Hiroshi Takeda

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 100u2009vol.%), using an unambiguous plagioclase absorption band recorded by the SELENE Multiband Imager. If the upper crust indeed consists of nearly 100u2009vol.% plagioclase, this is significantly higher than previous estimates of 82–92u2009vol.% (refs 2, 6, 7), providing a valuable constraint on models of lunar magma ocean evolution.

Ultramafic impact melt sheet beneath the South Pole–Aitken basin on the Moon

[1]xa0The Spectral Profiler onboard the Japanese lunar explorer Kaguya conducted the first hyperspectral survey around the South Pole-Aitken basin (SPA) on the Moon that is one of the largest impact structures in the Solar System. The radiative transfer modeling revealed that fresh crater central peaks inside SPA are commonly composed of an ultramafic assemblage dominated by Magnesium rich orthopyroxene, suggesting a homogeneous layer buried under SPA. The extensive ultramafic layer could be a relic of a gigantic impact melt pool produced by the SPA-forming impact. The preponderance of orthopyroxene could have resulted from significant fractional crystallization of the lunar magma ocean prior to the onset of plagioclase floatation.

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.

Asteroid-Meteorite Links: The Vesta Conundrum(s)

Although a direct link between the HED meteorites and the asteroid 4 Vesta is generally acknowledged, several issues continue to be actively examined that tie Vesta to early processes in the solar system. Vesta is no longer the only basaltic asteroid in the Main belt. In addition to the Vestoids of the Vesta family, the small asteroid Magnya is basaltic but appears to be unrelated to Vesta. Similarly, diversity now identified in the collection of basaltic meteorites requires more than one basaltic parent body, consistent with the abundance of differentiated parent bodies implied by iron meteorites. The timing of the formation of the Vestoids (and presumably the large crater at the south pole of Vesta) is unresolved. Peaks in Ar-Ar dates of eucrites suggest this impact event could be related to a possible late heavy bombardment at least 3.5 Gyr ago. On the other hand, the optically fresh appearance of both Vesta and the Vestoids requires either a relatively recent resurfacing event or that their surfaces do not weather in the same manner thought to occur on other asteroids such as the ordinary chondrite parent body. Diversity across the surface of Vesta has been observed with HST and there are hints of compositional variations (possibly involving minor olivine) in near-infrared spectra.

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.

First Results of High Performance Ge Gamma-Ray Spectrometer Onboard Lunar Orbiter SELENE (KAGUYA)

The high precision gamma-ray spectrometer (GRS) is carried on the first Japans large-scaled lunar explorer, SELENE (KAGUYA), successfully launched by the H-IIA rocket on Sep. 14, 2007. The GRS consists of a large Ge crystal as a main detector and massive bismuth germanate crystals and a plastic scintillator as anticoincidence detectors. After a series of initial health check of the GRS, it started a regular observation on December 21, 2007. Energy spectra including many clear peaks of major elements and trace elements on the lunar surface have been measured by the GRS. Global measurement of thorium counting rate on the lunar surface is presented. The region showing the highest count rate of thorium extends from Kepler to Fra Mauro region in the Procellarum. And Apennine Bench and Aristillus region and the northwestern region of Mare Imbrium are high in thorium count rate. Second high count rate region is located in the South Pole-Aitken basin of the farside. Arago and Compton/Belkovich craters are also e...

Periodic intensity distribution (PID) of mica polytypes: symbolism, structural model orientation and axial settings

Following a preliminary revisitation of the nomenclatures in use for mica polytypes, the properties of the periodic intensity distribution (PID) function, which represents the Fourier transform of the stacking sequence, are analysed. On the basis of the relative rotations of neighbouring layers, mica polytypes are classified into three types; for each type, the PID exists in different subspaces of the reciprocal space. A revised procedure to compute the PID, in which further restrictions on the structural model orientation are introduced, is presented. A unifying terminology based upon the most common symbols used to describe mica polytypes (RTW, Z and TS) is derived; these symbols represent the geometrical basis for the computation of the PID. Results are presented for up to four layer polytypes and are compared with the reflection conditions derived by means of Zvyagins functions. Both the PID values and the reflection conditions are expressed in suitable axial settings and compared with previous partial reports, revealing some errors in previous analyses. A computer program to compute PID from the stacking symbols is available.

The widespread occurrence of high‐calcium pyroxene in bright‐ray craters on the Moon and implications for lunar‐crust composition

[1]xa0We investigated the continuous spectral features of fresh craters on the Moon accompanied by distinctive bright rays, with cavity diameters between 8 and 24 km. We used the data from the Spectral Profiler onboard SELENE (Kaguya) to gain a better understanding of the composition of the lunar highland crust. We found that the observed spectra exhibited strong symmetric absorption around 1 μm and recognizable absorption around 1.3 μm. The spectra around a few craters showed a drastic change in the relative strengths of these two absorption bands s1.3/1.0 at different locations in and around the craters, indicating differences in the abundance of plagioclase and mafic minerals. In contrast, the spectra around most of the craters showed no significant variation in spectral shape, with an essentially constant s1.3/1.0. We analyzed the absorption features of the craters with an essentially constant s1.3/1.0 using the Modified Gaussian Model. We found that the strongest symmetric absorption bands were centered at 0.97–1.01 μm with s1.3/1.0 ≈ 0.2–0.6. Comparing these values with data from known samples, we concluded that high-calcium pyroxene (HCP) is the most plausible dominant mafic mineral identified from the observed spectra. The fact that we detected such HCP-dominant spectra among rayed craters widely spaced across the lunar highland implies that the major mafic component of some portions of the lunar crust is HCP rather than low-calcium pyroxene (LCP).