Apollo 11月壤样品中太阳风成因的水及其意义

Solar wind-implanted water in Apollo 11 lunar soils and its implications

为研究太阳风成因水在月表低纬度地区的分布特征,本文对中国科学院地质与地球物理研究所博物馆珍藏的一份Apollo 11月壤样品开展了单颗粒原位纳米离子探针H同位素和水含量深度剖面分析。结果表明月壤颗粒表层(<200nm)具有较高的水含量(剖面最大水含量变化范围为0.35%~1.59%,平均值为0.82%),该结果与前人对Apollo样品的报导类似。除一颗斜长石的H同位素组成(δD=-262‰)落在月幔范围内,其余颗粒表层均非常贫D(δD变化范围为-987‰~-642‰)。该强烈贫D的同位素组成与太阳风一致,完全不同于地球大气水,不仅证明这些样品的月球来源,并且主要是太阳风注入的贡献。矿物颗粒之间的水含量和δD值变化,很可能与太阳风注入后的扩散丢失程度不同有关。另外,我们观察到橄榄石、单斜辉石和长石的水含量剖面比较类似,整体呈现随深度逐渐递减的趋势。相比之下,玻璃质颗粒的水含量剖面呈现随深度先上升再降低的峰形特征,峰位在25~43nm深度。这两者之间的差异很可能跟H在矿物和玻璃中的扩散速率、辐射损伤层等的差异有关。本项研究也证明,即使对于H这样易于受地球污染的元素,在普通条件下(北京、瓶中密封)经过长达50多年的时间,仍能很好地保存注入月壤颗粒中的太阳风信息。但是低温、干燥和真空/惰性气体环境仍是长期保存的有利条件。

To investigate the distribution of solar-wind implanted H in the low latitude regions of the lunar surface,we carried out NanoSIMS depth-profiling measurements of H isotope and water content on Apollo 11 soils that were stored in the Geological Museum,Institute of Geology and Geophysics,Chinese Academy of Sciences.The results show that the surface of all lunar grains(<200nm)has a high water content(the maximum water content varying from 0.35%to 1.59%),which is similar to the results of previous studies on Apollo 11 samples.Except for one plagioclase(A11-021-1)whose surface H isotope composition(δD)is-262‰,the other grains have very lowδD values ranging from-987‰to-642‰.These characteristics indicate that the high water content in the surface layer of lunar grains is the result of solar-wind implantation rather than terrestrial contamination.The different water contents andδD values may be caused by H diffusion after initial solar-wind implantation.In addition,we find that the water-content depth profiles of olivine,clinopyroxene and plagioclase are similar,and their water contents gradually decrease with increasing depth.By contrast,two glassy grains show simple bell-shaped water-content distributions as a function of depth that peak ca.25~43nm below the grain surface.Such a phenomenon may be related to the difference of H diffusion rate in crystalline grain and glass and the difference of the damaged layers.As a specific study,this work also suggests that low temperature,dry and vacuum/inert gas environments are still favorable conditions for long-term preservation of extraterrestrial samples.