Lunar X-Ray Fluorescence Spectrometry from SELENE Lunar Polar Orbiter

Introduction: We have been developing an X-ray fluorescence spectrometer, XRS, for SELENE mission, a Japanese lunar polar orbiter that awaits its launch in FY2006. The XRS will map major elemental composition of lunar surface with less than 20km in spatial resolution and 90% surface coverage except for polar region. The XRS has been designed to improve energy resolution and detection efficiency at soft x-rays by adopting new technologies such as X-ray CCD and ultra-thin beryllium window. The current status of the instrumental development is also described. As was proven during the Apollo 15 and 16 missions, major elemental composition can be determined through remote XRF method for atmosphere-free planetary surfaces. Solar X-rays irradiate planetary surface to excite each atom of the uppermost surface materials. Immediately those atoms settle to the ground state and X-rays characteristic of major elements are illuminated off the surface. However, intensity and spectral profiles of solar X-rays varies time to time, which affects those of XRF off the planetary surfaces as well. Therefore major elemental composition can be mapped from the orbiting altitude with remote XRF spectrometry, together with concurrent monitoring of solar X-rays. Remote XRF spectrometry is the commonly used method to map major elemental composition of atmosphere-free planetary surface that have explored and will explore the Moon, Mercury, and asteroids such as the Apollo 15 and 16, ESA’s Smart-1, ISAS/JAXA’s SELENE, Chinese Chang’e, and Indian Chandrajaan-1 for the Moon, NASA’s Messenger and ESA-JAXA’s Bepi Colombo for the Mercury, and NEAR-Shoemaker, ISAS/JAXA’s Hayabusa (MUSES-C) for near-earth asteroids. As for the SELENE mission, lunar science is the first priority by conducting lunar global mapping with combinations of panoramic and high resolution imagery, X-ray and gamma-ray spectrometry, laser altimetry, radar surface and subsurface sounding, gravity anomaly, and remnant and responded magnetic field as well as its surrounding plasma environment. Then the XRF spectrometry aims at mapping major elemental composition, especially in Mg, Al, and Si, of the lunar uppermost surface with higher accuracy, with better spatial resolution and at larger coverage than those of that have been done in the previous lunar missions. To achieve those purposes, the XRS instrument consists of CCD-based main detector with a direct monitor of solar X-rays as well as XRF calibrator aboard. We present here the scientific objectives and instrumentation of the XRS as well as its current status of development. Scientific Objectives: Scientific objectives of the XRS observation are (1) global mapping of major elements of lunar surface materials except for polar regions through remote XRF spectrometry during day time observation, (2) understanding the physical processes of lunar X-ray illumination in the night time that happens by impact of solar wind particles and comic rays as well as natural radioactivity in the uppermost layer of lunar surface, and (3) regional variation of surface microscopic roughness as the results of particle size effect on XRF intensities. Lunar XRF experiments have mapped about 10 % of the lunar equatorial regions during the Apollo 15 and 16 missions in 1971-1972 and implied that lunar maria are covered with lava flows in basaltic composition and lunar highlands are dominantly occupied with aluminous anorthosetic materials (Adler and Trombka, 1977). Tsiolkovsky crater shows more mafic, mare-basaltic composition relative to its surrounding anorthosetic area. Impact ejecta of Picard crater that is located even in the mare basin reveals remarkably mafic composition in comparison to the average composition of Mare Crisium. The effective spatial resolution of the Apollo XRF map is about 30km after compilation of data from several orbits. ESA’s Smart-1 will map lunar surface composition with using array of newly developed SCD detectors in 2004-2005, but its ecliptic orbit of 300km x 10000km might limit its spatial resolution (Grande et al., 2002). Then the XRS onboard SELENE mainly aims at global