5:15 PM - 6:45 PM
[PPS09-P18] Water molecule creation by solar wind on the Moon's surface modeled by deuterium irradiation to anhydrous mineral
Keywords:the Moon
Water production and maintenance processes in the lunar surface layer are crucial, for human exploration resources and understanding the origin of water on Earth. Observations by the Infrared Observatory SOFIA (Reach et al. 2023) have detected 6.1 μm emission line spectra characteristic of water molecules in the lunar high-latitude surface layer. Return samples from the 1.1-3.2 billion-year-old region of Chang'e-5 have revealed traces of hydration within glass beads formed by meteorite impacts (He et al. 2023). The LCROSS mission has found water ice in the lunar surface layer in the permanent shadow of the South Pole (Colaprete et al. 2010). Two processes have been proposed as the origin of lunar surface water (Schörghofer et al. 2021); the supply of water by meteorite impacts (Watson et al. 1961) and the chemical reaction between hydrogen ions in the solar wind and surface rocks (Zeller et al. 1966). In the latter process, surface rocks irradiated with hydrogen ions form the hydroxyl. The H2O molecule is likely formed from the hydroxyl by further solar wind ion irradiation. However, the contribution of each theory to the total amount of lunar surface layer has not been quantified, leaving the question of which is the more likely source unresolved. Due to the complex physicochemical reactions in the lunar surface layer, the water production by solar wind irradiation cannot be accurately evaluated by theoretical models, and its contribution to the total amount of water remains unquantified.
Here we reproduce solar wind-induced water production by experiments irradiating deuterium ions onto samples modeling lunar surface minerals. Powder samples of Enstatite (Mg2Si2O6), an anhydrous silicate mineral that is a representative lunar surface mineral, were irradiated with deuterium molecular ions at 5 keV to simulate the solar wind irradiation. Deuterium was irradiated to separate the influence of the water adsorbed on the sample based on measuring the produced water such as D2O and HDO. The sample was irradiated for 36 and 300 minutes at a D2 equivalent flux of 5.2 × 1013 (cm-2s-1). The pressure of H2O, which is adsorbed water, reached a peak of 2.6 × 10-6 Pa at 25 min after the start of irradiation and then decreased to 3.0 × 10-7 Pa at 300 min. HDO, which is a product of deuterium irradiation, monotonically increased from the background and reached 4.0 × 10-8 Pa at 150 min. The D2O pressure also monotonically increased and reached 8.0 × 10-8 Pa after 300 min of irradiation; an increase to 4.0 × 10-8 Pa was also detected for the 36 min irradiation. In the mid-infrared reflectance spectrum analysis, an absorption structure of about 10% at 4.3 μm was observed after irradiation, which is included in the wavelength range of liquid phase D2O absorption band at 3.64-4.44 μm (NIST Chemistry Web Book). In the future, we are going to estimate the yield of D2O from the pressure measurements and derive the water production rate from surface minerals by solar wind irradiation. The infrared spectra will be also investigated in detail to explore the obtained absorption structure corresponding to OD groups bound. Based on these analyses, we will discuss the contribution of solar wind to the total amount of water accumulated in the lunar surface layer. Experiments with different mineral species and under more realistic conditions, such as temperature, will be conducted to clarify the parameter dependence of the water production efficiency. In this presentation, we report on the current status of the above.
Here we reproduce solar wind-induced water production by experiments irradiating deuterium ions onto samples modeling lunar surface minerals. Powder samples of Enstatite (Mg2Si2O6), an anhydrous silicate mineral that is a representative lunar surface mineral, were irradiated with deuterium molecular ions at 5 keV to simulate the solar wind irradiation. Deuterium was irradiated to separate the influence of the water adsorbed on the sample based on measuring the produced water such as D2O and HDO. The sample was irradiated for 36 and 300 minutes at a D2 equivalent flux of 5.2 × 1013 (cm-2s-1). The pressure of H2O, which is adsorbed water, reached a peak of 2.6 × 10-6 Pa at 25 min after the start of irradiation and then decreased to 3.0 × 10-7 Pa at 300 min. HDO, which is a product of deuterium irradiation, monotonically increased from the background and reached 4.0 × 10-8 Pa at 150 min. The D2O pressure also monotonically increased and reached 8.0 × 10-8 Pa after 300 min of irradiation; an increase to 4.0 × 10-8 Pa was also detected for the 36 min irradiation. In the mid-infrared reflectance spectrum analysis, an absorption structure of about 10% at 4.3 μm was observed after irradiation, which is included in the wavelength range of liquid phase D2O absorption band at 3.64-4.44 μm (NIST Chemistry Web Book). In the future, we are going to estimate the yield of D2O from the pressure measurements and derive the water production rate from surface minerals by solar wind irradiation. The infrared spectra will be also investigated in detail to explore the obtained absorption structure corresponding to OD groups bound. Based on these analyses, we will discuss the contribution of solar wind to the total amount of water accumulated in the lunar surface layer. Experiments with different mineral species and under more realistic conditions, such as temperature, will be conducted to clarify the parameter dependence of the water production efficiency. In this presentation, we report on the current status of the above.