Boulder stranding in ejecta launched by an impact generated seismic pulse

Abstract

We consider how an impact generated seismic pulse affects the surface of an asteroid distant from the impact site.With laboratory experiments on dry polydisperse gravel mixtures, we track the trajectories of particles ejected from the surface by a single strong upward propagating pressure pulse. High speed video images show that ejecta trajectories are independent of particle size, and collisions primarily take place upon landing. When they land particles are ballistically sorted, as proposed by Shinbrot et al. (2017), leaving larger particles on the surface and smaller particles more widely dispersed. A single strong pulse can leave previously buried boulders stranded on the surface. Boulder stranding due to an impact excited seismic pulse is an additional mechanism that could leave large boulders present on the surface of rubble asteroids such as 162173 Ryugu, 101955 Bennu and 25143 Itokawa.

Reconstructing the formation history of top-shaped asteroids from the surface boulder distribution

“…The boulders of various sizes, modelled as massive polyhedra, are located at four latitudes {15 • , 45 • , 75 • , 87 • } as representatives of low, mid-low, mid-high, and high latitudes, respectively. We assumed these boulders are initially on top of the regolith layer, which could be the direct result of the re-accumulation that formed the asteroids after the catastrophic disruption of their parent bodies 12,13 , or much later by ballistic sorting 19 or Brazil-nut effect 20 during impactinduced seismic events. Two different types of regolith grains were used: one with moderate friction (internal friction angle of ∼ 25 • ) and one with gravel-like friction (internal friction angle of ∼ 30 • ).…”

“…Some of the buried boulders in the equatorial region on Ryugu could be exhumed again by migrating grains from the ridge towards the current geopotential lows at higher latitudes 12 . Other mechanisms including Brazil nut effect 20 and ballistic sorting 19 could also further modify these landforms.…”

Propagation and attenuation of pulses driven by low velocity normal impacts in granular media

“…When they reach a granular surface, a strong seismic pulse can cause the surface to deform, induce landslides and loft particles off the surface (e.g., Tancredi et al 2012;Wright et al 2020). These processes would also reduce the amplitude of reflected waves and increase the attenuation rate.…”

“…The rapidly attenuated seismic pulse or 'jolt' model (Nolan et al, 1992;Greenberg et al, 1994Greenberg et al, , 1996Nolan et al, 2001;Thomas and Robinson, 2005) is consistent with strong attenuation in dry laboratory granular materials at kHz frequencies (Hostler and Brennen, 2005;O'Donovan et al, 2016), but qualitatively differs from the slowly attenuating seismic reverberation model (Cintala et al, 1978;Cheng et al, 2002;Richardson et al, 2004Richardson et al, , 2005, that is supported by measurements of slow seismic attenuation rates in lunar regolith (Dainty et al, 1974;Toksöz et al, 1974;Nakamura, 1976). While both impact-induced seismic jolt and reverberation can cause crater erasure and crater rim degradation (Veverka et al, 2001;Nolan et al, 2001;Richardson et al, 2004Richardson et al, , 2005Thomas and Robinson, 2005;Asphaug, 2008), size segregation induced by the Brazil-nut effect could depend on sustained vibrations or reverberation (e.g., Miyamoto et al 2007;Tancredi et al 2012;Matsumura et al 2014;Tancredi et al 2015;Perera et al 2016;Maurel et al 2017;Chujo et al 2018), though a single seismic pressure pulse can also leave boulders on the surface (Wright et al, 2020) via ballistic sorting (Shinbrot et al, 2017).…”

Seismic waves in the asteroid environment

“…• If seismic shaking due to small meteorite impacts on small bodies is not produced, or at least not to the extent that has previously been portrayed, the BNE might not be a possible explanation for global particle size segregation. However, the surface could be segregated [16,17].…”