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We present a framework to study regolith segregation on rubble-pile asteroids—self-gravitating granular aggregates—due to seismic shaking induced by impacts sustained during their lifetimes. We first relate the amplitude and frequency of surface vibrations to the location and severity of an impact, and the rubble body's geometry and bulk properties. For illustration, the body is taken to be an ellipsoid with size and spin close to that of Itokawa, although more complex asteroid shapes may be incorporated. We then model the body's collisional history stochastically given the variability in the impact activity on an asteroid. Finally, we use discrete element simulations to investigate the regolith's response to impacts. In these simulations, in any sample collisional history, every time an impact occurs, a bin filled with a grain mixture and located at the region of interest on the asteroid is vibrated at that impact's associated amplitude and frequency. Using this framework, we find that impact-driven seismicity is sufficient to drive size segregation on small rubble-piles, but the segregation quality depends on several aspects, e.g. total impact energy supplied, placement of the region of interest, bulk wave speed and seismic diffusivity. [ABSTRACT FROM AUTHOR]

We performed cratering experiments on targets composed of glass beads with a power‐law size distribution that simulated the surface of rubble‐pile asteroids, and we improved the previously studied reduction factor on the crater size scaling relationship including the armoring effect using the momentum transfer efficiency. Cratering experiments were conducted using light gas guns at Kobe University and ISAS/JAXA to control the impact velocity, vi ${v}_{\mathrm{i}}$, from 50 to 4,400 m s−1. Two kinds of mixed targets were prepared by mixing glass beads with diameters of 0.1, 1, 3, and 10 mm—one with the smallest diameter beads, and one without. The size ratio of the target bead to the projectile (diameter of 1–3 mm), ϕ $\phi $, changed from 0.03 to 10. The crater size scaling relationships for the mixed targets were found to depend on the first contacted bead size. Notably, first contact with a 10 mm‐sized bead reduced the crater radius by 35% in maximum. The reduction factor due to this armoring effect on the crater size scaling relationship is written as follows: f(ϕ)=1+8.99×10−3vi0.630.34ϕ−0.19 $f(\phi)={\left[1+\left(8.99\times {10}^{-3}\right){v}_{\mathrm{i}}^{0.63}\right]}^{0.34}{\phi }^{-0.19}$; it decreased with the increase of the size ratio of the target bead to the projectile, while it increased with the increase of the impact velocity and approached unity. Our improved crater size scaling relationship that includes the reduction factor could be used to reconstruct the crater size frequency distribution on rubble‐pile asteroids such as Ryugu, and it may lead to a revision of the crater chronology of asteroids. Plain Language Summary: A rubble‐pile asteroid, 162173 Ryugu, was explored by Hayabusa2, and the crater size frequency distribution on the surface was examined. Ryugu's surface age can be estimated from this size distribution using the impact crater chronology, but the size of the asteroids forming the craters is necessary for this estimate. The crater size scaling law is used to estimate the impactor size, but this law was established using homogeneous grains such as granular sand, so it might not be applicable for rubble‐pile asteroids covered with boulders of various sizes. Moreover, because it is well known that cratering efficiency is significantly reduced by an armoring effect when a smaller asteroid collides with a larger boulder, a scaling law that includes the armoring effect is required. In this study, we performed cratering experiments on the surface of simulated rubble‐pile asteroids and examined the armoring effect on the crater size. Based on the results, we improved the previously studied armoring effect and introduced the momentum transfer efficiency, β $\beta $, into the effect, instead of a restitution coefficient, which was assumed to be zero. Using this improved scaling relationship, a more accurate chronology can be established for the asteroid craters. Key Points: We performed cratering experiments on glass bead targets with a power‐law bead size distribution at 50–4,400 m s−1The size of craters formed in the mixed bead target was speculated to be changed with the size of the first contacted beadThe armoring effect is less efficient for granular targets with a size distribution, compared with equal‐sized granular targets [ABSTRACT FROM AUTHOR]

This paper considers the energy required for collections of finite-density bodies to undergo escape under internal gravitational interactions alone. As the level of the system energy is increased, there are different combinations of components that can escape, until the total energy becomes positive, when the entire system can undergo mutual disruption. The results are also defined for bodies modeled as a continuum. These results provide rigorous constraints for the disruption of rubble-pile asteroids when only considering gravitational interaction effects, with the energy provided by rotation of an initial collection of the system. These issues are considered for discrete particles in the N-body problem and for size distributions of discrete particles in the continuum limit. [ABSTRACT FROM AUTHOR]

We manually mapped particles ranging in longest axis from 0.3 cm to 95 m on (101955) Bennu for the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) asteroid sample return mission. This enabled the mission to identify candidate sample collection sites and shed light on the processes that have shaped the surface of this rubble-pile asteroid. Building on a global survey of particles, we used higher-resolution data from regional observations to calculate particle size-frequency distributions (PSFDs) and assess the viability of four candidate sites for sample collection (presence of unobstructed particles ≤ 2 cm). The four candidate sites have common characteristics: each is situated within a crater with a relative abundance of sampleable material. Their PSFDs, however, indicate that each site has experienced different geologic processing. The PSFD power-law slopes range from −3.0 ± 0.2 to −2.3 ± 0.1 across the four sites, based on images with a 0.01-m pixel scale. These values are consistent with, or shallower than, the global survey measurements. At one site, Osprey, the particle packing density appears to reach geometric saturation. We evaluate the uncertainty in these measurements and discuss their implications for other remotely sensed and mapped particles, and their importance to OSIRIS-REx sampling operations.

Quasi-2D, low-velocity experiments of colliding granular projectiles against granular targets were performed by means of a 7 m-long Hele-Shaw cell. The processes involved in the crater-opening mechanism of low-velocity granular-against-granular collisions are described. We show that the crater is opened mainly by a compaction process of the target. The projectile is fragmented and its lower section suffers a severe compaction; this projectile remnant forms a central dome or peak inside the crater. When the target reaches its maximum degree of compaction, the excess of momentum generates fast avalanches sliding on the slopes of the confined material, and exerts pressure on the crater walls, increasing its diameter. We propose that low-velocity collisions between granular aggregates are a possible mechanism that allows the growth of small planetary objects or the aggregation after catastrophic or high-energy collisions. [ABSTRACT FROM AUTHOR]

Small asteroids are often considered to be rubble-pile objects, and such asteroids may be the most likely type of Near Earth Objects (NEOs) to pose a threat to Earth. However, impact cratering on such bodies is complex and not yet understood. We perform three low-velocity (≈ 400 m/s) impact experiments in granular targets with and without projectile-size boulders. We conducted SPH simulations that closely reproduced the impact experiments. Our results suggest that cratering on heterogeneous targets displaces and ejects boulders, rather than fragmenting them, unless directly hit. We also see indications that as long as the energy required to disrupt the boulder is small compared to the kinetic energy of the impact, the disruption of boulders directly hit by the projectile may have minimal effect on the crater size. The presence of boulders within the target causes ejecta curtains with higher ejection angles compared to homogeneous targets. At the same time, there is a segregation of the fine ejecta from the boulders, resulting in boulders landing at larger distances than the surrounding fine grained material. However, boulders located in the target near the maximum extent of the expanding excavation cavity are merely exhumed and distributed radially around the crater rim, forming ring patterns similar to the ones observed on asteroids Itokawa, Ryugu and Bennu. Altogether, on rubble-pile asteroids this process will redistribute boulders and finer-grained material heterogeneously, both areally around the crater and vertically in the regolith. In the context of a kinetic impactor on a rubble-pile asteroid and the DART mission, our results indicate that the presence of boulders will reduce the momentum transfer compared to a homogeneous, fine-grained target. • Lab impacts successfully validate the SPH code aimed to simulate the DART impact. • Shots into rubble-pile analog target are closely reproduced by the SPH simulation. • Boulders in the target affect ejection, crater shape, and decrease momentum transfer. • When not hit by the projectile, boulders are displaced or ejected, not fragmented. • Results show crater patterns similar to ones observed on Ryugu and Bennu. [ABSTRACT FROM AUTHOR]

We manually mapped particles ranging in longest axis from 0.3 cm to 95 m on (101955) Bennu for the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) asteroid sample return mission. This enabled the mission to identify candidate sample collection sites and shed light on the processes that have shaped the surface of this rubble-pile asteroid. Building on a global survey of particles, we used higher-resolution data from regional observations to calculate particle size-frequency distributions (PSFDs) and assess the viability of four candidate sites for sample collection (presence of unobstructed particles ≤ 2 cm). The four candidate sites have common characteristics: each is situated within a crater with a relative abundance of sampleable material. Their PSFDs, however, indicate that each site has experienced different geologic processing. The PSFD power-law slopes range from −3.0 ± 0.2 to −2.3 ± 0.1 across the four sites, based on images with a 0.01-m pixel scale. These values are consistent with, or shallower than, the global survey measurements. At one site, Osprey, the particle packing density appears to reach geometric saturation. We evaluate the uncertainty in these measurements and discuss their implications for other remotely sensed and mapped particles, and their importance to OSIRIS-REx sampling operations. [ABSTRACT FROM AUTHOR]