Duplicating the Effects of Space Weathering

Introduction: Several lunar simulants have been produced in recent years [1], and each of them has qualities that simulate various properties of lunar surface materials. Here we report on ORBITEC simulants which are intended to reproduce the nanophase Fe and agglutinitic content of typical lunar dust. Background: There is no single simulant that meets the needs of all users. However, it is the goal of the NASA Simulants program to produce a basic feedstock that can be modified in various ways to meet diverse needs [2-4]. JSC-1A is the oldest of the recent products. It is designed to be chemically similar to a low-titanium lunar mare soil [3], and is rich in volcanic glass. This makes it a good engineering simulant and also useful for resource utilization experiments that are aimed at producing oxygen from lunar materials. NU-LHT-1 and NU-LHT-2, also produced under the direction of the NASA Marshall Space Flight Center, aim to simulate the composition of the lunar highlands areas, and also to contain agglutinitic material. NU-LHT is a good engineering simulant because it will help engineers understand how lunar surface materials might change as vehicles drive across it. (It is expected that the agglutinates will break down, and that the soil will be irreversibly compressed with repeated traffic [5].) OB-1, produced by NORCAT [6], is a good engineering simulant for drilling tests because it mimics the abrasiveness of lunar material more closely than do the other simulants mentioned above. Simulants will also be needed to conduct dust toxicity studies [7]. Because the mineral chemistry of the dust grains are important in determining their toxicity, lunar dust simulants designed for biological study will have to be produced under more stringent requirements than that meant for testing of equipment [7]. Lunar dust contains abundant Fe metal nano-particles (also called “nanophase Fe [8]”) contained within the matrix of glassy soil agglutinates that are formed by micrometeorite impacts, and also within vaporor sputter-deposited coatings on other soil grains. An open question is whether the nano-phase Fe, when absorbed through the skin or lungs, will result in unacceptably high levels of iron in the crew’s blood [9]. Consequently, several groups have become interested in synthesizing nanophase Fe for toxicology studies, e.g. [10]. Grain Size Distribution: The portion of the lunar soil that is of most concern to toxicologists is the finest fraction (less than 10 μm diameter [11]). For inhalation studies, materials should be less than three μm in size. We measured the ORBITEC simulants with a Microtrac laser diffraction particle measurement system at NASA’s Lyndon B. Johnson Space Center. We also made new measurements of the JSC-1A simulants. Table 1 shows the grain size properties of the simulants that we measured, including the ORBITEC simulants that are under development, as well as highland simulant NU-LHT-1M (measurement provided by Horiba [12]). ORBITEC-06, ORBITEC-01, and JSC1AVF are all within a size range that is useful for dust toxicology studies. Table 1. Properites of ORBITEC Simulants and other well-known simulants for comparison. Material Mean Std. Dev. Mode ORBITEC-06 3.43 μm 1.383 μm 3.20 μm ORBITEC-01 6.23 μm 3.06 μm 5.34 μm JSC-1AVF 6.39 μm 4.24 μm 4.71 μm JSC-1AF 24.34 μm 10.6 μm 21.05 μm JSC-1A 84.23 μm 68.47 μm 52.35 μm NU-LHT-1M 163.1 μm * N.R. 148 μm ORBITEC “Feedstock” 212.6 μm 140.7 μm 197.40 μm * Average of two samples