Composition, Structure and Origin of the Moon

Paolo A. Sossi, Miki Nakajima, Amir Khan
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Abstract

Here we critically examine the geophysical and geochemical properties of the Moon in order to identify the extent to which dynamical scenarios satisfy these observations. New joint inversions of existing lunar geophysical data (mean mass, moment of inertia, and tidal response) assuming a laterally- and vertically homogeneous lunar mantle show that, in all cases, a core with a radius of 300$\pm$20 km ($\sim$0.8 to 1.5 % the mass of the Moon) is required. However, an Earth-like Mg# (0.89) in the lunar mantle results in core densities (7800$\pm$100 kg/m$^3$) consistent with that of Fe-Ni alloy, whereas FeO-rich compositions (Mg# = 0.80--0.84) require lower densities (6100$\pm$800 kg/m$^3$). Geochemically, we use new data on mare basalts to reassess the bulk composition of the Moon for 70 elements, and show that the lunar core likely formed near 5 GPa, 2100 K and $\sim$1 log unit below the iron-w\"ustite buffer. Moreover, the Moon is depleted relative to the Earth's mantle in elements with volatilities higher than that of Li, with this volatile loss likely having occurred at low temperatures (1400$\pm$100 K), consistent with mass-dependent stable isotope fractionation of moderately volatile elements (e.g., Zn, K, Rb). The identical nucleosynthetic (O, Cr, Ti) and radiogenic (W) isotope compositions of the lunar and terrestrial mantles, strongly suggest the two bodies were made from the same material, rather than from an Earth-like impactor. Rb-Sr in FANs and Lu-Hf and Pb-Pb zircon ages point Moon formation close to $\sim$4500 Ma. Taken together, there is no unambiguous geochemical or isotopic evidence for the role of an impactor in the formation of the Moon, implying perfect equilibration between the proto-Earth and Moon-forming material or alternative scenarios for its genesis.
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月球的组成、结构和起源
在此,我们对月球的地球物理和地球化学特性进行了批判性研究,以确定满足这些观测结果的动力学方案的程度。对现有月球地球物理数据(平均质量、惯性矩和潮汐响应)进行新的联合反演,假设月球地幔横向和纵向均质,结果表明,在所有情况下,都需要一个半径为 300$pm$20 km 的内核($\sim$0.8-1.然而,月幔中类似地球的 Mg#(0.89)会导致与铁镍合金一致的内核密度(7800$\pm$100 kg/m$^3$),而富含氧化铁的成分(Mg# = 0.80--0.84)则需要较低的密度(6100$\pm$800 kg/m$^3$)。在地球化学方面,我们利用关于赤泥玄武岩的新数据重新评估了月球上70种元素的组成,结果表明月核可能在5 GPa、2100 K和低于铁-乌斯托缓冲区1个对数单位的地方形成。此外,与地球地幔相比,月球上挥发度高于锂的元素消耗殆尽,这种挥发损失很可能是在低温(1400K/pm100K)下发生的,与中度挥发元素(如Zn、K、Rb)的质量依赖性稳定同位素分馏相一致、月幔和地幔的核合成(O、Cr、Ti)和辐射成因(W)同位素组成完全相同,这有力地表明这两个天体是由相同的物质构成的,而不是由类似地球的撞击物构成的。FANs中的Rb-Sr以及Lu-Hf和Pb-Pb锆石的年龄表明月球的形成接近于4500Ma。总之,没有明确的地球化学或同位素证据表明撞击物在月球形成过程中的作用,这意味着原地球和月球形成物质之间完全平衡,或者月球形成的其他情况。
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