The spherical Brazil Nut Effect and its significance to asteroids

TL;DR

This study demonstrates that seismic shaking can cause size sorting of particles in a spherical asteroid model, with larger particles rising to the surface even without friction, impacting our understanding of asteroid surface processes.

Contribution

It introduces a spherical, self-gravitating model of an asteroid showing size sorting due to seismic shaking, including effects of friction and internal sorting limitations.

Findings

Larger particles rise to the surface above a seismic threshold.

Size sorting occurs even with zero friction.

Inner regions remain unsorted despite vigorous shaking.

Abstract

Many asteroids are likely rubble-piles that are a collection of smaller objects held together by gravity and possibly cohesion. These asteroids are seismically shaken by impacts, which leads to excitation of their constituent particles. As a result it has been suggested that their surfaces and sub-surface interiors may be governed by a size sorting mechanism known as the Brazil Nut Effect. We study the behavior of a model asteroid that is a spherical, self-gravitating aggregate with a binary size-distribution of particles under the action of applied seismic shaking. We find that above a seismic threshold, larger particles rise to the surface when friction is present, in agreement with previous studies that focussed on cylindrical and rectangular box configurations. Unlike previous works we also find that size sorting takes place even with zero friction, though the presence of friction does aid the sorting process above the seismic threshold. Additionally we find that while strong size sorting can take place near the surface, the innermost regions remain unsorted under even the most vigorous shaking.