An official website of the United States government
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.
more »
« less
FROM NEOS TO TNOS: HOW REGOLITH IS SHAPING THE SMALL WORLDS OF OUR SOLAR SYSTEM
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.
more »
« less
- Award ID(s):
- 1830609
- PAR ID:
- 10475060
- Publisher / Repository:
- Lunar and Planetary Institute Contributions
- Date Published:
- Journal Name:
- Asteroids, Comets, Meteors Conference 2023
- Page Range / eLocation ID:
- 2851
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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.more » « less
-
Abstract The surfaces of many planetary bodies, including asteroids and small moons, are covered with dust to pebble-sized regolith held weakly to the surface by gravity and contact forces. Understanding the reaction of regolith to an external perturbation will allow for instruments, including sensors and anchoring mechanisms for use on such surfaces, to implement optimized design principles. We analyze the behavior of a flexible probe inserted into loose regolith simulant as a function of probe speed and ambient gravitational acceleration to explore the relevant dynamics. The EMPANADA experiment (Ejecta-Minimizing Protocols for Applications Needing Anchoring or Digging on Asteroids) flew on several parabolic flights. It employs a classic granular physics technique, photoelasticity, to quantify the dynamics of a flexible probe during its insertion into a system of bi-disperse, centimeter-sized model grains. We identify the force chain structure throughout the system during probe insertion at a variety of speeds and for four different levels of gravity: terrestrial, Martian, lunar, and microgravity. We identify discrete, stick-slip failure events that increase in frequency as a function of the gravitational acceleration. In microgravity environments, stick-slip behaviors are negligible, and we find that faster probe insertion can suppress stick-slip behaviors where they are present. We conclude that the mechanical response of regolith on rubble-pile asteroids is likely quite distinct from that found on larger planetary objects, and scaling terrestrial experiments to microgravity conditions may not capture the full physical dynamics.more » « less
-
Abstract Frictional sliding along grain boundaries in brittle shear zones can result in the fragmentation of individual grains, which ultimately can impact slip dynamics. During deformation at small scales, stick–slip motion can occur between grains when existing force chains break due to grain rearrangement or failure, resulting in frictional sliding of granular material. The rearrangement of the grains leads to dilation of the granular package, reducing the shear stress and subsequently leading to slip. Here, we conduct physical experiments employing HydroOrbs, an elasto-plastic material, to investigate grain comminution in granular media under simple shear conditions. Our findings demonstrate that the degree of grain comminution is dependent on both the normal force and the size of the grains. Using the experimental setup, we benchmark Discrete Element Method (DEM) numerical models, which are capable of simulating the movement, rotation, and fracturing of elasto-plastic grains subjected to simple shear. The DEM models successfully replicate both grain comminution patterns and horizontal force fluctuations observed in our physical experiments. They show that increasing normal forces correlate with higher horizontal forces and more fractured grains. The ability of our DEM models to accurately reproduce experimental results opens up new avenues for investigating various parameter spaces that may not be accessible through traditional laboratory experiments, for example, in assessing how internal friction or cohesion affect deformation in granular systems.more » « less
-
El Mohtar, Chadi; Kulesza, Stacey; Baser, Tugce; Venezia, Michael D. (Ed.)Piles socketed into rock are frequently utilized to carry large loads from long-span bridges and high-rise buildings into solid ground. The pile design is derived from internal shear and moment magnitudes following code recommendation and numerical predictions. Little experimental data exist to validate code prescriptions and design assumptions for piles embedded in rock. To help alleviate the lack of large-scale test data, the lateral response behavior of three 18-in. diameter, 16 ft long, reinforced concrete piles was evaluated. The pile specimens were embedded in a layer of loose sand and fixed in “rock-sockets,” simulated through high strength concrete. The construction sequence simulated soil-pile interface stress conditions of drilled shafts. The pile reinforcement varied to satisfy the internal reaction forces per (1) code requirements, (2) analytical SSI predictions, and (3) structural demands only. The pile specimens were tested to complete structural failure and excavated thereafter. Internal instrumentation along with crack patterns suggested a combined shear-flexural failure, but do not support the theoretically predicted amplification and de-amplification of shear and moment forces at the boundary, respectively.more » « less
