High Pressure Shock Metamorphism in Rubble-pile Asteroids using Numerical Simulations

Introduction: Shock metamorphism in ordinary chondrites [1] is driven by impact events between asteroids. One effect of shock is the darkening of the lithology, which happens at two different stages of the shock intensity scale, at shock stage C-S7 (>70 GPa, whole rock melting, [1]) or between shock-stage 5 and 6 (40–60 GPa, metals and iron sulfides melt veins [2−7]). The darkening affects reflectance spectra of ordinary chondrites and, consequently, of asteroids. If an asteroid fragment of the S-complex asteroids is shock-darkened, its spectra will be similar to C/X-complex asteroids. This would affect the generally accepted spectroscopy-derived distribution of asteroids in the Main Asteroid Belt [7,8].

We investigate the impact conditions on rubble-pile asteroids in order to study the shock stage distribution and the mass of possibly shock-darkened asteroidal fragments. The distribution of porosity in rubble-pile asteroids depends on the internal structure [9]. The aim of the research is to highlight what type of rubble-pile asteroid collisions can explain the abundance of high shock stage materials [10] and the abundance of shock-darkened asteroidal fragments within the asteroid population.

Methods: We use the iSALE code [11] to simulate hypervelocity impact processes. We used the ε-α compaction model [11] for porosity, strength properties from [12] and an ANEOS for dunite to represent the material of the projectile and asteroid. We applied cylindrical-symmetry 2-D Eulerian gridwith Lagrangian tracers to study the distribution of peak shock pressures [13,14]. We apply numerical models of simplified rubble-pile structures of asteroids and study: a) The distribution of peak shock pressures (shock metamorphism). b) Quantification of shock-darkened material in the rubble-pile asteroid as well as in the escaping material. The model of a rubble-pile asteroid of 5 km in diameter is represented by several porous boulders of varying sizes surrounded by loose material. For the different impact scenarios, we varied the impact velocities (4−10km/s), the projectile diameters (800-1600m) and the porosities of the boulders (10−30%) as well as the porosity of the loose material (75−100%).

Results and discussion: The distribution of peak shock pressures strongly depends on both: impact velocity and projectile size (Fig.1a, b). Scenarios with cases considering higher boulder porosities (30%) only result in a small decrease of pressure (Fig. 1c, left panel) and thus lower shock stages are experienced as the shock wave is dampened by the compaction of porosity.

On the one hand porosity is responsible for an overall energy absorption but can on the other hand lead to localized pressure amplifications. Larger porosities of loose material (Figure 1c, right panel; here empty space) do not lead to significant differences compared to the case with porosities of 75% (Fig. 1a, left panel). We also observe that the rubble-pile asteroid is almost completely destroyed and porosity of the loose material is crushed out by the shock wave.

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