Lunar Water Formation by Solar Wind: Nuclear Reaction Analysis of Deuterium Ion Interactions with Lunar-Like Glass

LUNAR WATER FORMATION BY SOLAR WIND: NUCLEAR REACTION ANALYSIS OF DEUTERIUM ION INTERACTIONS WITH LUNAR-LIKE GLASS. J. Simčič1, S. Markelj1, M. Vrabec2, T. Sotelšek2, J. Medved3, A. Peschel4 and J. M. Neumann5, 1Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia, [email protected], 2University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Geology, 3University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Materials and Metallurgy, 4Technical University Munich, Professorship for Lunar and Planetary Exploration, 5University of Ljubljana, Faculty of Mathematics and Physics, Department of Physics. Introduction: The origin and transport of water on the Moon have been the subjects of many studies over the past few decades. Nevertheless, many questions remain unanswered [1], particularly the absence of a substantial subsurface water reservoir predicted by many models, which would enable water molecules to be observed in the tenuous lunar exosphere [2]. Recent findings from the analysis of impact glass beads, delivered to the Earth by the Chang’e 5 mission [3], suggest that this material, scattered across the surface of the moon, can generate and store significant amounts of water when exposed to solar wind bombardment. Furthermore, the water molecules contained within these beads are easily extractable during the lunar daytime, supporting the hypothesis that impact glass beads may serve as the missing reservoir supplying the lunar exosphere with water molecules. To investigate this process, we are conducting a series of laboratory experiments to quantify the ability of glass samples to produce and retain water and to measure the corresponding diffusion and extraction rates under temperature conditions dictated by the lunar diurnal cycle. Experiment: In our laboratory experiments, the samples are exposed to deuteron bombardment following a similar approach to that described in [4], with a total fluence of 1018 deuterium ions per cm² — equivalent to approximately 300 years of lunar surface exposure to solar wind protons. Deuterium is used to distinguish between water-group molecules generated by irradiation and those inherently present in the samples. Additionally, we utilize the D(3He,p)α nuclear reaction between the energetic 3He2+ beam and implanted deuterium, which produces MeV protons and alpha particles, enabling us to measure the concentration and depth profiles of implanted ions via Nuclear Reaction Analysis (NRA) and Rutherford Backscattering Spectroscopy (RBS). The irradiated samples undergo thermal cycling between -150°C and +150°C and are periodically analyzed using NRA and RBS. To obtain depth profiles of water-group molecules such as OD and D O, we₂ are working on employing MeV Time-of-Flight Secondary Ion Mass Spectrometry (MeV TOF SIMS) [5,6]. These analyses will allow us to investigate the formation, depth distribution, and migration of water-group molecules under conditions simulating the lunar diurnal cycle. The three Ion Beam Analytical (IBA) techniques employed for the analysis of irradiated samples are illustrated in Figure 1. The irradiation, thermal cycling, and analysis all take place within the same experimental station, ensuring that the samples remain in a continuous vacuum environment. Figure1: IBA techniques for analyzing irradiated samples. Each technique requires different energetic ion projectile: NRA – 3He, ERDA – 7Li, SIMS – 35Cl. Figure 2 shows recent results on deuterium depth profile measurement in an olivine sample, before and after thermal cycling between 300K and 600K. Samples: In addition to Earth analogues of impact glass beads, such as tectite and olivine, we produce our own lunar-like glass with minimal water content. A carefully selected mixture of metal oxides is melted in a high-power induction vacuum oven and rapidly quenched by pouring it into a liquid nitrogen-cooled copper vessel. This process prevents crystallization and ensures the formation of an appropriate glass structure. Our method replicates the conditions under which lunar impact glass beads form — where regolith is instantaneously melted in a vacuum upon impact, followed by rapid cooling as molten glass droplets settle on the lunar surface. By synthesizing glass samples with varying compositions, we can systematically investigate the role of individual oxides in the production and mobility of water-group molecules. Figure 2: Deuterium depth profile measurement in an olivine sample, before (blue) and after (red) a thermal cycle from 300K to 600K and back. A reduction of approximately 50% in deuterium content is observed. Vertical lines indicate the position of the center of gravity of the corresponding deuterium depth distribution. A shift of ~5nm to the right indicates slight diffusion into the bulk. Figure3: Schematic view of the setup for lunar-like glass production. Summary: We will present scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) images of lunar-like glass samples, along with results from Nuclear Reaction Analysis (NRA) and Rutherford Backscattering Spectrometry (RBS) deuterium depth profile measurements in both Earth analogues and lunarlike glass samples after extended thermal cycling. Additionally, we will report on the progress of establishing a dedicated experimental station equipped with MeV Secondary Ion Mass Spectrometry (SIMS) instrumentation and a Residual Gas Analyzer (RGA) to monitor the release of water-group molecules from the samples. This research aims to provide crucial insights into the role of impact glass beads in lunar water formation and migration, contributing to our understanding of the Moon’s water cycle and its potential utilization for future lunar exploration. References: [1] N. Schörghofer et al. (2021) Space Science Reviews 217:74. [2] Benna et al. (2019) Nat. Geosci. 12, 333–338. [3] He et al. (2023) Nat. Geosci.16, 294–300. [4] Zhu et al. (2019) , Proc. Natl. Acad. Sci. U.S.A. 116 (23), 11165-11170. [5] M. Mayer (1999) Proc. of the 15th Int. Conf. on the Appl. of Accelerators in Research and Industry 475. [6] Calligaro et al. (2004) Chapter 5, Ion beam microanalysis, Comprehensive Analytical Chemistry, 42, 227-276.