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The Forefront of Space Science

Lunar Interior Material Revealed by KAGUYA
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Olivine Ean important clue to discovering the lunar interior

One important objective of solar-system exploration is to elucidate how solid planetary bodies such as the Moon, Earth, Mercury, Venus and Mars have formed and evolved. Of these planets, it is thought that the Moon best preserves information on its initial evolution process since it was cooled earlier than the others due to its smaller size. For this reason, understanding the Moon is important for us to understand the formation and evolution processes of the Earth we live on and other planets.

To date, based on research by remote-sensing observation by lunar explorers, direct observations from the earth and examination of the Moons rocks and meteorites, various models have been proposed for the Moons formation and evolution. One typical theory is the lunar magma-ocean theory. This theory presumes that, at its initial evolutionary stage, the Moon was covered with a magma ocean, as its surface rocks melted. According to this model, as the magma ocean cooled, olivine (a silicate mineral present in the magma) sank deep into the magma to become a major component of the lunar mantle. Meanwhile, plagioclase (another silicate mineral) rose to the surface and formed the Moons crust.

No clear explanation, however, has been provided regarding the temporal changes to the lunar mantle and crust when the current Moon was formed. The main reason this question remains is that olivine, believed to compose the lunar mantle, is usually present deeper within the Moon, and does not rise to the surface. On the other hand, it is believed that the deep-seated mantle was brought to the surface by the formation of craters by collisions with huge meteoroids. To understand the lunar interior, the olivine has been explored by lunar-explorer and Earth-based observations. Past research on olivine, however, is very limited, with only a few places on the lunar surface discovered. There is still no clear answer to the question, Does olivine on the Moon come from the mantle, or from the magma in the lower crust above the mantle?E/span>

Spectroscopic observation of the entire Moon by Spectral Profiler onboard KAGUYA

The answer may be provided by the Spectral Profiler (SP) (Fig. 1) onboard KAGUYA, the lunar orbiting explorer launched in 2007. The SP is an instrument to perform continuous spectroscopic observation of sunlight reflected on the lunar surface in the wavelength range of 0.5 to 2.6µm. By referring absorption bands of the reflection spectrum obtained by SP to the specific absorption bands belonging to each mineral, we can investigate mineral distribution (e.g., type, volume/ratio, and chemical composition) on the lunar surface. There are four main minerals present on the surface: pyroxene, olivine, plagioclase and ilmenite. It is known that olivine has specific absorption bands of 0.85, 1.05, and 1.25µm. The SP has sufficient high wavelength resolution and wide wavelength range to identify such absorption bands. It has successfully retrieved up to 70 million spectral reflectance data across the entire Moon by orbital observation over about 18 months.

Figure 1
Figure 1. Spectral Profiler (SP) onboard lunar explorer SELENE/KAGUYA
SP is circled in red and wrapped with silver cover.

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