Collisional Modification of Metal‑Rich Asteroids and the Influence of Pre‑impact Rotation

Impact cratering is one of the primary processes influencing major landscape evolution on an asteroid’s surface. The largest craters offer a unique natural laboratory to investigate an asteroid's interior structure through the exhumation and redistribution of material via hypervelocity impacts. Metal-rich asteroids, thought to be the leftover cores of differentiated planetesimals [1], exhibit variable metal concentrations indicated by radar albedo measurements, and heavily cratered surfaces; however, a detailed understanding of their collisional modification is currently a key unanswered question. In addition, several of the M-types in the main belt, such as asteroid (16) Psyche, (216) Kleopatra, and (22) Kalliope, have rapid spin rates [2, 3]. As previously shown for asteroid (4) Vesta, ejecta deposition is not hemispherically symmetric when the target is rotating before impact [4]. Instead, the ejecta is deposited over multiple rotations, and the majority of the ejected material is reaccumulated by the asteroid, which may form features like folds and thrusts on the surface of the body. In this work, we demonstrate how the formation of large impact craters on metal-rich worlds, through hydrocode modeling, can provide the foundational framework for understanding their surface and interior morphology, including the influence of pre-impact rotation.

We have developed a workflow using the realistic shape models of asteroids, with asteroid (16) Psyche as an example, to conduct our high-resolution impact simulations, as we find that asteroid shape combined with pre-impact rotation plays a critical role in influencing the final ejecta distribution. Our 3D models use the Bern SPH code [5], which incorporates detailed and validated material treatments, including a strength and cohesion model, and a robust P-α porosity model, subject to a crushing curve [6, 7].

We consider two end-member metal-rich interior structures, a layered metal-silicate interior with a large iron core surrounded by a dunite mantle, with variable porosity throughout the target. Second, a stony-iron meteorite interior with variable porosity. We have developed a new Equation of State (EOS) for the latter, using a modified version of the Tillotson EoS, as one does not exist for more exotic materials like stony-iron. All impacts are performed at 5 km/s impact speed, reflective of the modern main belt. We also conduct additional simulations using idealized spherical targets with varying impact angle, and rotation rate (3-5 hours) to specifically analyze variable ejecta emplacement patterns. We will present provenance maps which track the excavation depth from these impacts on metal-rich targets and comment on the efficiency of material retention as a function of the material characteristics of the target (porosity/crushing strength), and rotation rate.

 

References

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[2] Marchis, F., Jorda, L., Vernazza, P., Brož, M., Hanuš, J., Ferrais, M., ... & Yang, B. (2021). (216) Kleopatra, a low density critically rotating M-type asteroid. Astronomy & Astrophysics, 653, A57.

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[6] Jutzi, M., Benz, W., & Michel, P. (2008). Numerical simulations of impacts involving porous bodies: I. Implementing sub-resolution porosity in a 3D SPH hydrocode. Icarus, 198(1), 242-255.

[7] Jutzi, M., Michel, P., Hiraoka, K., Nakamura, A. M., & Benz, W. (2009). Numerical simulations of impacts involving porous bodies: II. Comparison with laboratory experiments. Icarus, 201(2), 802-813.