P037-09
Simulation of seismic wave propagation on Ryugu induced by the SCI impact experiment of the Hayabusa2 mission: Limited seismic shaking due to low yield strength of the surface regolith
Simulation of seismic wave propagation on Ryugu induced by the SCI impact experiment of the Hayabusa2 mission: Limited seismic shaking due to low yield strength of the surface regolith
Thursday, 10 December 2020: 07:32
Virtual
Abstract:
Recently, multiple space exploration missions have revealed that topography of asteroids is affected by active surface renewal. While several processes can account for both lateral and vertical mass transport of asteroid regolith, the most plausible hypothesis has been global vibration, that is, impact-induced seismic shaking. The Hayabusa2 was expected to observe such impact-induced seismic shaking through the Small Carry-on Impactor (SCI) operation based on previous numerical simulations and laboratory experiments which suggest that the energy of this impact experiment is high enough to generate boulder movement around a newly formed crater. However, images of the periphery of the SCI crater taken by the Hayabusa2 Optical Navigation Camera (ONC) show that boulders located around the edge of the crater appear moved little by the seismic waves supposedly produced by the SCI impact. Aiming to explain this unexpected observation, we conduct seismic wave propagation modelling with a wide range of elastic parameters of Ryugu.
Using a fast modelling code for 3D seismic wave propagation, AxiSEM3D, we numerically simulate seismic shaking on asteroids, changing rigidities, quality factors, and seismic efficiencies (the conversion ratio of impactor’s kinetic energy to seismic wave energy), assuming an isotropic seismic moment tensor of explosion form. Through the comparison between simulations and observations, we find that seismic efficiency smaller than 10-7 is required. Such a low seismic efficiency can result from the powdery behavior of regolith. Seismic waves propagate through powdery materials only if the seismic stress is lower than its yield strength. Assuming that the yield strength of regolith is 1 kPa as indicated by laboratory experiments, calculated boulders’ lateral movement would be always smaller than 1 m. This hypothesis may account for the absence of boulder lateral movement observed in the SCI operation during the Hayabusa2 mission and indicate that the low yield strength of regolith possibly limits the seismic wave propagation on asteroids, making the apparent seismic efficiency small. This result also indicates that global and long-lasting seismic shaking on rubble-pile asteroids is not efficient, consistent with the presence of boulders on boulders.
Using a fast modelling code for 3D seismic wave propagation, AxiSEM3D, we numerically simulate seismic shaking on asteroids, changing rigidities, quality factors, and seismic efficiencies (the conversion ratio of impactor’s kinetic energy to seismic wave energy), assuming an isotropic seismic moment tensor of explosion form. Through the comparison between simulations and observations, we find that seismic efficiency smaller than 10-7 is required. Such a low seismic efficiency can result from the powdery behavior of regolith. Seismic waves propagate through powdery materials only if the seismic stress is lower than its yield strength. Assuming that the yield strength of regolith is 1 kPa as indicated by laboratory experiments, calculated boulders’ lateral movement would be always smaller than 1 m. This hypothesis may account for the absence of boulder lateral movement observed in the SCI operation during the Hayabusa2 mission and indicate that the low yield strength of regolith possibly limits the seismic wave propagation on asteroids, making the apparent seismic efficiency small. This result also indicates that global and long-lasting seismic shaking on rubble-pile asteroids is not efficient, consistent with the presence of boulders on boulders.