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Most smaller asteroids (1 km diameter) are granular material loosely bound together primarily by self-gravity known as rubble piles. In an effort to better understand the evolution of rubble-pile asteroids, we performed bulk measurements using granular simulant to study the effects of the presence of fine grains on the strength of coarse grains. Our laboratory samples consisted of fine–coarse mixtures of varying percentages of fine grains by volume of the sample. We measured the material’s angle of repose, Young’s Modulus, angle of internal friction, cohesion, and tensile strength by subjecting the samples to compressive and shear stresses. The coarse grains comprising the fine–coarse mixtures ranged from 1 mm to 20 mm (2 cm) and the fines were sieved to sub-millimeter sizes (1 mm). The measured angles of repose varied between 32–45 which increased with increasing fine percentage. In compression, samples generally increased in strength with increasing fine percentage for both confined and unconfined environments. In all cases, the peak strengths were not for purely fine grains but for a mixture of fine and coarse grains. Shear stress measurements yielded angles of internal friction ranging between 25 and 45 with a trend opposite that of the angle of repose, 300–550 Pa for bulk cohesion, and 0.5–1.1 kPa for tensile strength. Using other published works that include data from telescopic and in-situ observations as well as numerical simulations, we discussed the implications of our findings regarding rubble-pile formation, composition, evolution, and disruption. We find that the presence of fine grains in subsurface layers of regolith on an asteroid (confined environment) aids the avoidance of disruption due to impact. However these same fines increase an asteroid’s chance to disrupt or deform from high rotation speeds due to reduced grain interlocking. In surface layers (unconfined environments), we find that the presence of fine grains between coarse ones generates stronger cohesion and aids in the prevention of mass loss and surface shedding. 

Introduction: With the capture of the first high- resolution, in-situ images of Near-Earth Objects (NEOs) a couple of decades ago [1–4], the ubiquity of regolith and the granular nature of small objects in the Solar System became apparent. Benefiting from an increased access to high computing power, new numerical studies emerged, modeling granular structures forming and evolving as small bodies in the Solar System [5–7]. Now adding laboratory studies on granular material strength for asteroid and other small body applications [8,9], we are steadily progressing in our understanding of how regolith is shaping the interiors and surfaces of these worlds. In addition, our ever-more powerful observation capabilities are uncovering interesting dust-related phenomena in the outer skirts of our Solar System, in the form of activity at large heliocentric distances and rings [10–12]. We find that our recent progress in understanding the behavior of granular material in small body environments also has applications to the more distant worlds of Centaurs and Trans-Neptunian Objects (TNOs). Internal Strength: We currently deduce internal friction of rubble piles from the observation of large numbers of small asteroids and their rotation rates, combined with the associated numerical simulations [13,14]. In the laboratory, we study internal friction of simulant materials using shear strength measurements [8]. Combining observations, modeling, and laboratory work, the picture emerges of rubble pile interiors being composed of coarse grains in the mm to cm range. The irregular shapes of the grains lead to mechanical interlocking, thus generating the internal friction required to match observations of the asteroid population [8,9]. We find that the presence of a fine fraction in the confined interior of a rubble pile actually leads weaker internal strength [9]. Surface Strength: Deducing surface regolith strength for NEOs is usually performed via average slope measurements [15–17] or, most notably, observing the outcome of an impact of known energy [18]. In the laboratory, we measure the angle of repose of simulant material via pouring tests, as well as its bulk cohesion using shear strength measurements [8]. In some cases, this allows us to infer grain size ranges for various regions of the surface and subsurface of pictured NEOs, beyond the resolution of their in-situ images. Surface Activity: The Rosetta mission revealed that a number of activity events on comet 67P/Churyumov–Gerasimenko were linked to active surface geology, most notably avalanches and cliff collapses [19]. In addition, the role of regolith strength in asteroid disruption patterns has been inferred from numerical simulations of rotating rubble piles [20]. By studying strength differences in simulant samples, it becomes apparent that a difference in cohesion between a surface and its subsurface layer can lead to activity events with surface mass shedding, without the presence of volatiles sublimating as a driver [8]. We show that such differences in surface strength can be brought upon by a depletion in fine grains or a change in composition (e.g. depletion in water ice) and could account for regular activity patterns on small bodies, independently of their distance to the Sun. This is of particular interest to the study of Centaur activity and a potential mechanism for feeding ring systems. 

Several lines of evidence indicate that most of the smaller asteroids (< 1 km) consist of granular material loosely bound together primarily by self-gravity; these are commonly called rubble piles. While the strength of these rubble piles is valuable information on their origin and fate, it is still debated in the literature. We report on a laboratory measurement campaign on fine-coarse mixtures of simulated asteroid regolith. In a series of table-top measurements, we have determined sample compression and shear strengths for various fine-fractions within coarse-grained samples. We used confined setups (less than 10cm in length) to measure the strength of the material in constricted environments such as an asteroid’s core and unconfined setups (greater than 10cm in length) to simulate open environments such as the surface of an asteroid. Using CI Orgeuil high fidelity asteroid soil simulant, we performed three measurement types to determine the strength of our samples: shear yield, which in turn provided values for the Angle of Internal Friction (AIF), bulk cohesion, and tensile strength of the samples; compression strength, which allowed to calculate the Young’s Modulus (YM); and the Angle Of Repose (AOR). From the AOR, we determined the coefficient of friction of each sample. Samples of regolith were created by measuring percentage by volume amounts of both coarse and fine grains into the measurement container. We prepared coarse grains in two size distributions, mm-sized and cm-sized. The fine fraction was composed of grains sieved between 100 and 250 μm. For compression and AOR measurements, we find that the strength of the coarse grain samples increases with the addition of a fine fraction. However, we find that the increase of the fine fraction in a sample of coarse grains does not consistently increase the sample shear strength. With increasing fine fractions, the AIF and bulk cohesion of the mixed samples decrease (until a point of saturation). This could be indicative of the fine grains acting as a lubricant as the larger grains move across each other, aiding rolling and reducing interlocking strength. Our findings suggest that in the case of the surface of an asteroid, the presence of fine grains does indeed increase the strength of coarse regolith material. However, fine grains in the regolith sublayers or the asteroid interior will reduce material strength due to grain interlocking and ease disruption. Therefore, rubble piles that are depleted in fine grains will have higher internal strength compared to those composed of grain size distributions that include sub-mm sized particles.