[Paper Review] Granular physics in low-gravity environments using DEM

G. Tancredi, Andrea Maciel+3 more|arXiv (Cornell University)|Nov 26, 2011

Planetary Science and Exploration2 references3 citations

TL;DR

This paper uses the Discrete Element Method (DEM) to simulate granular dynamics in low-gravity environments typical of asteroids and comets, demonstrating that repeated shaking induces size and density segregation (Brazil nut effect) and that surface explosions or impacts can eject particles at low velocities. Key results show that even weak shocks can trigger particle ejection and segregation on km-scale bodies like Itokawa and P/Hartley 2, with ejection velocities exceeding escape velocity in low-gravity conditions.

ABSTRACT

Granular materials of different sizes are present on the surface of several atmosphere-less Solar System bodies. The phenomena related to granular materials have been studied in the framework of the discipline called Granular Physics; that has been studied experimentally in the laboratory and, in the last decades, by performing numerical simulations. The Discrete Element Method simulates the mechanical behavior of a media 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, explosions like 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.

Motivation & Objective

• To investigate how granular materials behave under low-gravity conditions typical of minor Solar System bodies.
• To model size and density segregation processes induced by repeated surface shaking or impacts.
• To assess the feasibility of particle ejection from surfaces due to seismic waves or explosions in low-gravity environments.
• To evaluate the role of impact-induced shocks in triggering physical processes like segregation and ejection on small bodies.
• To provide a numerical framework for studying the physical evolution of agglomerated rock surfaces in space using DEM.

Proposed method

• Discrete Element Method (DEM) is used to simulate the mechanical behavior of granular media composed of discrete particles interacting via contact forces.
• Simulations model spherical particles with varying sizes and densities, using physical laws for contact mechanics and gravity.
• Low-gravity environments are simulated by setting surface gravity to 10−2 to 10−4 m/s², representative of asteroids and comets.
• Shaking is induced by applying repeated, localized impulses to the base of the granular layer, mimicking seismic waves from impacts or internal stress release.
• Explosion events are modeled as sudden, localized energy releases at the surface, generating shock waves that propagate through the granular medium.
• The simulations track particle trajectories, velocities, and ejection events, with results analyzed for escape velocity thresholds and segregation patterns.

Experimental results

Research questions

• RQ1 Can size segregation (Brazil nut effect) occur in low-gravity environments typical of asteroids like Eros and Itokawa?
• RQ2 How does particle density segregation manifest under repeated shaking in low-gravity conditions, particularly on cometary bodies like P/Hartley 2?
• RQ3 To what extent can impact-induced seismic waves or surface explosions lead to particle ejection at low relative velocities?
• RQ4 What is the relationship between explosion energy, body size, and the fraction of particles achieving escape velocity?
• RQ5 How do gravity levels and body size affect the propagation of shock waves and the resulting surface dynamics?

Key findings

• Size segregation (Brazil nut effect) occurs in low-gravity environments on km-sized bodies like Itokawa and Eros, even with weak shaking.
• Density segregation is observed in simulations, though not complete, indicating differential migration of high- and low-density particles under shaking.
• Surface explosions can eject particles at velocities exceeding escape velocity, particularly on small bodies (e.g., 250 m radius), with up to significant fractions of near-surface particles achieving ejection speeds.
• For the smallest body (250 m radius), even low-velocity explosions (100 m/s) produce ejection velocities over escape velocity for many particles far from the explosion point.
• Shock waves from impacts or internal releases propagate through the entire body, inducing global shaking that triggers surface particle lifting and ejection.
• The results suggest that repeated impacts or internal energy releases are sufficient to drive segregation and particle loss on small, low-gravity bodies like Itokawa and P/Hartley 2.