Impacts and Ejecta in Natural Multi-scale Granular Material

Rubble-pile asteroids are a conglomeration of weakly gravitationally bound granular material. Missions such as OSIRIS-REx, Hyabusa, and Hyabusa2 to asteroids 101995 Bennu, 25143 Itokawa, and 162173 Ryugu have shown that their surfaces are strewn with material ranging from decimeters to tens of meters in size [1, 2]. The polydisperse nature of their surfaces complicates the dynamics of a low velocity secondary impacts (~m/s) and spacecraft landing on these bodies. Many experiments of impacts are performed in monodisperse granular material with spherical grains. We present experiments of low velocity impacts into a polydisperse mixture of sand and gravel, measuring the ejecta launch angles, velocities, and asymmetry in the ejecta flow field. We then compare these ejecta curtains to those made by impacts into monodisperse sand.

The granular media is held in an 11-gallon (41.6 liter) galvanized washtub that is predominately filled with monodisperse sized playground sand that is surrounding a polydisperse mixture as shown in Figure 1. The polydisperse mixture is positioned directly below the impact site. We film the resulting ejecta curtain using two Krontech Chronos 2.1high-speed cameras at 1000 frames per second (fps) positioned above and to the side of the impact site. The sand grains are nearly uniform in size with diameters on the order of micrometers. Larger gravel grains have sizes of a few centimeters. The large gravel grains are painted using fluorescent paint and lit with blue LEDs to contrast against the grains against the sand.

We use a glass marble as our impactor with diameter of 3.5 cm, painted black with green dots, and have a mass of about 60g. The impactor is released from a height of ~200 cm resulting in an impact velocity of ~6.2 m/s. Two experimental sets are presented, impacts into monodisperse sand, and impacts into the polydisperse mixture.

We analyze our ejecta curtain using three different methods: particle tracking to measure the velocity field of the ejecta curtain, a histogram of oriented gradients (HOG) to investigate the ejecta angle and structure, and particle image velocimetry (PIV) to measure the azimuthal distribution of ejecta around the impact site.

We find that the ejecta curtain produced by impacts into a polydisperse media is very different from those produced in impacts into monodisperse sand. We use particle tracking to measure the velocity of the ejecta from the side of the impact. Plotting the velocity components of the ejecta from our experiments shows asymmetry in the ejecta velocities with different slopes bounding the locus of points. Our HOG analysis highlights the complex structure in the ejecta curtain caused by the large gravel grains buried below the surface interacting with the smaller sand grains. With PIV, we see a clear azimuthal asymmetry around the impact point with impacts into sand displaying more evenly distributed ejecta direction. However, impacts into polydisperse media have a multi-modal distribution. These experiments qualitatively reproduce results of the Small Carry-on Impactor (SCI) impact into the asteroid Ryugu [3, 4].

[1] DellaGiustina, D.N., et. al (2019) Properties of rubble-pile asteroid (101955) Bennu from osiris-rex imaging and thermal analysis. Nature Astronomy 3, 341–351.

[2] Michikami, T., et. al (2019) Boulder size and shape distributions on asteroid Ryugu. Icarus 331, 179–191.

[3] Honda, R., et. al (2021) Resurfacing processes on asteroid (162173) Ryugu caused by an artificial impact of Hayabusa 2’s small carry-on impactor. Icarus 366, 114530.

[4] Arakawa, M., et. al (2020) An artificial impact on the asteroid (162173) Ryugu formed a crater in the gravity-dominated regime. Science 368, 67–71. doi:10.1126/science.aaz1701