A joint research team led by Prof. WEN Zhi from the Northwest Institute of Eco-Environment and Resources (NIEER) of the Chinese Academy of Sciences (CAS), in collaboration with researchers from Lanzhou Institute of Space Technology Physics in China and Tel-Aviv University in Israel, has investigated the migration of water molecules in lunar soil pores to reveal how the lunar regolith hinders water migration.

The study was published in Journal of Geophysical Research: Planets.

The Moon has long been considered a "water-free" body due to its high vacuum and low gravitational binding. However, the latest lunar remote sensing exploration has confirmed the presence of water in permanently shadowed areas at the north and south poles of the Moon, as well as in limb regions beyond the shadowed areas.

It has been suggested that lunar surface water is probably in the form of a mixture of water ice and lunar soil (commonly known as dirty ice), and lunar soil may be the reason that prevents the escape of lunar surface water molecules and leads to the expansion of the distribution range of lunar surface water ice.  

However, the internal mechanisms of lunar soil layer that affect the distribution of water ice on lunar surface are not well understood. 

In this study, the researchers quantitatively analyzed the effects of pore characteristics and water adsorption characteristics of lunar soil on the process of water molecules dispersion and water ice retention in lunar soil. "We present the diffusion coefficient equation and compare the results of our model with the results derived from previous research regarding diffusion coefficients in porous media," said Prof. WEN.

They found that heat adsorption effects and pore characteristics of lunar soil could potentially impede the loss of water molecules in lunar regolith, which may lead to the expansion of potential water ice retention area from the permanently shadowed areas to wider areas.

Furthermore, in contrast to the results of previous studies, the potential water retention area may extend to some regions covered by regolith with large adsorption heat. Regolith-covered areas with both large adsorption heat and low temperatures are most likely to retain water ice.

"The study not only reveals the contribution of water absorption of lunar soil to water ice retention, but also theoretically explains the phenomenon of water molecule signal in the non-permanent shadow area of the moon, which can provide theoretical guidance for lunar water ice detection," said Prof. WEN.