelength correction, and the correction of the radiometric calibration coefficients affected by the water vapor. After all the corrections, including the photometric correction, we obtained the reflectance spectra for the lunar surface. Finally, we examined the stability of the SP using the SP data near the Apollo 16 landing site observed at four different times. The difference in reflectance among these four observations was less than ~ ±1% for most of the bands, suggesting that the degradation of the SP is not significant over the mission period.

Impact process of boulders on the surface of asteroid 25143 Itokawa— fragments from collisional disruption

The subkilometer-size asteroid 25143 Itokawa is considered to have a gravitationally bounded rubble-pile structure. Boulders appearing in high-resolution images retrieved by the Hayabusa mission revealed the genuine outcome of the collisional event involving the asteroid’s parent body. Here we report that the boulders’ shapes and structures are strikingly similar to laboratory rock impact fragments despite differences of orders of magnitude in scale and complexities of the physical processes. These similarities suggest the universal character of the process throughout the range of these scales, and the brittle and structurally continuous nature regarding the parent body of the boulders. The similarity was likely preserved because of relatively lesser comminuting processes acting on individual boulders; the close assemblages of similar appearing boulders (a boulder family) represent the impact destruction of boulders on the surface.

Testing hypotheses for the origin of steep slope of lunar size-frequency distribution for small craters

The crater size-frequency distribution of lunar maria is characterized by the change in slope of the population between 0.3 and 4 km in crater diameter. The origin of the steep segment in the distribution is not well understood. Nonetheless, craters smaller than a few km in diameter are widely used to estimate the crater retention age for areas so small that the number of larger craters is statistically insufficient. Future missions to the moon, which will obtain high resolution images, will provide a new, large data set of small craters. Thus it is important to review current hypotheses for their distributions before future missions are launched. We examine previous and new arguments and data bearing on the admixture of endogenic and secondary craters, horizontal heterogeneity of the substratum, and the size-frequency distribution of the primary production function. The endogenic crater and heterogeneous substratum hypotheses are seen to have little evidence in their favor, and can be eliminated. The primary production hypothesis fails to explain a wide variation of the size-frequency distribution of Apollo panoramic photographs. The secondary craters are likely the major source of the steepening of the distribution. It is ambiguous, however, which primary craters can produce sufficiently numerous secondary craters. The regional variation of the size-frequency distributions shows that few large impacts produce enough secondary craters to affect the distributions in the surrounding area. We emphasize that a crater size-frequency distribution of small craters on the moon should not be taken as an indication of the surface age. More data obtained from future lunar missions should be viewed in this context, and continued to be examined for further insight into the possible formation mechanism for secondary craters.

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.