[P-PS08] Lunar Science and Exploration

[PPS08-P13] Surface geometry dependence of water creation by airless body-solar wind interactions investigated based on deuterium ion irradiation experiments on anhydrous minerals

*Yudai Arai¹, Tomoki Kimura¹, Morita Risa¹, Shun-ichiro Karato², Qinting Jiang², Yoshifumi Saito³, Yusuke Nakauchi⁴ (1.Tokyo University of Science, 2.Yale University, 3.Japan Aerospace Exploration Agency, 4.Ritsumeikan University)

Keywords:Solar Wind, Moon, Olivine, Ion irradiation

The water production and maintenance in the airless body universally take place on Moon, Mercury, and asteroids, which may have been a water source for the early Earth (Daly et al., 2021). The elucidation of the lunar and Mercury water creation process is of great importance for understanding the origin of water on Earth. On Mercury, observations with the neutron spectrometer onboard MESSENGER have suggested the water in the polar permanent shadow (Lawrence et al., 2013). On Moon, infrared spectroscopic observations with the M3 onboard Chandrayaan-1 have suggested the polar permanent shadow water (Li et al., 2018). Recently, observations with the infrared observatory SOFIA (Stratospheric Observatory for Infrared Astronomy) have detected a water molecule emission line at 6.1 μm in the high-latitude surface layers of Moon (Reach et al., 2023). These observations highly suggest the presence of water in the permanent shadows on Moon and Mercury. Two hypotheses have been proposed as the origin of water on Moon and Mercury (Schörghofer et al., 2021), which are the supply by bombardment of water-bearing meteorites, asteroids, and comets (Watson et al., 1961; Chabot et al., 2018) and the chemical reaction between the solar wind hydrogen ions and the oxygen atoms in surface minerals (Zeller et al., 1966; Jones et al., 2020). In the latter hypothesis, surface minerals exposed to hydrogen ion irradiation form OH groups, and further hydrogen ion irradiation produces H₂O (Jones et al., 2020) by detaching the formed OH groups. However, which hypothesis is quantitatively more significant as a water source is unresolved. To evaluate the solar wind hypothesis, some previous studies have quantified the water production rate by irradiating powder samples modeling the lunar and Mercury surface material with hydrogen and deuterium ions modeling the solar wind (e.g. Kitano, Kimura et al., JpGU, 2023; Morita, Kimura et al., JpGU, 2024). However, the dependence of water production rate on various surface morphologies (e.g. crystals, regolith, etc.), which have been observed on the lunar and Mercury surfaces, is still unresolved.

In this study, deuterium ions are irradiated onto samples that model the bulk crystal structure of the lunar and Mercury surface materials to reproduce the solar wind hypothesis. The water production rate is compared with the results of the previous powder sample experiments to evaluate the surface topography dependence. Samples of anhydrous silicate minerals (olivine, (Mg, Fe)₂SiO₄), which are the lunar and Mercury representative surface minerals, were irradiated with deuterium ions at 5 keV after mirror polishing with colloidal silica. By measuring the pressures of D₂O and HDO during the deuterium irradiation, we evaluated only the water production due to irradiation eliminating the influence of the sample's attached H₂O water. Deuterium ions were irradiated at a flux of 5.66e+13 ion/cm²/s and a fluence of 6.11e+17 ion/cm². The partial pressure of H₂O reached a peak of 3.3e-6 Pa 10 minutes after the start of irradiation and then settled down to the background of 3.0e-6 Pa after 150 minutes. HDO increased from the background of 1.4e-7 Pa and reached 1.5e-7 Pa after 60 minutes. D₂O increased to 4.0e-8 Pa immediately after the irradiation start, but after 10 min it settled to 4.0e-8 Pa, which was the same as the background value, and it was below the background partial pressure value after 90 min. The production rate of H₂O, HDO, and D₂O in this experiment, which was almost on the background level, was found to be significantly lower than the water production rate in the powder sample experiment (yield: 2.9e-2 D₂O/incident ion, Morita, Kimura et al., JpGU, 2024). This can be attributed to differences in the surface area of the sample material interacting with the irradiated particles. The powder sample has a larger interacting area than the bulk crystal sample and, accordingly, may lead to a possible higher water production rate. In the future, the surface area dependence will be quantitatively evaluated by modeling the surface morphology, calculating the area ratio between the powder and bulk crystal samples, and correcting the water production rate. This presentation will report on the current status of our study.