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Lunar farside Th distribution measured by Kaguya gamma-ray spectrometer

https://doi.org/10.1016/j.epsl.2012.05.007Get rights and content

Abstract

Kaguya gamma-ray spectrometer measured thorium (Th) distribution on the lunar farside with the highest sensitivity among past gamma-ray remote sensing missions. The newly obtained Th map has revealed that two regions near the equator on the farside have the lowest Th abundances. We found that the variation of the Th abundance perfectly correlates with the crustal thickness in the farside and the southern nearside, and it could be a result of the crystallization of the lunar magma ocean.

Highlights

► Kaguya gamma-ray spectrometer accurately measured Th distribution of the lunar surface. ► We found that the two regions near the equator on the farside have the lowest Th abundances. ► We found the perfect correlation between the Th abundance and the crustal thickness. ► The correlation could be vestige of the crystallization of the lunar magma ocean.

Introduction

It has long been known that the anorthositic crust of the Moon was formed by crystallization and floatation of low density plagioclase from a lunar magma ocean (LMO) about ∼4.6–4.4 Ga (Shearer and Papike, 1999, Wieczorek et al., 2006), but the actual formation process and the relationship between the earliest crust directly crystallized from the LMO and the present lunar geological properties are still uncertain. Most of the farside and the southern nearside (latitude=60-90°S) of the Moon are currently regarded as the largest geologically distinctive province named Feldspathic Highland Terrane (FHT) (Jolliff et al., 2000). The important characteristic of the FHT is that the surface regolith is depleted in Th as revealed by the gamma-ray spectrometers onboard Apollo 15 and 16 (AGRS) (Metzger et al., 1977, Trombka et al., 1973) and Lunar Prospector (LPGRS) (Lawrence et al., 2000, Lawrence et al., 1998). This low Th characteristic is probably attributed to the incompatibility of Th during the crystallization stage of the LMO. Th would be hardly incorporated into crystal structures of major rock forming silicates, such as plagioclase, because of the large ionic radius of Th (Korotev, 1998). Thus, the Th abundance of the anorthositic floatation cumulate in the LMO that would constitute the surface of the FHT became low, while the residual liquid was enriched in Th (Wieczorek et al., 2006). According to this scenario, the earliest crust crystallized from the LMO had the lowest Th abundance, and the Th concentration in the crust gradually increased with the degree of the crystallization of the LMO (cf. Snyder and Taylor, 1993). Some previous studies indicate that there is an inverse correlation between the Th abundance and the crustal thickness (Lawrence et al., 2000, Metzger et al., 1977, Trombka et al., 1973), which might show the crystallization process of the LMO. On the other hand, there has been a claim that Th-rich ejecta due to Imbrium impact onto the high Th region known as Procellarum KREEP Terrane (PKT; Jolliff et al., 2000) of the nearside covers the FHT (Haskin, 1998, Lawrence et al., 1998). If this is the case, the Th distribution on the farside is not connected with the LMO crystallization. The final conclusion has been postponed.

It has been difficult to measure the distribution of Th precisely and accurately within the FHT by a gamma-ray remote sensing because of the low Th abundance of the FHT. Recently, Kaguya gamma-ray spectrometer (KGRS; Hasebe et al., 2008) onboard Japanese lunar explorer (SELENE, Kaguya; Kato et al., 2010) has obtained gamma-ray spectra of the global lunar surface with the highest energy resolution among the past lunar gamma-ray spectrometers. The energy resolution of the KGRS is about ten times as high as the previous lunar gamma-ray spectrometers (Hasebe et al., 2009, Kobayashi et al., 2010a). The excellent energy resolution of the KGRS lowers the detection limit of a characteristic gamma ray from Th and helps to distinguish the Th peak more clearly from the background continuum. In addition, the KGRS is able to identify at least three characteristic gamma rays from the lunar Th with the energies of 239, 911 and 2615 keV (see Fig. 1). This provides us with the method for checking the reliability of a measured Th distribution by comparing each Th map independently derived from the three Th peaks (Kobayashi et al., 2010a). The KGRS is able to provide the most reliable Th map on the FHT. Here we produce the Th maps by using the KGRS data and discuss the Th distribution in the FHT. Then the relationship between the Th abundance and the crustal thickness in the FHT is investigated.

Section snippets

Method

The energy spectra of gamma rays from the lunar surface were obtained by the KGRS from the lunar orbit of Kaguya. The data obtained from December 14, 2007 to February 16, 2008 (Period 1) and from July 7 to December 14, 2008 (Period 2) at 100 km altitude of the lunar orbit were used. The total observation time was 111.4 days (live time). The Th peaks of 239, 911 and 2615 keV (Fig. 1) were analyzed to obtain the global distribution of Th. The gamma-ray peaks of 2615 keV and 911 keV are caused by 208

Th distribution outside of the PKT and the inverse correlation between the Th and the crustal thickness

Here we report Th distribution measured by the KGRS with a particular focus on the outside of the PKT, i.e. FHT and South-Pole Aitken basin (SPA). The Th distributions determined by analyzing the 2615 keV, 911 keV and 239 keV are shown in Fig. 2a, b, and c, respectively. The three Th maps are consistent with each other and reveal several characteristics of Th distribution on the FHT. Low Th region locates in low- and mid-latitude on the farside and Th enhancement is observed in SPA. The two

Conclusion

We have obtained the Th distributions on the lunar farside by using KGRS, which has the highest sensitivity to the lunar farside Th so far. The newly obtained map shows that there are two lowest Th regions within the farside, and two moderately high Th belts. We found the Th distribution perfectly corresponds to the crustal thickness variations and there is an inverse correlation between the Th abundance and the crustal thickness. The inverse correlation could be the vestige of the LMO

Author contributions

N.H., E.S., C.U., and R.R. suggested the original design of Kaguya Gamma-ray Spectrometer. N.H., M.H, M.K., S.K., Y.K., N.Y., E.S., C.U., O.G., O.F., R.R. and students in Waseda University discussed the primary calibration method and developed the basic data processing tools. S.K. conducted the calibration and the analyses of data for this paper. S.K., Y.K., T.M., H.T, and O.G. especially contributed to the data interpretation and the paper writing. All the authors including K.K. and Y.I.,

Acknowledgments

The SELENE (Kaguya) mission was conducted by the Japan Aerospace Exploration Agency (JAXA). S. Kobayashi and T. Morota were supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. This work was partly supported by the Japan Society for the Promotion of Science under a Grant-in-Aid for JSPS Fellows (2110122). The participation of the French team members to the KGRS mission is supported by CNES and of the American team member is supported by NASA.

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    1

    Present address: National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan.

    2

    Present address: Graduate School of Environmental Studies, Nagoya University, Rigakukan 203-1, Furocho, Chigusa-ku, Nagoya city, Aichi 464-8601, Japan.

    3

    Present address: Planetary Science Institute, 6509 Caballero Pkwy. NW, Los Ranchos de Albuquerque, NM 87107, USA.

    4

    Present address: National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba 305-8506, Japan.

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