Boulder reaccumulation on Dimorphos after the DART Impact
Authors: Schwartz Stephen R. , Campo Bagatin Adriano , Liu Po-Yen , Parro Laura M. 
Published: July 2024

&#160; &#160; &#160; &#160; &#160; NASA&#8217;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&#8211;50 m/s by analyzing LICIACube images, taken by DART&#8217;s onboard CubeSAT provided by the Italian Space Agency (the CubeSAT was released from DART 15 days prior to DART&#8217;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.&#160; &#160; &#160; &#160; &#160; &#160;The upcoming Hera mission will characterize the surfaces of the two bodies in great detail after it arrives to the Didymos system&#8212;slated to occur at the end of 2026&#8212;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. &amp; 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.&#160; &#160; &#160; &#160; &#160; A common feature in solar system solid-body disruption-reaccumulation simulations (e.g., in SSDEM: Schwartz et al. 2018; Michel P. and Ballouz, R.&#8211;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.&#8211;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.&#160; &#160; &#160; &#160; &#160; 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.&#160;Bibliography:Graykowski, A., Lambert, R.A., Marchis, F. et al. (2023). 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