First report
Updated : 2009.03.16
Observation date :

The KAGUYA Gamma-Ray Spectrometer (GRS), which employs a high-purity Ge semiconductor detector, is observing the distribution of element concentrations (K, Th, U, O, Mg, Al, Si, Ca, Ti, and Fe) in the lunar subsurface with its spatial resolution of approximately 135 km.

In the left figure, an energy spectrum of gamma rays observed by the KAGUYA GRS (red line) is shown with one observed by the GRS of Lunar Prospector (blue line), which is a past American lunar mission. The figure is plotted with counting rates of gamma rays as a vertical axis and energies of gamma rays as a horizontal axis, and the individual peaks indicate the existence of elements. As seen from the comparison of these two energy spectra, the energy spectrum measured by the KAGUYA GRS shows much sharper peaks, because of its excellent energy resolution of the KAGUYA GRS. It is clear that the KAGUYA GRS provides more detail information on elements than the Lunar Prospector GRS. The data of the KAGUYA GRS used in the figure were obtained from December 2007 (when its regular observation began) through February 2008, and the data of the Lunar Prospector GRS were from December 1998 through July 1999.

The right figure shows two energy spectra measured by the KAGUYA GRS in two regions: One is on the nearside of the Moon and another is on the farside. In the figure, a "Th" marked peak indicates gamma rays originated from radioactive thorium contained in the lunar subsurface. The higher peak of Th gamma rays is seen in the energy spectrum measured in the nearside region, indicating the nearside region contains the higher concentration of Th than the farside region. In contrast, two peaks in both sides of the "Th" peak indicate background gamma rays from aluminum materials used in the spacecraft body, showing no difference in the nearside and the farside.

The observation of the KAGUYA GRS will determine the concentrations of the major and trace elements in the lunar subsurface and it is expected to deepen our understanding of the origin and evolution of the Moon.

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