Water-Ice Weathering in Permanently Shadowed Craters on the Moon

On Earth, rocks and soil are weathered by wind and water, but what weathers surfaces on the Moon? New research investigates how ice might alter lunar soil in permanently shadowed lunar craters.

The Importance of Lunar Soil

The Moon is blanketed in a layer of material called regolith, made up of fragments of rock, soft lunar soil, and fines: particles less than 2 microns across (1 micron = 10^-6 meter). Apollo-era astronauts encountered fines during moonwalks; the fines clung relentlessly to every surface and caused sneezing and sore throats when brought in to the lunar module. Lunar fines aren’t just annoying — studies using simulated lunar material show that fines can damage lung cells and DNA.

NASA’s upcoming Artemis missions will bring astronauts to the lunar south pole, which is dotted with steep-sided craters that lie in permanent shadow. Because some of these craters are thought to harbor water ice, it’s important to know if the ice is likely to weather the lunar surface material, creating an abundance of harmful and irritating fines.

A Lunar Experiment on Earth

Autumn Shackelford (University of Central Florida) and collaborators examined how lunar regolith might be weathered by water ice. If a tiny bit of liquid water seeps into fine cracks in lunar surface particles, the expansion of the water as it freezes will widen the cracks. The mechanical stress caused by this expansion may cause the crack system to grow, potentially reshaping the particle.

To understand how water ice plays a role in the weathering process, Shackelford’s team mixed samples of simulated lunar soil with water and placed the samples in a freezer. Half of the samples contained minerals similar to the lunar highlands (the lighter regions of the Moon’s surface), while the other half were similar to the lunar “seas,” or maria (the darker regions of the Moon’s surface).

The team varied the amount of water added to the samples from no water — the control samples — to 30% water by weight, and left the samples in a freezer for 1, 3, or 6 months. After the samples finished hibernating, the team characterized the size and shape of the particles, counted the number of fine particles, and took spectra to search for possible spectral changes.

Water Weathering

Contact with water ice changed the shapes and sizes of the simulated lunar particles, and the changes were dependent upon the amount of water, the time spent in the freezer, and whether the sample was similar to the lunar highlands or the lunar maria. Crucially, the amount of fine particulate matter — the material that was so perplexing during the Apollo missions — generally increased with both water content and time spent in the freezer. The lunar mare samples produced more fine particles than the lunar highlands samples in all cases, showing that composition plays an important role. Curiously, while the highlands simulant showed changes in its spectrum in the 5–22.5-micron range after exposure to ice, the mare samples showed no spectral changes.

With humans potentially exploring lunar regions where water ice lingers in shadowy craters, this work provides an important first look at how ice plays a role in producing fine particles. Future astronauts, take heed!

Citation

“Morphological and Spectral Characterization of Lunar Regolith Breakdown Due to Water Ice,” A. Shackelford et al 2024 Planet. Sci. J. 5 1. doi:10.3847/PSJ/ad0041