Several decades ago scientists discovered that the Moon, long thought to have no atmosphere, actually does have an extremely thin exosphere. Scientists generally believe that the ions that make up the lunar exosphere are generated at the Moon's surface by interaction with solar photons, plasma in the Earth's magnetosphere, or micrometeorites. However, scientists have been uncertain about which processes are the main contributors of lunar exosphere ions.
Using instruments aboard the Japanese lunar orbiter SELENE (also known as Kaguya), Tanaka et al. made the first spacecraft-based observations of the lunar exosphere when the Moon was inside Earth's magnetosphere. They detected ions of several elements at 100-kilometer (62-mile) altitude above the lunar surface.
Previous studies have detected Moon-originating ions when the Moon was in the solar wind; this new study is the first to detect such ions when the Moon was not affected by solar wind particles or the Earth's magnetotail plasma.
The results, which provide new evidence about the origin of the lunar exosphere, are consistent with the idea that solar photon-driven processes dominate in supplying exosphere components.
The research is published in Geophysical Research Letters. Authors include Takaaki Tanaka, Yoshifumi Saito, Shoichiro Yokota, Kazushi Asamura, Masaki N. Nishino, Hideo Tsunakawa, Masaki Matsushima, and Futoshi Takahashi: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan; Hidetoshi Shibuya: Department of Earth Science, Kumamoto University, Kumamoto, Japan Hisayoshi Shimizu: Earthquake Research Institute, University of Tokyo, Tokyo, Japan; Masaki Fujimoto and Toshifumi Mukai: Institute of Space and Astronautical Science, Japan; Aerospace Exploration Agency, Sagamihara, Japan; and Toshio Terasawa: Department of Physics, Tokyo Institute of Technology, Tokyo, Japan.
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