1,6,7,8,12- 1Yamagata University, Institute of Arts and Sciences, Yamagata, Japan (kei.masunaga@cc.yamagata-u.ac.jp)
- 2Graduate School of Science, Osaka University, Toyonaka, Japan
- 3Institute of Space-Earth Environmental Research, Nagoya University, Nagoya, Japan
- 4Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo, Japan
- 5Department of Geophysics, Graduate School of Science, Tohoku University, Sendai, Japan
- 6Faculty of Environment and Information Studies, Keio University, Fujisawa, Japan
- 7Data Analysis Center for Geomagnetism and Space Magnetism, Graduate School of Science, Kyoto University, Kyoto Japan
- 8Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan
- 9Institute for Planetary Materials, Okayama University, Misasa, Japan
- 10Earthquake Research Institute, The University of Tokyo, Tokyo, Japan
- 11Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, Fukuoka, Japan
- 12Department of Earth and Planetary Sciences, Institute of Science Tokyo, Tokyo, Japan
Water on the Moon is crucial for understanding its origin and evolution and is also of interest for future human exploration. Recent observations show that water ice and hydroxyl are mainly concentrated in permanently shadowed regions near the lunar poles, and these species have also been detected in the lunar exosphere. However, how exospheric water is distributed and how it is coupled to the surface remain poorly constrained.
Using the Ion Mass Analyzer onboard the Kaguya spacecraft, we report the detection of water ions originating from the lunar exosphere. We identify statistically significant low-energy water ions and characterize their latitudinal distribution, showing that their flux is enhanced in the polar regions compared with lower latitudes, consistent with previous remote-sensing observations of lunar water. From the observed flux, we estimate near-surface exospheric water density, comparable to previous in-situ measurements.
These results demonstrate that the spatial distribution of surface water is reflected in the exospheric ion population, indicating a dynamically coupled surface–exosphere system. Ion measurements therefore provide a powerful tool for investigating the release, transport, and loss of water on airless bodies.
How to cite: Masunaga, K., Yokota, S., Harada, Y., Keika, K., Terada, N., Sakai, S., Matsuoka, A., Nakagawa, H., Arai, Y., Saito, Y., Nishino, M., Ohtake, M., Asamura, K., Shimizu, H., Takahashi, F., and Matsushima, M.: Polar enhancement of lunar water ions observed by Kaguya: Implications for surface-exosphere coupling, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-424, https://doi.org/10.5194/epsc2026-424, 2026.
