Contrary to previous assumptions of the Moon being an inert object in space, recent lunar missions have revealed that the Moon appears bright when viewed in energetic neutral atoms (ENAs). These ENAs are the result of solar wind plasma impinging freely onto the lunar surface, where (i) a significant part of the plasma is neutralized and reflected back to space as ENAs and where (ii) it liberates lunar surface atoms as ENAs through a process called sputtering. Together, these two processes lead to the Moon emitting ~240 metric tons of hydrogen and ~50 metric tons of oxygen into interplanetary space per year (McComas et al. 2009; Vorburger et al. 2013, 2014). Lunar ENAs have been measured by ENA instruments on board NASA’s Interstellar Boundary Explorer (IBEX) (McComas et al. 2004) and ISRO’s Chandrayaan-1 (Goswami and Annadurai 2009) under different plasma conditions, i.e., when the Moon is fully exposed to the solar wind, when the Moon is located within Earth’s magnetosheath, and when...
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Vorburger, A., Wurz, P. (2019). Lunar Atmosphere, Energetic Neutral Atoms. In: Cudnik, B. (eds) Encyclopedia of Lunar Science. Springer, Cham. https://doi.org/10.1007/978-3-319-05546-6_221-1
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