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1. Strength of fine-coarse mixtures in rubble pile asteroids

   Cox, C.; Brisset, J.; Partida, A.; Madison, A.; Bitcon, O. (October 2022, Bulletin of the AAS)

   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. 

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2. FROM NEOS TO TNOS: HOW REGOLITH IS SHAPING THE SMALL WORLDS OF OUR SOLAR SYSTEM

   Brisset, J. (June 2023, Asteroids, Comets, Meteors Conference 2023)

   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. 

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3. Strength-dilatancy and critical state behaviours of binary mixtures of graded sands influenced by particle size ratio and fines content

   https://doi.org/10.1680/jgeot.20.p.320  

   Yilmaz, Y.; Deng, Y.; Chang, C. S.; Gokce, A. (January 2021, Geotechnique)

   null (Ed.)

   Binary granular soil mixtures, as common heterogeneous soils, are ubiquitous in nature and man-made deposits. Fines content and particle size ratio are two important gradation parameters for a binary mixture, which have potential influences on mechanical behaviours. However, experimental studies on drained shear behaviour considering the whole range of fines content and different particle size ratios are scarce in the literature. For this purpose, we performed a series of drained triaxial compression tests on dense binary silica sand mixtures with 4 different particle size ratios to systematically investigate the effects of fines content and particle size ratio on the drained shear behaviours. Based on these tests, the strength-dilation behaviour and critical state behaviour were examined. It was observed that both fines content and particle size ratio have significant influence on the stress-strain response, the critical state void ratio, the critical state friction angle, the maximum dilation angle, the peak friction angle, and the strength–dilatancy relation. The underlying mechanism for the effects of fines content and particle size ratio was discussed from the perspective of the kinematic movements at particle level. 

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4. Bennu and Ryugu: diamonds in the sky

   https://doi.org/10.1007/s10035-021-01152-z  

   Sabuwala, Tapan; Chakraborty, Pinaki; Shinbrot, Troy (September 2021, Granular Matter)

   Abstract Rapidly spinning and loosely aggregated asteroids appear to take on diamond-shaped profiles, with elevated poles as well as equators. The evolutionary processes that form these characteristic shapes remain a matter of debate. In this paper, we propose a novel model, based on debris accretion, to explain these diamond-shaped profiles. We derive an analytic expression for the shapes of such rapidly spinning rubble piles based on the principle that as rubble is deposited it assumes a critical angle of repose. We show that this expression correctly reproduces diamond shaped profiles. We also conduct granular simulations of debris deposition and show that simulated shapes are in striking accord with both observations and analytical results. Our results suggest that non-uniform debris accumulation, which is overlooked in current models, may play a cardinal role in the formation of diamond-shaped asteroids. 

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5. An Efficient Numerical Approach to Modeling the Effects of Particle Shape on Rubble-pile Dynamics

   https://doi.org/10.3847/PSJ/ad0467  

   Marohnic, Julian C.; DeMartini, Joseph V.; Richardson, Derek C.; Zhang, Yun; Walsh, Kevin J. (December 2023, The Planetary Science Journal)

   Abstract We present an approach for the inclusion of nonspherical constituents in high-resolutionN-body discrete element method (DEM) simulations. We use aggregates composed of bonded spheres to model nonspherical components. Though the method may be applied more generally, we detail our implementation in the existingN-body codepkdgrav. It has long been acknowledged that nonspherical grains confer additional shear strength and resistance to flow when compared with spheres. As a result, we expect that rubble-pile asteroids will also exhibit these properties and may behave differently than comparable rubble piles composed of idealized spheres. Since spherical particles avoid some significant technical challenges, most DEM gravity codes have used only spherical particles or have been confined to relatively low resolutions. We also discuss the work that has gone into improving performance with nonspherical grains, building onpkdgrav's existing leading-edge computational efficiency among DEM gravity codes. This allows for the addition of nonspherical shapes while maintaining the efficiencies afforded bypkdgrav's tree implementation and parallelization. As a test, we simulated the gravitational collapse of 25,000 nonspherical bodies in parallel. In this case, the efficiency improvements allowed for an increase in speed by nearly a factor of 3 when compared with the naive implementation. Without these enhancements, large runs with nonspherical components would remain prohibitively expensive. Finally, we present the results of several small-scale tests: spin-up due to the YORP effect, tidal encounters, and the Brazil nut effect. In all cases, we find that the inclusion of nonspherical constituents has a measurable impact on simulation outcomes. 

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