Grain-scale mechanical properties of lunar plagioclase and its simulant: Initial experimental findings and modeling implications

Preparations for lunar missions involve the extensive use of simulants of lunar surface materials. Since many planned operations engage the mechanical properties of the regolith, it is of utmost importance to assure that the simulants adequately represent the engineering properties of actual lunar materials. To address this critical issue, we are conducting an experimental and modeling effort to quantify and compare the grain-scale properties of lunar materials and their simulants. Given the scarcity of lunar regolith for experimentation, we employ an approach to material characterization based on grain-to-grain contact experiments and use numerical modeling to extend the work to the engineering scale. This paper describes the experimental methods, compares the contact properties of the materials under study, and discusses our on-going DEM modeling efforts. We present the results of an initial set of contact experiments conducted on the plagioclase component of the lunar simulant NU-LHT, and a number of highlands plagioclase grains obtained during the Apollo 17 mission. Our initial experiments indicate that the normal contact stiffness of the simulant plagioclase can be up to 2-3 times higher than the lunar highlands plagioclase, which appears to be primarily in the form of relatively weak micro-breccias. This difference is substantially lower for more intact lunar grains. Sliding contact experiments indicate that the shear stiffness at low force levels is close to the normal stiffness. These preliminary experiments indicate that lunar highland simulants produced by simple crushing and grinding of plagioclase rock may not capture the actual strength of lunar highlands plagioclase. Continuous micro-meteorite space weathering of soil on the Moon can weaken grains considerably more than simple crushing/grinding. Additional experiments are in progress with plagioclase from contrasting maturity of soils, as well as with lunar rock plagioclase. INTRODUCTION The testing and development of equipment that will be deployed in planned lunar missions to the surface of the Moon will rely heavily on the use of simulants of the lunar regolith. Since these missions will involve many operations that engage the mechanical properties of the lunar regolith (e.g., surface mobility, drilling, excavation, and construction operations), it is of utmost importance to assure that the simulants adequately represent the engineering properties of actual lunar materials. An unusual aspect of this situation is the very limited supply of Apollo lunar material for use in experiments to validate the simulant properties. To address this issue, we are conducting grain-to-grain contact experiments on the components of the NU-LHT family of lunar simulants being produced by the USGS and comparing those to results of contact experiments on grains of Apollo highland lunar materials. Our 74 Earth and Space 2010: Engineering, Science, Construction, and Operations in Challenging Environments © 2010 ASCE