Shape is an important property of lunar regolith particles. It substantially affects the regolith's strength, the angle of repose, the packing density, and the ability of regolith particles to attach to and abrade spacecraft materials or clog air filters. For these reasons, simulants are needed to reproduce regolith particle shapes for testing spacecraft that interacts with the lunar surface. Difficulties in assessing “how good” a simulant reproduces the shape of lunar particles stem from the lack of shape measurements obtained on lunar particles, compounded by the lack of a strict definition of shape or a way to quantifiably measure shape. This paper presents a general method for measuring and comparing particle shapes, and provides shape measurements for six lunar-regolith simulants.
The method of evaluating particle shape presented here involves Fine Particle Analysis and corresponding image analysis to extract the maximum Feret diameter, Feret diameter of the minor axis of the inertial ellipse, mean diameter, sieve diameter, area, and perimeter length from images with an approximately 4.4 µm pixel resolution. From those, aspect ratio and Heywood factor are computed. Data processing involves calculating the frequency distributions of particle shapes according to their sieve diameter, aspect ratio, and Heywood factor. The resulting graphical representations indicate that the majority of the lunar regolith simulant particles, in two-dimensional projection, can be well represented by a mixture of ellipses and rectangles of varying aspect ratios. The differences between simulant particle shapes, and lunar regolith particle shapes measured in the future, can be quantified by differencing the accumulated distributions. Thus, this paper presents a new method of particle shape analysis and a new way to quantifiably compare particle shapes.