High abundance of solar wind-derived water in lunar soils from the middle latitude

Abstract

Remote sensing data revealed that the presence of water (OH/H 2 O) on the Moon is latitude-dependent and probably time-of-day variation, suggesting a solar wind (SW)-originated water with a high degassing loss rate on the lunar surface. However, it is unknown whether or not the SW-derived water in lunar soil grains can be preserved beneath the surface. We report ion microprobe analyses of hydrogen abundances, and deuterium/hydrogen ratios of the lunar soil grains returned by the Chang’e-5 mission from a higher latitude than previous missions. Most of the grain rims (topmost ~100 nm) show high abundances of hydrogen (1,116 to 2,516 ppm) with extremely low δD values (−908 to −992‰), implying nearly exclusively a SW origin. The hydrogen-content depth distribution in the grain rims is phase-dependent, either bell-shaped for glass or monotonic decrease for mineral grains. This reveals the dynamic equilibrium between implantation and outgassing of SW-hydrogen in soil grains on the lunar surface. Heating experiments on a subset of the grains further demonstrate that the SW-implanted hydrogen could be preserved after burial. By comparing with the Apollo data, both observations and simulations provide constraints on the governing role of temperature (latitude) on hydrogen implantation/migration in lunar soils. We predict an even higher abundance of hydrogen in the grain rims in the lunar polar regions (average ~9,500 ppm), which corresponds to an estimation of the bulk water content of ~560 ppm in the polar soils assuming the same grain size distribution as Apollo soils, consistent with the orbit remote sensing result.

A solar wind-derived water reservoir on the Moon hosted by impact glass beads

“…However, finding this water reservoir has remained elusive, despite several studies having investigated the water inventory of fine mineral grains in lunar soils (for example, refs. 34,35 ), impact-produced agglutinates 27 , volcanic rocks (for example, ref. 26 ) and pyroclastic glass beads 21,36 .…”

“…Importantly, this estimate provides a minimum quantity of water available at the lunar subsurface as it does not take into account the water contribution from permanently shadowed regions [4][5][6]11 , indigenous lunar rocks (for example, ref. 26 ), delivery by meteoritic impacts 13,27 and solar wind-derived water preserved in the space-weathered rims of lunar soils 34,35 . These findings indicate that the lunar soils contain a much higher amount of solar wind-derived water than previously thought, which could be a water reservoir for in situ utilization in future lunar exploration.…”

“…Considering the scooped samples were collected from less than 3 cm deep 38 , diffusion simulations were performed using a peak temperature of 360 K, relevant for a latitude of 45° N on the Moon 32 , because the peak temperature is the main controlling factor for the diffusion time 35 . The H diffusion coefficients of water/hydroxyl (D H ) determined on silica glass 48 , D H = 20.84 μm 2 yr −1 at 360 K, were used in our simulations.…”

NanoSIMS sulfur isotopic analysis at 100 nm scale by imaging technique

“…In order to reduce the impact of image drift, each area has been raster in 30 frames, with 256 × 256 pixels and 5 ms/pixel dwell time. The total duration is 9,900 s for each area and the sputter depth is ∼5 nm calculated using the sputter rate (0.2 nm*μm 2 pA−1 s−1) report in the previous study ( Xu et al, 2022 ). The acquired ion images were processed and analyzed using ImageJ with the Open MIMS plugin.…”

Transport of Volatiles in Agglutinates from Lunar Regolith of Chang’E-5 Mission

“…In particular, the agglutinates usually contain pure Fe metal nanoparticles and solar wind implanted gases (mainly H and He) in comparison with other components of the regolith. [12,13] However, the formation of agglutinates is still mysterious. Based on previous research, the most popular model can provide some insights: agglutinates are formed through complex processes resulted from micrometeoroids impaction into the regolith rich in solar wind elements.…”