Dust Clumps Dynamics of the Dimorphos Ejecta Plume

Introduction:   The NASA Double Asteroid Redirection Test (DART) impact [1], was the first space mission that successfully demonstrated the kinetic impactor technique for planetary defense. It was at the same instant, on 26^th September 2022, when ASI/Light Italian Cubesat for Imaging of Asteroids (LICIACube) [2] was the first Cubesat to image the plume coming from Dimorphos, the smaller body of the binary asteroid (65803) Didymos. The DART impact into Dimorphos [3] caused ejecta plume propagation with high velocity and very filamentary structure, composed of dust particles from μm to cm sizes in size [4]. The large aperture and observed spikes did not prevent propagation of larger excavated material, namely, boulders up to ~2 m [5]. Far-field observations such as HST clearly showed dust tail formed from the low-speed ejecta dust due to solar radiation pressure (SRP) [4].

The scientific objectives:   The estimation of the size distribution and velocity distribution of the plume in close vicinity to Dimorphos, captured in the LICIACube images is still an unanswered question. While the long-term monitoring of the tail can reveal the size distribution up to tens of cm in size, the impact simulations can constrain the initial velocity of the excavated material. Near and mid – field simulations considering different dynamical properties at local scale can address the complex collimated but inhomogeneous distribution of the dust within the plume. Here, we discuss some of the dynamical properties of the plume using the available observational DART and LICIACube data of the plume propagation. We try to constrain the particle sizes within the collimated plume structures.

The model:   We apply the 3D+t model – LIMARDE [6,7] constrained with laboratory observations [8], impact simulations and near- and far- field observations such as the LICIACube [9] images and HST [2] dust observations, respectively. The model computes single particle trajectories, the dust rotational frequencies and velocity as well as the particle orientation at any time and distance. We compute the dust velocity distribution based on the physical properties (size, mass and shape) derived from the LICIACube observations. The results are useful to check what is the role of the fragmentation of the particles and to constrain the physical properties based on the dynamical properties of the ejected dust in the near- and mid- environment.

Fig. 1. The dust speed and rotation frequency of particles with different shapes as constitutes of the dust clumps shown in the observations of ASI/LICIACube/LUKE, 2022-09-26T23:17:04.

Discussion: The LICIACube observations suggest that we have the locations of accumulation of different particles along the collimated plume streamers. The latter may contain particles of the same density and shape but with different velocity and rotation due to the initial ejected position and form not-linear motion within the collimated filament – like structures. In Fig. 1 we show LIMARDE simulations with particles of different shapes that result with different velocities suggesting a scenario where the dusty clumps could occur at the same location due to motion of particles with different shapes. The study discusses what is the probability that these dust clumps are formed owing to fragmentation, or their location is a result of their motion history of the ejected particles.

Acknowledgements: This research was supported by the Italian Space Agency (ASI) within the LICIACube (ASI-INAF agreement AC n. 2019-31-HH.0).

References: [1] Rivkin, A.S. et al. 2021, PSJ, 2, 24pp; [2] Dotto, E. et al. 2021, PSS 199, [3] Daly, R.T. et al. (2023) Nature. [4] Li, J.-Y., et al. (2023) Nature. [5] Farnham et al. LPSC abs. [6] Ivanovski et al. 2023, u.rev.; [7] Fahnestock et al. 2022, PSJ; [8] Ormo et al. 2022, E&PSL [9] Dotto et al. 2023, Nature