13:45 〜 14:00
[PPS02-01] Moganite in a lunar meteorite as a trace of H2O ice in the lunar regolith
★Invited Papers
キーワード：モガナイト、月隕石、氷、レゴリス
A large amount of H2O ice has been observed on the regolith surface near the lunar poles by remote sensing spacecraft. However, only sparse evidence for H2O ice beneath the top few millimeters of the regolith exists because neutron spectrometers cannot distinguish forms of H and near-infrared spectrometers are only sensitive to the optical surface. Furthermore, traces of H2O ice have not yet been found from the lunar samples. Here, 13 different lunar meteorites with various lithologies have been investigated using micro-Raman spectrometry, electron microscopy, and synchrotron angle-dispersive x-ray diffraction (SR-XRD) to find a trace of H2O ice in the subsurface. Consequently, moganite has been discovered from the KREEP (high potassium, rare-earth element, and phosphorous)-like gabbroic-basaltic brecciated meteorite NWA 2727 that may originate from the local crater sites within the Procellarum terranes.
Moganite (monoclinic SiO2 phase that belongs to the I2/a space group) coexisting with coesite and stishovite (high-pressure SiO2 polymorphs) was discovered only in NWA2727 by the micro-Raman spectrometry. NWA 2727 is mainly composed of gabbroic and basaltic clasts containing their breccia matrix, where only the breccia matrix contain moganite-bearing silica micrograins (2 to 13 mm in radius) between the constituent minerals (olivine, pyroxene and plagioclase). The SR-XRD analyses of these silica micrograins in the breccia matrix indicate a characteristic diffraction pattern that is in good agreement with the structure of moganite. However, no moganite was discovered from the basaltic and gabbroic clasts of NWA 2727 as well as in the other lunar meteorites selected here. Transmission electron microscopy of the silica micrograins demonstrates the aggregates of numerous euhedral or subhedral moganite nanoparticles (average radius of 4.5 nm).
Moganite was discovered in only one of those 13 samples. If terrestrial weathering had produced moganite in the lunar meteorites, there should be moganite present in all the samples, especially lunar meteorites fell to Earth around the same time (NWA 2977, 3333, and 6950 as measured here), but this was not the case. Furthermore, many previous researches on natural occurrences and laboratory experiments conclude that moganite is restricted to formation by precipitating from alkaline silicic acid (H4SiO4) water under high-pressure consolidation at >100 MPa, which is distinctly different environment from the desert that NWA 2727 found. A part of the moganite had changed into the coesite and stishovite, indicating their formation through heavy impact collisions on the Moon. These facts confirm our theory that it could not have formed in the desert.
Formation of lunar moganite can be interpreted based on the present and previous works as follows. Alkaline water-bearing carbonaceous chondrite collisions delivered abundant alkaline water to the lunar surface at <2.67 Ga. After the collisions, the delivered water was captured as fluid inside the breccia during the shock-induced consolidation. On the sunlit surface, lunar moganite precipitated from this captured alkaline water. Simultaneously, such captured water got cold-trapped at the lunar regolith in the subsurface and may still remain as ice underneath the local crater sites within the Procellarum terranes. Lunar moganite coexists with coesite and stishovite, thereby implying that a trace of the subsurface water ice was brought from the Moon by the recent impact at <1–30 Ma. Our moganite-precipitation simulation modeling concludes a subsurface H2O concentration higher than the estimated bulk content of 0.6 wt % is expected to still remain as ice. This value is in excellent agreement with the concentrations of H2O ice on the poles by remote sensing spacecraft observation. Thus, the subsurface is expected to be the abundant and available water resource for future lunar explorations.
Moganite (monoclinic SiO2 phase that belongs to the I2/a space group) coexisting with coesite and stishovite (high-pressure SiO2 polymorphs) was discovered only in NWA2727 by the micro-Raman spectrometry. NWA 2727 is mainly composed of gabbroic and basaltic clasts containing their breccia matrix, where only the breccia matrix contain moganite-bearing silica micrograins (2 to 13 mm in radius) between the constituent minerals (olivine, pyroxene and plagioclase). The SR-XRD analyses of these silica micrograins in the breccia matrix indicate a characteristic diffraction pattern that is in good agreement with the structure of moganite. However, no moganite was discovered from the basaltic and gabbroic clasts of NWA 2727 as well as in the other lunar meteorites selected here. Transmission electron microscopy of the silica micrograins demonstrates the aggregates of numerous euhedral or subhedral moganite nanoparticles (average radius of 4.5 nm).
Moganite was discovered in only one of those 13 samples. If terrestrial weathering had produced moganite in the lunar meteorites, there should be moganite present in all the samples, especially lunar meteorites fell to Earth around the same time (NWA 2977, 3333, and 6950 as measured here), but this was not the case. Furthermore, many previous researches on natural occurrences and laboratory experiments conclude that moganite is restricted to formation by precipitating from alkaline silicic acid (H4SiO4) water under high-pressure consolidation at >100 MPa, which is distinctly different environment from the desert that NWA 2727 found. A part of the moganite had changed into the coesite and stishovite, indicating their formation through heavy impact collisions on the Moon. These facts confirm our theory that it could not have formed in the desert.
Formation of lunar moganite can be interpreted based on the present and previous works as follows. Alkaline water-bearing carbonaceous chondrite collisions delivered abundant alkaline water to the lunar surface at <2.67 Ga. After the collisions, the delivered water was captured as fluid inside the breccia during the shock-induced consolidation. On the sunlit surface, lunar moganite precipitated from this captured alkaline water. Simultaneously, such captured water got cold-trapped at the lunar regolith in the subsurface and may still remain as ice underneath the local crater sites within the Procellarum terranes. Lunar moganite coexists with coesite and stishovite, thereby implying that a trace of the subsurface water ice was brought from the Moon by the recent impact at <1–30 Ma. Our moganite-precipitation simulation modeling concludes a subsurface H2O concentration higher than the estimated bulk content of 0.6 wt % is expected to still remain as ice. This value is in excellent agreement with the concentrations of H2O ice on the poles by remote sensing spacecraft observation. Thus, the subsurface is expected to be the abundant and available water resource for future lunar explorations.