Boulder reaccumulation on Dimorphos after the DART Impact

          NASA’s DART spacecraft impacted the secondary of the Didymos asteroid binary system, Dimorphos, on 26 September 2023 at 23:14 UTC, releasing several thousand tons of asteroid material (Graykowski et al. 2023). Thirty-seven individual boulders, meters in size, were observed by Hubble Space Telescope (Jewitt et al. 2023) moving at roughly 30 cm/s, and Farnham et al. (2023) observed several dozens more moving at 20–50 m/s by analyzing LICIACube images, taken by DART’s onboard CubeSAT provided by the Italian Space Agency (the CubeSAT was released from DART 15 days prior to DART’s impact to image the event). Langner et al. (2024) and Moreno et al. (2024) consider the fate of such boulders, but dependent on the size- and velocity-frequency distribution of the ejected material, it is likely that many more boulders of order this size were lofted at speeds below ~24 cm/s (the approximate escape speed of the system from the surface of Dimorphos), many of which may have impacted Didymos or reimpacted Dimorphos at these comparably low speeds.

           The upcoming Hera mission will characterize the surfaces of the two bodies in great detail after it arrives to the Didymos system—slated to occur at the end of 2026—and the effects of such low-speed impacting boulders may be evident in what we observe with Hera. Using a soft-sphere discrete element method (SSDEM) contact model (Cundall, P.A. & Strack, O.D.L. 1979) introduced by Schwartz et al. (2012), and with additional contact physics (Zhang et al. 2017), in the n-body software package pkdgrav (Stadel, J.G. 2001; Richardson, D.C. et al. 2000), we are analyzing the mechanics of these secondary impacts and the post-DART implications they may have on the surfaces of Dimorphos and Didymos.

          A common feature in solar system solid-body disruption-reaccumulation simulations (e.g., in SSDEM: Schwartz et al. 2018; Michel P. and Ballouz, R.–L., et al. 2020) is to have small components be last to settle back onto surfaces. This is due to their number and collisional energy partitioning during reaccumulation. However, while considering non-gravitational forces including solar radiation pressure (Yu et al. 2017), smaller particles may clear the system before burying marginally bound boulders, making a further case for Hera to potentially observe boulders reaccumulated after the DART impact. This reaccumulation process in general has larger implications for size-frequency distributions we observe on the surfaces of small bodies and correlations to the energetics of their recent impacts, including whether large/disruptive impacts help to clear the surfaces of smaller regolith. Given that n-body reaccumulation simulations that do not consider non-collisional and non-gravitational forces (e.g., Schwartz et al. 2018; Michel P. and Ballouz, R.–L., et al. 2020), convincing evidence of low-speed boulder deposition in the wake of the DART impact would imply a clearing of small components from the system, either due to lofting caused by the DART impact or occurring subsequent to it.

          We will report on our progress in characterizing this type of reaccumulation and consider observational signatures of low-speed boulder impacts informed by this work.

 

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