Daily variations of carbon, nitrogen and oxygen ions in a thin lunar atmosphere

Abstract On-site observations of volatile elements in the lunar exosphere provide robust evidence that volatiles such as carbon, nitrogen and oxygen are stored on the lunar surface and are being supplied to the exosphere. However, their sources and dynamics remain poorly understood. Here, we report the chemical composition and its time variations in the lunar exosphere using an Ion Mass Analyzer onboard the KAGUYA satellite. We first discovered sporadic changes in the C+/O+ ratio occurring immediately after meteoroid showers. We also observed daily variations in the C+/O+ and N+/O+ ratios, suggesting two possible sources: the reservoir with a higher N/C ratio than the solar value and CO/CO2 gasses. Our findings indicate that the crucial mechanism for the production of C⁺, N⁺, and O⁺ ions during the daytime is the sputtering of the nitrogen-rich reservoir by the solar wind, whereas micrometeoroid impacts and photon-stimulated desorption are not dominant processes. These new insights shed light on the reservoirs on the lunar surface and the 100-km-scale dynamics of volatile ions, supporting future human activities on the Moon.