Recently, we have been involved in a couple of studies of tiny glass droplets recovered from soil samples returned from the Moon in December 2020 by the China National Space Agency Chang’e-5 mission. These melt droplets are impact glass beads, which formed when some asteroidal and cometary impactors collided at high speed with the lunar surface. During these hypervelocity impact events, some of the material ejected melts because of the high temperatures involved (typically > 1200 °C), producing these impact glass beads.
In a study led by Tao Long from the Beijing SHRIMP Center, and published a few months ago, we investigated the chemistry of >200 impact glass beads, and found out that the majority of these likely formed in response to a few larger impact events in the local regolith developed on top of basaltic lavas. We also measured the formation ages of these beads, and obtained a range of ages over the past 2 billion years. Interestingly, the bead age distribution showed some prominent formation age peaks, for example around 65 and 35 million years ago, which also correspond to known impact cratering activity on Earth. This may suggest enhanced impact activity at specific time periods.
In another study led by Huicun He and Sen Hu from IGGCAS in Beijing, and published last week, we investigated the water content of these impact glass beads from the Chang’e-5 soils. This is the first study to measure the water abundance (and its isotope composition, that is the ratio of heavy D to light H in water) in lunar impact glass beads. A couple of previous studies had measured hydrogen and water in minerals and agglutinates in soils, and suggested that hydrogen implanted by the solar wind was a likely source. Here we took it a step further and used the very high spatial resolution of the NanoSIMS instrument to do transect across glass beads about 100 micrometres across (or about the width of a human hair!), allowing us to look at spatial variations across these small beads.
These profiles across the beads showed high water abundances at the rims and low abundances in the cores, associated with very low D/H ratios at the rims. This indicates that the source of the hydrogen in the beads is diffusion from the outside-in of hydrogen delivered to the Moon’s surface by solar wind, which is characterised by very low D/H ratios. Interestingly, the water abundance in the cores of the beads is very low, consistent with their formation at high temperature that resulted in degassing of any water originally present in the target rocks.
Finally, because we measured profiles across the beads, we were able to model how long it would take to reproduce these profiles at conditions relevant to the lunar surface, since diffusion is largely dependent on temperature. These calculations suggest that these hydration / dehydration features can be produced in ~10-20 years, in other words almost instantaneously compared to typical geological timescales. Therefore, hydration and dehydration of these impact glass beads are probably a key feature of a water cycle at the lunar surface, which may also include water ice trapped in craters at the Moon’s poles.
Once again, the number of studies and key discoveries that have been made on samples returned only just over 2 years ago by the Chinese Chang’e-5 mission highlight the key role of sample return missions in advancing our understanding of the Solar System and the bodies it hosts. We are very grateful to our Chinese scientist colleagues for these amazing collaborations.
The study led by Tao Long was published in Science Advances and can be read here: https://www.science.org/doi/10.1126/sciadv.abq2542.
The study led by Huicun He was published in Nature Geoscience and can be read here: https://www.nature.com/articles/s41561-023-01159-6.
Earth & Solar System 