Granular Mechanics in Asteroid Regolith: Simulating and Scaling the Brazil Nut Effect

Introduction It is well known that shaking can drive size sorting and segregation in granular media ( [1] and references therein). We consider the hypothesis that seismic shaking on asteroids could promote the size sorting of its constituent particles [2]. Such sorting is evident in the images sent by the Hayabusa mission of the surface of the asteroid Itokawa [3]. The simplest example of this sort of segregation is the well-known Brazil-Nut Effect (BNE), which has been studied in the last few years and validated using both simulations and laboratory experiments. However, all studies carried out to date have assumed a “uniform” terrestrial gravitational field as a constant [1]. On an asteroid this does not hold true. First, the gravitational field itself can vary by a factor of two or more across an asteroid surface. Second, the scale of the total gravitational accelerations are much smaller than those simulated in terrestrial laboratories. In order to properly analyze whether this effect plays a role in the evolution of Itokawa’s surface we have initiated a series of granular dynamics simulations that test this effect over a range of different conditions found on asteroids.