[P-PS06] Planetary Sciences

[PPS06-03] Terrestrial 40Ar implantation into lunar regolith via Earth wind

*Hiroya Abe¹, Akifumi Nakayama², Ryoya Sakata³, Kanako Seki⁴, Shotaro Sakai³, Hiroyuki Shinagawa⁵, Naoki Terada³, Ko Hashizume⁶, Hidenori Genda¹ (1.Earth-Life Science Institute, Tokyo Institute of Technology, 2. Department of Physics, Rikkyo University, 3.Department of Geophysics, Graduate School of Science, Tohoku University, 4.Graduate School of Science, University of Tokyo, 5.International Research Center for Space and Planetary Environmental Science, Kyushu University, 6.Faculty of Science, Ibaraki University)

Keywords:Moon, Regolith, Atmospheric escape, Archean

Some volatile elements in lunar regolith are implanted from external sources and preserved over geological timescales. Apollo samples exposed on the lunar surface from 3 to 4 billion years ago (Ga) exhibit similar 40Ar/36Ar ratios of approximately 12. In contrast, lunar meteorites exposed during the same period show 40Ar/36Ar ratios ranging from 2 to 18, indicating significant variability. This suggests that all Apollo samples and some lunar meteorites acquired a lot of 40Ar. We propose that the escape of 40Ar-rich terrestrial atmosphere into space (the Earth wind) and their subsequent implantation into the Moon could account for the high 40Ar/36Ar ratios on the lunar surface. Due to tidal locking, the Moon always faces the same side toward Earth, allowing the Earth wind to preferentially impact the lunar nearside. Consequently, 40Ar from the Earth wind is expected to be predominantly supplied to the nearside. Conversely, 36Ar, primarily originating from the solar wind, is distributed globally across the lunar surface. Apollo samples were exclusively collected from the lunar nearside, whereas lunar meteorites can originate from both the nearside and farside. The anisotropic supply of 40Ar via the Earth wind provides a natural explanation for the high 40Ar/36Ar ratios observed in all Apollo samples and some lunar meteorites. To test this hypothesis, we simulated the flux of 40Ar delivered to the lunar surface by the Earth wind under Archean atmospheric and solar XUV conditions from 3 to 4 Ga. Using a one-dimensional thermospheric model and an exospheric kinetic gas model, we calculated the atmospheric structure for varying CO₂ partial pressures, eddy diffusion coefficients, and solar XUV fluxes. Assuming that 40Ar⁺ ions generated above the exobase or ionopause are stripped by the solar wind, we estimated the 40Ar flux to the lunar surface. Our results demonstrate that, with an eddy diffusion coefficient five times that of present Earth’s, the Earth wind makes 40Ar/36Ar ratios on the nearside five higher than on the farside. This finding successfully explains the lower range of the observed 40Ar/36Ar ratios, which range from 5 to 15 in Apollo samples and some lunar meteorites. Furthermore, the results suggest that CO₂ constituted more than 50% of the Archean atmosphere between 3.5-4.0 Ga and gradually decreased during 3.0-3.5 Ga, supporting the conventional theory that there was plenty of CO₂ in the Archean atmosphere and that it has decreased over the present. To verify the 40Ar contribution from the Earth wind, we need to analyze samples with old exposure ages from the lunar farside, possibly brought back by Chinese lunar sample return mission Chang’e-6. Such samples could provide critical evidence for the proposed anisotropy in 40Ar supply. Future lunar missions and additional analyses may refine our understanding not only of the Moon, but also of the Archean Earth’s atmospheric composition and its evolution.