Inferring the interior of asteroid Dimorphos from hypervelocity DART-scale impact simulations
Authors: Senel Cem Berk , Karatekin Özgür , Luther Robert , Dai Kaiyi , Zhu Meng-Hua , Wünnemann Kai , Claeys Philippe 
Published: July 2024

Introduction:&#160; Impacts are prevalent across the Solar System, acting as one of the fundamental mechanisms shaping the evolution of celestial bodies from large planets to asteroids and comets. From small-scale Martian meteorites to larger biosphere-forming collisions, e.g., the Chicxulub event, impact events are essential for comprehending the dynamic history and evolution of planetary bodies. Collisions in space have been explored by missions to the near-Earth Asteroids (NEAs), leading to major advancements in characterizing Near-Earth Objects (NEOs), from JAXA's Hayabusa2 sample-return mission on asteroid Ryugu to NASA&#8217;s recent DART space mission that performed the first kinetic deflection on asteroid Dimorphos [1,2]. Up next, the Hera mission by the European Space Agency (ESA) is set to rendez-vous with Dimorphos to assess the effects of DART impact from a unique proximity in 2026 [3]. Understanding the aftermath of the DART impact, shock physics modeling emerges as a promising approach. Model simulations have thus far examined the impact cratering, ejecta plume, and momentum exchange resulting from DART-scale impactors. These analyses encompassed factors varying from the surface strength and porosity [4,5], to target layering [5,6], obliquity [7] and projectile shape [8], boulder distribution [6,9], and impact-induced gravity anomalies [10].Methods: Interior features of near-Earth asteroids are mostly unknown. To infer what lies below the surface of asteroid Dimorphos, here we tested various interior scenarios through the iSALE-2D shock physics modeling [11-13]. Regarding the model set-up, the latest material and mechanical parameters are documented in detail for the impactor and target asteroid [14-16]. To model the porosity compaction of the target material, we use the &#949;-&#945; model [13] in a wide porosity range (10-50%) for a monolithic and homogeneous interior. This process is iterated for heterogeneous interiors consisting of multiple inner layering with or without weak subsurface and strong core formation, and meter-scale boulders, including the&#160;surface boulder Atabaque, as observed by the DRACO camera onboard the DART spacecraft [17].Results: We document the consequences of the DART collision, performing a series of hyper-velocity impact simulations considering various interior compositions and structures in iSALE-2D. Results are depicted at t=1s after impact in terms of porosity and density fields. The crater size and morphology highly rely on the asteroid strength. In the case of a homogeneous interior with a cohesion of 100kPa, a 10-meter-wide crater forms at t=1s. At a lower strength of 1 kPa the crater is 20-meter-wide at t=1s. As expected, the lower the strength, the larger is the crater diameter.&#160; Besides, the results highlight the crucial role of boulder positioning and subsurface layering in shaping crater morphology. For instance, when a half-buried boulder the size of Atabaque [17] is positioned off-center, it results in substantially deeper craters. This stems from the impact on weak and high porosity regolith (Fig. 1h), in contrast to those formed from head-on collisions with a boulder (Fig. 1i).Perspectives and future work: For a rubble-pile asteroid with extremely low strength, the crater size can be much larger. We will include such cases as well, i.e. weak asteroids (Y0 = 1-100 Pa) with a homogeneous interior, in the next step. We will examine the impact of boulder representation in 2D axisymmetric simulations, exploring both single and double boulder configurations in three-dimensional setups using the output iSALE-3D. As the impact crater continues to collapse for several seconds after impact in the case of rubble-pile asteroids subject to low cohesions, we will comprehensively examine cratering formation over extended timeframes. To achieve this, a fast time integration scheme is implemented into the iSALE-2D code (similar to the approach documented in Ref. [15]), to thoroughly explore plausible cratering processes associated with the interior heterogeneity, as assisted by the available telescopic ejecta observations following the NASA DART impact.Figure 1: Effects of the DART-scale impact on the asteroid Dimorphos with different interior structures. Homogeneous interior with a cohesion of a 100 kPa and b 1 kPa with 10% porosity. c Heterogeneous interior with a strong core overlaid by a thin surface layer having a low cohesion of 1 Pa, Heterogeneous interior d with a subsurface boulder, e with a half-buried boulder, the size of boulder Atabaque, positioned off-center, f at the center. Results in d, e, and f show the initial state, while g, h, and i display corresponding cases at t=1s post-impact.Acknowledgments: This research is financially supported by the Research Foundation Flanders (FWO) with grant: 12AM624N to C.B.S. P.C. acknowledge the support of the Vrije Universiteit Brussel (VUB) strategic program. O.K. acknowledges the support of European Union&#8217;s Horizon 2020 research and innovation program, NEO-MAPP project (grant: 870377), and the PRODEX program managed by the ESA with the help of the Belgian Science Policy Office (BELSPO). Here, we gladly acknowledge the developers of iSALE-2D shock physics model, including G. Collins, K. W&#252;nnemann, D. Elbeshausen, T. Davison, B. Ivanov, &amp; J. Melosh. Plots were created using pySALEPlot tool developed by T. Davison available in iSALE-2D.References: [1] Daly et al.(2023). Nature,616(7957), 443-447. [2] Thomas et al.(2023). Nature,616(7957), 448-451. [3] Michel et al.(2022). PSJ,3(7),160. 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