The Brazil-nut effect and its application to asteroids

TL;DR

This paper investigates how seismic shaking causes size sorting of particles in asteroids through the Brazil Nut Effect, demonstrating its scalability to low-gravity environments and relevance to observed asteroid surface features.

Contribution

It provides a comprehensive analysis of the Brazil Nut Effect in asteroid-like conditions, including critical parameters, scalability, and application to real asteroid observations.

Findings

BNE is insensitive to restitution coefficients but sensitive to friction and oscillation speed.

Convection drives the BNE, with the intruder rising to the top.

Rise speed scales with the square root of gravity.

Abstract

Out of the handful of asteroids that have been imaged, some have distributions of blocks that are not easily explained. In this paper, we investigate the possibility that seismic shaking leads to the size sorting of particles in asteroids. In particular, we focus on the so-called Brazil Nut Effect (BNE) that separates large particles from small ones under vibrations. We study the BNE over a wide range of parameters by using the N-body code PKDGRAV, and find that the effect is largely insensitive to the coefficients of restitution, but sensitive to friction constants and oscillation speeds. Agreeing with the previous results, we find that convection drives the BNE, where the intruder rises to the top of the particle bed. For the wide-cylinder case, we also observe a "whale" effect, where the intruder follows the convective current and does not stay at the surface. We show that the non-dimensional critical conditions for the BNE agree well with previous studies. We also show that the BNE is scalable for low-gravity environments and that the rise speed of an intruder is proportional to the square root of the gravitational acceleration. Finally, we apply the critical conditions to observed asteroids, and find that the critical oscillation speeds are comparable to the seismic oscillation speeds that are expected from non-destructive impacts.