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

Abstract

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 low δD 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 and δD 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.