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Seismic Waves in the AsteroidEnvironment

Presentation #104.04 in the session “Main Belt Asteroids 2: Physical Properties”.

Published onOct 26, 2020
Seismic Waves in the AsteroidEnvironment

Through numerical simulations and theory we investigate impact generated seismic wave transmission in granular media under extremely low pressure (0.1-50000Pa). We simulate a number of cm size particles, inside a box, at a nominal overburden pressure to mimic asteroid conditions. We Tnd a dependency not only on the overburden pressure on the medium, but also on the speed of the impact. Though the simulations correctly predict the wave speeds at high pressures (> 10 kPa), these speeds plateaued at lower pressures (approx. 220 m/s), which was not expected for an impact speed of 0.5 m/s. The addition of cohesive forces between the particles was seen to produce no appreciable effects in the investigated range. This prompted us to change the initial vertical velocity of particles that generated the pressure wave to 0.01, 0.1 and 5 m/s. This produced wave speeds that also changed with pressure but plateaued at different values (approx. 125, 150 and 380 m/s respectively) with little dependence on cohesion. This has prompted us to believe that the change in the local pressure produced by the passing wave was responsible for its velocity. At high pressure, the additional pressure provided by the passing wave would be inconsequential, but at lower pressures, such as the one existent in small bodies, it is determining. Taking what we have observed in our DEM simulations, and if we accept them as a good enough representation of such a granular system, with the usual caveats about particle shape, size distribution and contact dynamics, there are some things that we could say about impacts and seismic wave transmission on small planetary bodies and possible observations made by the mentioned missions: 1. The landing of asteroid Itokawa must have hardly disturbed particles far from the landing sites. At a distance of about 1 m from this place, most of the energy should have been dissipated. 2. The impact produced on asteroid Ryugu by the SCI should not have had much of an effect on the terrain in a radius greater than 5-10 m for the same reason. 3. The sampling manoeuvre planned by the OSIRIS-REx mission, at a nominal speed of 10 cm/s, should not cause any disturbance in the terrain around it. Note, that this refers only to the impact of the TAGSAM (Touch-and-Go Sample Acquisition Mechanism) head on the surface of Bennu and not to the gas ejection that will drive the sample.

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