Chinese researchers find proof that solar wind creates water on the Moon's surface

This is consistent with lunar water from solar wind (SW).

The researchers simulated the retention of hydrogen in lunar soils at various temperatures. They discovered that water from the southwest could remain on the moon's surface in the middle and high latitudes.

The study author, Prof. LIN Yangting from IGG, stated that the polar lunar soils "could contain more water than Chang'e-5 samples."

Water on the Moon's surface appears to vary depending on latitude and the time of day

According to earlier research, the amount of water (OH/H2O) on the lunar surface changes with latitude and time of day (up to 200 parts per million, or ppm). Such a noticeable difference suggests a quick rate of lunar surface desorption.

The Chang'e-5 mission returned soil samples from a position at a middle latitude (43.06°N), in contrast to the six Apollo and three Luna missions, which all touched down at low latitudes (8.97°S–26.13°N).

The Chang'e-5 samples were also taken from the driest basaltic basement and the youngest lunar basalts. Chang'e-5 samples are needed to determine where and how long water from the SW stays in the moon's regolith.

The Chang'e-5 mission brought back 17 small pieces of lunar soil, which were used to determine the ratio of deuterium to hydrogen and the NanoSIMS depth profile.

Most of the grain rims (the top 100 nm) had low D values (-908 to -992) and high hydrogen contents (1,116–2,516 ppm), which pointed to an SW origin. Based on the size of the grains and the amount of hydrogen in the Chang'e-5 lunar soils, the bulk SW-derived water content was found to be 46 ppm.

This is similar to the result from remote sensing.

Heating some of the grains showed that the hydrogen added by the SW could still be there after the grains were buried. Using this information and what they already knew from other studies, the researchers made a model of the dynamic equilibrium between the implantation and outgassing of SW hydrogen in lunar soil grains. This model showed that temperature (due to latitude on the Moon's surface) is a key factor in how hydrogen is put into and moves through lunar soils.

Using this theory, they thought that the grain rims of the lunar poles would have even more hydrogen than they did.

According to Professor LIN, "this discovery is of great significance for the future utilization of water resources on the moon."

"Also, through particle sorting and heating, it is relatively easy to exploit and use the water contained in the lunar soil," he added.

You can view the study in the journal Proceedings of the National Academy of Sciences (PNAS).

Study abstract:

"Remote sensing data revealed that the presence of water (OH/H2O) 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."