Simulations of Post-Impact Granular Dilation on Airless Bodies

Abstract:

Cold Spots are thermophysically distinct halos found around young craters on the lunar surface, identified in 2014 by their low nighttime temperatures as compared to the surface average. The low nighttime temperatures are believed to be caused by a decrease in regolith density in the cold spots. The formation mechanism leading to these large distal regions of lower density is unknown. We propose that a post-impact granular shock causes dilation in the upper centimeters-thick layer of lunar regolith, leading to reduced regolith density. We use LIGGGHTS, an open source discrete element method solver, to simulate a granular shock in a channel filled with monodisperse, spherical particles. Appropriate channel dimensions are determined by analyzing the average force and velocity over time experienced by the particles, steady state dilation throughout the grid, and the shock’s evolution in strength and extent over time. We find that the granular shock causes dilation in the upper layer of the simulated regolith. A preliminary sensitivity analysis reveals that the steady state dilation is sensitive to the friction coefficient of the regolith as well as the shock strength. Future work will consider different particle size distributions to achieve varied initial regolith packing and more closely approximate the lunar regolith.