¹Yuta Otsuki, ¹Ken-ichi Bajo, ^1,2Tomoya Obase, ³Rainer Wieler, ¹Hisayoshi Yurimoto
Earth and Planetary Science Letters 683, 120000 Link to Article [https://doi.org/10.1016/j.epsl.2026.120000]
¹Department of Natural History Sciences, Faculty of Science, Hokkaido University, Sapporo, Hokkaido, 060-0810, Japan
²Department of Earth and Planetary Sciences, Institute of Science Tokyo, Meguro-ku, Tokyo, 152-8551, Japan
³Department of Earth and Planetary Sciences, ETH Zürich, 8092 Zürich, Switzerland
Copyright Elsevier

Lunar soils have been studied since the NASA/Apollo missions. Noble gas studies suggested that the soils contain two components of solar noble gases: solar wind (SW) implanted within <100 nm depths, and an isotopically heavier component attributed to solar energetic particles (SEPs) implanted to larger depths. Data from the NASA/Genesis mission revealed, however, that the isotopically heavy signature is a result of isotopic fractionation of solar wind ions upon implantation, thus the acronym fSW for fractionated solar wind. However, previous analyses lacked a quantitative depth scale for fSW. Here we report high-spatial-resolution depth profiling of He, Ne, and Ar from the SW in three ilmenite grains from Apollo 17 soil 71501, using a time-of-flight secondary neutral mass spectrometry. Noble gases are highly concentrated within the topmost 100 nm depth from the surface. The 20Ne/22Ne ratio decreases with increasing depth from the SW value of ∼14 to ∼11 at ∼50 nm depth. Our quantitative depth profiles give strong support that the isotopically heavy Ne observed in lunar samples is fSW, allowing to further refute the SEP hypothesis. The 4He/20Ne ratio from the three grains and 20Ne/36Ar from one of them are lower than the present-day SW composition, indicating a selective loss of light noble gases. Based on numerically simulated profiles, we suggest that this loss is caused by a defect-mediated effusion process.