著者

春山 純一 橋爪 光 鹿山 雅裕 長岡 央 仲内 悠祐 Haruyama Junichi Hashizume Ko Kayama Masahiro Nagaoka Hiroshi Nakauchi Yusuke

出版者

宇宙航空研究開発機構(JAXA)

雑誌

宇宙航空研究開発機構研究開発報告 = JAXA Research and Development Report (ISSN:24332216)

巻号頁・発行日

vol.JAXA-RR-18-001, pp.1-8, 2018-06-20

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将来の有人月面活動を目指した探査の重要課題の一つとして, 「月の極の水氷」の存否, その量の調査が挙げられる. 月の極に水氷が期待されるのは, 月に対して, いくつかの水の供給源が存在する可能性があり, 月面に供給された水が, 濃集し, かつまたは安定的に存在できる可能性が, 極低温域となる永久陰にあるからである. しかし, 隕石や彗星の落下衝突した際の衝撃加熱で失われることもあるだろう. 更に, 現在の永久陰は過去においては永久陰でなかった可能性が指摘されており, 地質学的に長期間永久陰となっていて水氷を集積できる場所は無いとも言われる. 実際, これまでの探査機による観測では, 水氷発見を報告するものがあるが, 月の極の水氷の存在について決定的証拠を出したといえるものがない. データや解釈を整理してみると, 数%もの「水氷」の存在というデータの解釈には多くの難点があるともいえる. 一方で, 太陽風起源の水素が月極域に打ち込まれ留まっている可能性もある. 最新の中性子分光計による計測結果だと, 最も濃集しているところ(40K以下の永久陰など)で470ppm程度と報告されているが, この水素濃集見積もりは(水の形を取るにしても)妥当なところではないかと思われる. 月は, 人類が宇宙へと活動の場を拡げていくときの橋頭堡で有り, その探査は重要不可欠である. だからこそ, 今後, 有人月面活動を目指した探査について議論, 企画していくうえで, 最新の科学的知見を十分に加味, 考慮した上で進めていくことが必要である.

著者

長岡 央 Fagan Timothy 鹿山 雅裕 長谷部 信行

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2016年大会

巻号頁・発行日

2016-03-10

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Based on previous study of lunar returned samples and meteorites, the main suites of pristine nonmare igneous rocks have been classified into the following four types: (1) ferroan anorthosite (FAN) or ferroan anorthositic-suite (FAS), (2) magnesian suite (Mg-suites), (3) alkali-anorthosite-suite and (4) KREEP basalt and possibly related rocks such as quartz-monzogabbro (QMG) /monzodiorite (QMD), granite and felsite. The latest suite type, the evolved rock samples related to KREEP, may have been derived from residue of the lunar magma ocean (urKREEP), or from low degrees of partial melting or some other process to account for their high incompatible trace element (high-ITE) compositions. Granite and felsite have Th-rich compositions (10 to 60 ppm), and such lunar samples with bulk SiO2 content of >60wt% originated from silicic volcanic or exposed intrusive material. Recent global remote sensing data have presented several candidates of silicic volcanism over the Moon based on indicators such as ITE-rich compositions, dome-like topography, characteristic infrared spectra (Christiansen Feature), and high albedo. Silica-rich, broadly granitic samples have been identified in lunar returned samples and lunar meteorites, but are rare.Lunar meteorite Northwest Africa (NWA) 2727 is a breccia paired with NWA 773 and the other meteorites of the NWA 773 clan. An olivine cumulate gabbro (OC) is common to most of these lunar meteorites within the NWA 773 clan; in fact NWA 2977 and 6950 consist entirely of OC lithology. However, in addition to the OC lithology, several clast types, including in olivine phyric basalt, pyroxene phyric basalt, pyroxene gabbro, ferroan symplectite, and alkali-rich-phase ferroan (ARFe) rocks have been discovered from the NWA 773 clan. The ARFe clasts have K-feldspar and/or felsic glass, a silica phase and minerals rich in incompatible elements such as merrillite. In this work, we characterize a felsic clast in NWA 2727 and compare our results with other lunar samples to discuss silicic volcanism.A polished thin section (PTS) of NWA 2727 was investigated by a combination of petrographic microscopy and electron probe micro-analysis. The NWA 2727 breccia includes a variety of large-scaled lithic clasts (>1mm) including: OC, ferrogabbro, pyroxene-phyric basalt, and the felsic igneous clast. The felsic clast has a modal composition of 37% silica, 34% plagioclase, 14% K-feldspar, 6% high-Ca pyroxene, 5% fayalite, 3% Ca-phosphate, 1% ilmenite, and traces of troilite and chromite. Feldspar compositions of the plagioclase are near An85-90. Two compositional types of pyroxene were identified—one near hedenbergite (Wo46Fs53, Mg#=1 [calculating Mg# as Mg/(Mg+Fe)x100]) and the other with zoning and more magnesian compositions (Wo25-30Fs55-65, Mg#=8~20). The K-feldspar is also zoned with variable concentrations of Ba, clearly detected in elemental X-ray maps (quantitative analyses of Ba are planned). The abundance of silica + feldspars (>80 mode%), the high proportion of K-feldspar to plagioclase, and the very ferroan compositions of mafic minerals attest to the felsic composition of this clast. Subhedral-euhedral olivine crystals up to 0.3 mm in maximum length are preserved, and silica and K- and Ba-feldspar occur in elongate parallel crystals indicating an igneous origin. These observations indicate that this clast was derived from silica-rich magma.Silicic volcanism is also interesting from the viewpoint of landing site candidates for future lunar landing mission. Global gamma-ray observations have presented several high-Th regions in PKT, but the main lithology of the Th-rich regions remains a subject of dispute; possibilities include mafic impact-melt breccia, KREEP basalt, QMD, and felsite/granite. If a lander/rover mission to a high-Th region is equipped for analysis of major elements, in situ analyses on the Moon can be compared with silica-rich samples such as the felsic clast in NWA 2727.