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

TL;DR

This study investigates how low velocity impacts generate seismic pulses in granular media, revealing rapid attenuation and anisotropic propagation, with implications for asteroid impact experiments like DART.

Contribution

The paper introduces an advection-diffusion model to describe pulse attenuation and shape evolution in granular media, supported by experimental data and applied to asteroid impact scenarios.

Findings

Pulse amplitude decreases with distance following a -2.5 power law.

Seismic energy attenuates rapidly, supporting a seismic jolt model.

Impact pulses can exceed surface gravity within an asteroid.

Abstract

We carry out experiments of low velocity normal impacts into granular materials that fill an approximately cylindrical 42 litre tub. Motions in the granular medium are tracked with an array of 7 embedded accelerometers. Longitudinal pulses excited by the impact attenuate and their shapes broaden and become smoother as a function of travel distance from the site of impact. Pulse propagation is not spherically symmetric about the site of impact. Peak amplitudes are about twice as large for the pulse propagating downward than at 45 degrees from vertical. An advection-diffusion model is used to estimate the dependence of pulse properties as a function of travel distance from the site of impact. The power law forms for pulse peak pressure, velocity and seismic energy depend on distance from impact to a power of -2.5 and this rapid decay is approximately consistent with our experimental measurements. Our experiments support a seismic jolt model, giving rapid attenuation of impact generated seismic energy into rubble asteroids, rather than a reverberation model, where seismic energy slowly decays. We apply our diffusive model to estimate physical properties of the seismic pulse that will be excited by the forthcoming DART mission impact onto the secondary, Dimorphos, of the asteroid binary (65803) Didymos system. We estimate that the pulse peak acceleration will exceed the surface gravity as it travels through the asteroid.