Granular physics in low-gravity environments using discrete element method

Abstract

Granular materials of different sizes are present on the surface of several atmosphereless Solar system bodies. The phenomena related to granular materials have been studied in the framework of the discipline called granular physics, both experimentally and, over the last few decades, by numerical simulations. The discrete element method simulates the mechanical behaviour of a medium formed by a set of particles which interact through their contact points.The difficulty in reproducing vacuum and low-gravity environments makes numerical simulations the most promising technique in the study of granular media under these conditions.In this work, relevant processes in minor bodies of the Solar system are studied using the discrete element method. Results of simulations of size segregation in low-gravity environments in the cases of the asteroids Eros and Itokawa are presented. The segregation of particles with different densities was analysed, in particular, the case of comet P/Hartley 2. The surface shaking in these different gravity environments could produce the ejection of particles from the surface at very low relative velocities. The shaking causing the above processes is due to impacts and explosions such as the release of energy by the liberation of internal stresses or the re-accommodation of material. Simulations of the passage of impact-induced seismic waves through a granular medium were also performed.We present several applications of the discrete element methods for the study of the physical evolution of agglomerates of rocks under low-gravity environments.

Distribution of boulders and the gravity potential on asteroid Itokawa

“…These results supports the hypothesis that the size segregation is a global process; there is a global relocation of boulders, with large boulders going into the high potential regions and small boulders into the low potential ones. The size sorting could be due the seismic shaking produced by repetitious impacts, as it was shown to operate in the low-gravity environment of Itokawa by Tancredi et al (2012). Shape segregation is observed on the distribution of boulders: more rounded boulders are found in the regions of high potential, while more elongated objects are more frequent in regions of low potential.…”

“…Nevertheless, they have not performed any experiments to test this. Tancredi et al (2012) performed numerical simulations based on the Discrete Element Methods (DEM) of a set of particles in different gravity environments under the action of several shaking processes. They have shown that the Brazilian nut effect is present even in low-gravity environments like the surface of Itokawa.…”

Size Sorting on the Rubble-Pile Asteroid Itokawa

“…the BNE 12,21,23,24,25 ), we focus on the results of a single mechanism while acknowledging that it is only one of several processes involved.…”

“…In the BNE, smaller grains either subduct through convection 17,18,19 or sift beneath larger neighbors 20 . Studies attributing Itokawa's segregation to convection 5,12 are constrained by the observation 21,22 that under its weak gravity (g/10 5 ), convection would be very slow, and by calculations 23,24 that show that required agitation velocities would be very close to escape velocities. Simulations investigating sifting 21,25 , on the other hand, produce surfaces uniformly dotted with boulders, in stark contrast the lateral segregation seen on Itokawa.…”

An implementation of the soft-sphere discrete element method in a high-performance parallel gravity tree-code

“…However, it is only very recently that it has started to be applied to the realm of planetary science (see e.g., Sànchez et al [16], Tancredi et al [17], although some groups, such as [17], make use of third-party software not fully under their control). An important characteristic of SSDEM in pkdgrav, in addition to its parallelization, is that it can cover a very wide range of gravity regimes and boundary conditions, as is required to study the large variety of environments encountered in the Solar System.…”