The Rheology of Lunar Regolith and its Simulants

Granular materials are composed of macroscopic solid particles with a discrete nature. Their complex particle interactions and structure lead to their observed material properties, complicating the prediction their behavior in various applications more difficult. Extreme environments, such as those found on the Moon, are very different from Earth and create a challenging environment for working with granular materials such as lunar regolith, the soil on the lunar surface. Lunar regolith simulants, which are materials that are designed to mimic the properties of lunar soil, are used on Earth for experiments investigating the behavior of lunar soil. Understanding the rheological and mechanical behaviour of granular materials in space environments like the Moon is essential for the advancement of space exploration, as it has implications for the design of infrastructure and the exploitation of resources on the surfaces of extraterrestrial bodies. This thesis investigates the rheological properties and soil mechanics of lunar regolith and its simulants, using data from NASA’s Apollo missions and experiments with EAC-1A and LHS-2E simulants. The objective is to explore key parameters that influence the material behavior and provide insight into the observed characteristics of lunar regolith. The experiments include classical ring shear tests and vertical wall collapse tests, inspired by the Apollo missions trench tests. These were conducted in both air and vacuum (20 mbar), with moist and dried simulants. The strength parameters in the Mohr-Coulomb framework - cohesion and internal friction angle - were derived using image analysis. Results indicate that bulk density, atmospheric pressure, and moisture content have a significant effect on cohesion and internal friction angle. Specifically, cohesion decreases, while internal friction angle increases in a vacuum in respect to an air environment. Comparing data from the lunar soil to regolith simulants suggests that in a vacuum environment, the angle of internal friction becomes less dependent on material properties and bulk density than cohesion. The analysis of data highlights the influence of environmental factors, such as gravity and pressure, as well as intrinsic material properties such as bulk density and particle size distribution. The tests performed exhibit a strong dependence on the environment, in contrast to universal material properties, which are expected to remain independent of environmental factors. Therefore, providing detailed information about the experimental framework, which includes environment and material, is essential to allow meaningful comparisons of the strength parameters obtained from different experiments.