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

Significance

The latitude-dependent and probably time-of-day variations of water (OH/H ₂O) on the Moon surface have previously been explored by reflectance spectroscopy. The lunar soils returned by the Chang’e-5 mission from a middle latitude, significantly higher than Apollo missions, provide a unique opportunity for studying the latitude-dependent implantation and retention of solar wind (SW)-derived water in lunar soils. We have conducted a combined NanoSIMS–FIB–TEM analysis of the CE-5 soil grains, complemented by the heating experiments. The high abundance of SW-derived hydrogen in the rims of the grains provides a significant constraint on the preservation of SW-derived water in lunar soils. The predicted water contents of bulk soils in the lunar polar regions are consistent with the remote sensing data.

Abstract

Remote sensing data revealed that the presence of water (OH/H ₂O) 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.