Growing small KBOs through gentle collisions: Implementing particle spin and SSDEM into REBOUND

Several lines of evidence indicate that small bodies of the outer solar system are highly porous and grew through gentle collisions of icy aggregates [1,2]. A potential formation process for these Kuiper Belt Objects (KBOs) is hierarchical growth over several orders of magnitude by sticking of monomer grains composed of dust-ice mixtures. Monomers form aggregates, which in turn form aggregates of aggregates, and so on until km-sized lobes are available for the formation of bilobate bodies as seen for comet 67P/Churyumov-Gerasimenko [3] and Arrokoth (2014 MU69) [4].

In order to evaluate the validity of such a formation process for small KBOs, we are planning to combine small-scale laboratory collision experiments with contact mechanics theory and numerical simulations to determine scaling laws for the collision outcomes in the appropriate regime. In this manner, we will be able to span over the required orders of magnitude in size and study the growth of cometesimals from their monomer grains. Here, we present the current effort to develop our own N-body code based on the existing open-source N-body code REBOUND [5]. As particle spin is currently not supported in REBOUND, we have been extending the code in order to include this feature, as well as to implement the soft-sphere discrete element method (SSDEM) that captures relevant aspects such as rolling and twisting friction. Working with REBOUND will allow us to share our code adjustments with the planetary science community at large thus enabling other researchers to perform their own SSDEM N-body simulations of dust aggregate growth. We show our methodology and present first test results for this new extension of REBOUND.

[1] Kofman, et al. (2015), Science, 349(6247), p. aab0639.

[2] McKinnon, et al. (2020), Science, 367(6481).

[3] Davidsson, et al. (2016), Astronomy & Astrophysics, 592, p. A63.

[4] Stern, et al. (2019), Science, 364(6441), p. eaaw9771

[5] Rein & Liu, (2012), Astronomy & Astrophysics, 537, p. A128.

[6] Wada, et al. (2011), The Astrophysical Journal, 737, article id. 36.