Three-dimensional shape distribution of lunar regolith particles collected by the Apollo and Luna programs

Abstract

The shapes of regolith particles on airless bodies, such as the Moon and asteroids, are important to understand their formation and evolution on surfaces. Limited studies have shown that the three-dimensional (3D) shapes of lunar regolith particles are, on average, more equant (spherical) than those of asteroid Itokawa or fragments by impact experiments. Therefore, more studies are required to determine whether such a feature is common. Accordingly, we performed X-ray microtomography imaging of lunar regolith particles collected by the Apollo program by NASA and the Luna program by the Soviet Union to obtain their 3D shapes. The ten samples (65 to 1108 particles/sample) examined had varieties of sampling sites (maria and highlands), reflecting the difference in materials (basalts and anorthosites, respectively, in general), regolith maturities, particle size ranges (< 74 to 450 µm), and petrographic textures (monomineralic, polymineralic, and agglutinate). The 3D particle shape distributions regarding three-axial length ratios (L:I:S, where L, I, and S are the longest, intermediate, and shortest lengths, respectively) showed that the average three-axial ratios were almost similar among the samples, irrespective of the sampling sites, maturities, and the size ranges [S/I = 0.770(8), I/L = 0.758(10), and S/L = 0.581(11) for whole samples]. The 3D shapes of lunar particles were more equant (spherical) than those of the particles collected from asteroid Itokawa and fragments by hypervelocity impact experiments which had the average ratios similar to the 2D silver ratio (S/I = I/L = 0.707 and S/L = 0.500). These findings showed that the balance between impact fragmentation and mechanical abrasion controls the 3D shapes of lunar particles because impact and particle motion on the Moon’s surface occur for a longer duration; however, impact fragmentation on this small asteroid surface primarily controls those of Itokawa particles. We also found shape dependence on petrographic textures of the lunar particles, and this could be explained by the strength of the materials against abrasion. The results obtained in this study will be the basic data to be compared with upcoming new results, such as particles collected from asteroid Ryugu, possibly from asteroid Bennu and Martian moons. Graphical Abstract: [Figure not available: see fulltext.].