Rubble Pile Research Articles

Heat transfer in granular media with weakly interacting particles

We study the heat transfer in weakly interacting particle systems in vacuum. The particles have surface roughness with self-affine fractal properties, as expected for mineral particles produced by fracture, e.g., by crunching brittle materials in a mortar, or from thermal fatigue or the impact of micrometeorites on asteroids. We show that the propagating electromagnetic (EM) waves give the dominant heat transfer for large particles, while for small particles both the evanescent EM-waves and the phononic contribution from the area of real contact are important. As an application, we discuss the heat transfer in rubble pile asteroids.

Implications for the Collisional Strength of Jupiter Trojans from the Eurybates Family

In this work, we model the collisional evolution of the Jupiter Trojans and determine under which conditions the Eurybates-Queta system survives. We show that the collisional strength of the Jupiter Trojans and the age of the Eurybates family and by extension Queta are correlated. The collisional grinding of the Jupiter Trojan population over 4.5 Gy results in a size–frequency distribution (SFD) that remains largely unaltered at large sizes (>10 km) but is depleted at small sizes (10 m to 1 km). This results in a turnover in the SFD, the location of which depends on the collisional strength of the material. It is to be expected that the Trojan SFD bends between 1 and 10 km. Based on the SFD of the Eurybates family, we find that the family was likely the result of a catastrophic impact onto a 100 km rubble pile target. This corresponds to objects with a rather low collisional strength (10 times weaker than that of basaltic material studied in Benz & Asphaug). Assuming this weak strength, and an initial cumulative slope of the size–frequency distribution of 2.1 between diameters of 2 m and 100 km when the Trojans were captured, the existence of Queta, the satellite of Eurybates, implies an upper limit for the family age of 3.7 Gy. Alternatively, we demonstrate that an unconventional collisional strength with a minimum at 20 m is a plausible candidate to ensure the survival of Queta over the age of the solar system. Finally, we show how different collisional histories change the expected number of craters on the targets of the Lucy mission and that Lucy will be able to differentiate between them.

The mid-Ordovician meteorite flux to Earth shortly before breakup of the L-chondrite parent body

The breakup of the L-chondrite parent body (LCPB) in the asteroid belt at 466 Ma ago is the largest asteroid breakup documented in Earth's geological record for the past ca. three billion years. Recovery of abundant macroscopic fossil L chondrites in mid-Ordovician marine sediments as well as reconstructions of the flux of micrometeoritic chrome spinel through the ages have given much new information on the precise timing of the breakup and its effects on Earth. In the present study, we focus on the flux of large micrometeorites to Earth shortly (in the 2 Ma time interval) before the LCPB breakup (pre-LCPB), which may be crucial for understanding the dynamical evolution of the asteroid belt leading up to the breakup. We present chrome-spinel data (32–355 μm grain size) from two mid-Ordovician limestone sections in Sweden (Kinnekulle and Öland, 300 km apart) and one section in western Russia (Lynna River), ca. 1100 km from Kinnekulle. One aim is also to test the level of reproducibility of chrome-spinel flux reconstructions between different sites.Between 300 and 600 kg of limestone were collected from each section in the stratigraphic interval corresponding to ca. 2 Ma before up to immediately before the LCPB breakup. The relations between H, L and LL meteorites from Kinnekulle (38.7 ± 6.3%, 33.2 ± 6.1% and 28.1 ± 5.8%), Öland (46.0 ± 5.6%, 31.2 ± 3.1% and 24.5 ± 4.8%) and Lynna River (38.2 ± 5.5%, 32.8 ± 5.3% and 29.0 ± 5.1%) sections are indistinguishable from each other within uncertainties, revealing a globally homogeneous influx of H, L and LL meteorites. This gives support for the validity of previous reconstructions for the meteorite flux based on chrome-spinel reconstructions for fifteen time windows through the Phanerozoic.All the pre-LCPB samples from the three regions show a collective dominance of H-chondritic grains (42 ± 3%) over L (31 ± 3%) and LL grains (27 ± 3%), largely similar to the Phanerozoic background flux. Intriguingly, the presence of background concentrations of L-chondritic material also in the pre-LCPB flux demonstrates that the idea of a largely intact LCPB still existing before the final breakup may be far from reality. Apparently, a substantial amount of equilibrated chondritic material from deep levels of an L-chondritic body reached Earth even before the inferred catastrophic disruption at 466 Ma ago. This would concur with a “rubble pile” structure of the L-chondrite parent body and exposure of abundant deep-seated material as a result of earlier disruption and re-accretion events. The LL-chondritic contribution in the pre-LCPB flux is higher than at other Phanerozoic time windows, including the Cambrian. This anomalously enhanced flux thus cannot be ascribed to the Neoproterozoic breakup of the large LL-chondritic Flora asteroid. Previously observed high concentrations of chrome-spinel grains of achondritic origin can be reproduced only in our samples from immediately (< 1 Ma) before the LCPB breakup. We speculate that this, together with the high LL percentage before the breakup, may be explained by dynamic perturbations, possibly in the near-Earth region, leading up to the LCPB event.

Dynamics of a deforming planetary body

Self-gravitating planetary bodies possess non-spherical shapes and various density distributions. The dynamics of such bodies are treated using a rigid-body assumption, where bodies do not deform throughout their motion, or simplified force models and shapes like a sphere. These assumptions make problems simple, though it essentially ignores the response of deformation to translation and rotation, and vice versa, limiting its applicability. Describing the coupling of three dynamics modes (deformation, rotation, and translation) is essential to detailing the dynamical evolution of planetary bodies. However, a continuum mechanics approach such as finite element modeling (FEM) needs additional cautions because ill-defined body forces may cause unrealistic stress and displacement conditions. Here, we propose a theoretical framework for characterizing the dynamics of a deforming body, applicable to the numerical implementation of continuum mechanics models. We introduce the kinetics and mechanics to derive the governing equations representing all the dynamics modes. The advantage of this technique is that the equation of deformation no longer deals with translation and rotation included in body forces, which removes difficulties in computing stress and displacement fields around boundary conditions. We demonstrate this technique by considering a system that consists of two masses connected by a spring and a damper. An example of this application discusses the dynamical response of the Cold Classical Kuiper Belt Object (CCKBO) 486958 Arrokoth to the Sky Crater-forming impact event on its small lobe. When the dissipation quality factor, known as the Q factor, is about 100, a typical value for a rubble pile object, the deformation dissipation timescale is just a few days. Considering the yield stress and strain to be ∼10 kPa and ∼0.01, respectively, we derive a displacement of up to tens of meters and normal stress of up to ∼5 kPa if the impact direction consists of the components along the two lobes. Without it, the rotational state change gives no significant displacement and normal stress; however, the major part of the imparted momentum may be converted to shear.

Momentum Enhancement from a 3 cm Diameter Aluminum Sphere Striking a Small Boulder Assembly at 5.4 km s−1

An impact experiment was performed with a target of relevance to the upcoming DART impact. In this experiment, a collection of stones that is similar to a rubble pile was the target, though it was necessary to hold the stones in place (in this case with cement) since the target was hung vertically to perform the experiment. The stones–cement target has a higher density and a lower porosity than expected for Dimorphos, with the density being 2.92 g cm−3. A 3 cm diameter aluminum sphere was launched at a speed of 5.44 km s−1, which is similar to the anticipated 6.1 km s−1 impact speed of DART. The stones–concrete target was completely disassembled by the impact. The target was mounted on a pendulum. The swing of the pendulum was measured and from it the momentum enhancement was measured. Due to possible lateral expansion of debris material, this value is a lower bound on the momentum enhancement that would be imparted to an extended target.

The influence of interparticle cohesion on rebounding slow impacts on rubble pile asteroids

The ballistic sorting effect has been proposed to be a driver behind the observed size sorting on the rubble pile asteroid Itokawa. This effect depends on the inelasticity of slow collisions with granular materials. The inelasticity of a collision with a granular material, in turn, depends on grain size. Here we argue that determining the inelasticity of such collisions in an asteroid-like environment is a nontrivial task. We show non-monotonic dependency of the coefficient of restitution (COR) on target particle size using experiments in microgravity. Employing numerical simulations, we explain these results with the growing influence of adhesion for smaller-sized particles. We conclude that there exists an optimum impactor to target particle size ratio for ballistic sorting.

Inferring interiors and structural history of top-shaped asteroids from external properties of asteroid (101955) Bennu

Asteroid interiors play a key role in our understanding of asteroid formation and evolution. As no direct interior probing has been done yet, characterisation of asteroids’ interiors relies on interpretations of external properties. Here we show, by numerical simulations, that the top-shaped rubble-pile asteroid (101955) Bennu’s geophysical response to spinup is highly sensitive to its material strength. This allows us to infer Bennu’s interior properties and provide general implications for top-shaped rubble piles’ structural evolution. We find that low-cohesion (≲0.78 Pa at surface and ≲1.3 Pa inside) and low-friction (friction angle ≲ 35∘) structures with several high-cohesion internal zones can consistently account for all the known geophysical characteristics of Bennu and explain the absence of moons. Furthermore, we reveal the underlying mechanisms that lead to different failure behaviours and identify the reconfiguration pathways of top-shaped asteroids as functions of their structural properties that either facilitate or prevent the formation of moons.

Dynamical Evolution of the Didymos−Dimorphos Binary Asteroid as Rubble Piles following the DART Impact

Previous efforts have modeled the Didymos system as two irregularly shaped rigid bodies, although it is likely that one or both components are in fact rubble piles. Here, we relax the rigid-body assumption to quantify how this affects the spin and orbital dynamics of the system following the DART impact. Given the known fundamental differences between our simulation codes, we find that faster rigid-body simulations produce nearly the same result as rubble-pile models in scenarios with a moderate value for the momentum enhancement factor, β (β ∼ 3) and an ellipsoidal secondary. This indicates that the rigid-body approach is likely adequate for propagating the post-impact dynamics necessary to meet the DART Mission requirements. Although, if Dimorphos has a highly irregular shape or structure, or if β is unexpectedly large, then rubble-pile effects may become important. If Dimorphos’s orbit and spin state are sufficiently excited, then surface particle motion is also possible. However, these simulations are limited in their resolution and range of material parameters, so they serve as a demonstration of principle, and future work is required to fully understand the likelihood and magnitude of surface motion.

Cold Atom Interferometry for Enhancing the Radio Science Gravity Experiment: A Phobos Case Study

Interplanetary missions have typically relied on Radio Science (RS) to recover gravity fields by detecting their signatures on the spacecraft trajectory. The weak gravitational fields of small bodies, coupled with the prominent influence of confounding accelerations, hinder the efficacy of this method. Meanwhile, quantum sensors based on Cold Atom Interferometry (CAI) have demonstrated absolute measurements with inherent stability and repeatability, reaching the utmost accuracy in microgravity. This work addresses the potential of CAI-based Gradiometry (CG) as a means to strengthen the RS gravity experiment for small-body missions. Phobos represents an ideal science case as astronomic observations and recent flybys have conferred enough information to define a robust orbiting strategy, whilst promoting studies linking its geodetic observables to its origin. A covariance analysis was adopted to evaluate the contribution of RS and CG in the gravity field solution, for a coupled Phobos-spacecraft state estimation incorporating one week of data. The favourable observational geometry and the small characteristic period of the gravity signal add to the competitiveness of Doppler observables. Provided that empirical accelerations can be modelled below the nm/s2 level, RS is able to infer the 6 × 6 spherical harmonic spectrum to an accuracy of 0.1–1% with respect to the homogeneous interior values. If this correlates to a density anomaly beneath the Stickney crater, RS would suffice to constrain Phobos’ origin. Yet, in event of a rubble pile or icy moon interior (or a combination thereof) CG remains imperative, enabling an accuracy below 0.1% for most of the 10 × 10 spectrum. Nevertheless, technological advancements will be needed to alleviate the current logistical challenges associated with CG operation. This work also reflects on the sensitivity of the candidate orbits with regard to dynamical model uncertainties, which are common in small-body environments. This brings confidence in the applicability of the identified geodetic estimation strategy for missions targeting other moons, particularly those of the giant planets, which are targets for robotic exploration in the coming decades.

Load distributions in the ice-structure interaction process in shallow water

Model-scale experiments were performed to explore the process of level ice interacting with a, in model-scale, ten-meter-wide inclined structure in shallow water. During each experiment, an initially intact ice sheet was pushed against the sloping structure, failed against it and accumulated into a grounded rubble pile in front of it. The strength of the model ice used in the experiments was varied. The spatial and temporal ice load distributions were measured at two scales: the structure was divided into ten identical segments along its waterline with the horizontal load on each segment measured, while the local pressure distributions were simultaneously measured with tactile sensors from two of the segments. This paper focuses on the relationships between the loads on different segments and the correlation between global and local loads. The load records of the individual segments during the whole experiment correlated with each other. The Pearson correlation coefficient for the load records of two neighboring segments was about 0.8, and for segments having a distance of 5 m or more, about 0.4. The magnitudes of both the maximum and the mean load on each segment were approximately equal. Even if the segment loads correlated during the whole experiment, the load records of the individual segments showed also transient ice load peaks, which did not correlate. These were used to define local peak load events and related local line loads, defined by dividing the magnitude of a peak load by its width in space. Roughly 2/3 of the local peak load events concentrated on one segment and only about 5% of them covered more than three segments. Narrow local peak load events caused the local line loads of the highest magnitude, with the magnitude and the relative frequency of the events increasing with the ice strength. Narrow events resulted in local line loads up to four times higher than the global line load, which was defined by dividing the total load on the structure by its width. The investigation of the local peak load events also showed that an inclined structure having a width of approximately 5 m (the ice thickness to structure width ratio of 1:100) appeared infinitely wide and gave similar results as a wider structure. Ice pressure measurements showed a line-like pressure distribution at the waterline of the structure with the weakest ice, whereas with stronger ice, zones of high ice pressure were observed at upper parts of the segments.

Possible particle ejection contributions to the shape and spin stability of small near-Earth asteroids

Top Shaped Asteroids (TSAs) have proven to be common amongst the near-Earth rubble pile population, with multiple examples confirmed via groundbased radar and spaceborne optical sensors through the past 20 years. A substantial body of literature has developed, exploring the formation of these unique shapes either through rotation-induced landslides and creep, or collisional reaccumulation. Models of such mass movements can provide good explanations for mid and low latitude material redistribution, but Bennu also shows a significant increase in radius in the high polar regions, which is harder to explain with these processes. The discovery of repeated and probably ongoing particle ejections around the 500 m diameter asteroid Bennu by the OSIRIS-REx mission suggests that we need to consider an alternate or additional mechanism which, we show, can anticipate the detailed variety of TSA shapes. This paper explores asteroid shape evolution as the result of particle ejections, modeled as being simply correlated with latitude via diurnal heating (or meteorite impacts), and re-accumulation using simulations including gravity and solar radiation pressure. Asteroid outlines are evolved with time as a function of particle ejection velocities and asteroid rotation rates. Bennu’s shape can be anticipated by our model with RMS surface errors of less than 1.1% (2.7 m) although some southern latitudes have errors up to 10 m. Straightforward variation in conditions can produce shapes matching other TSAs. However, the observed particle fluxes on Bennu are approximately 3 orders of magnitude too low for this to be the only shaping mechanism on Bennu. The time necessary to form these shapes by our mechanism alone is far longer than the lifetimes of near-Earth asteroids, unless fluxes were once much greater, or there was an underlying oblate shape.

Possible thermal evolutionary pathways of irregular shaped small asteroids and planetesimals

Two distinct thermal evolutionary pathways of irregular shaped small planetesimals in the early solar system have been studied. We have taken a case study of two S-type asteroids; (243) Ida and (951) Gaspra, on the basis of their precise physical dimensions accessed by the Philip Stooke small body 3-D shape models of the NASA Planetary Data System. The 3- D shape models for the two asteroids are based on the Galileo spacecraft fly-by mission. Based on our novel thermal evolutionary code for the precise shape of the asteroids we found that the small planetary bodies that accreted within the initial 2-3 million years (Myr) experienced sintering, whereas, the bodies formed afterwards were left unconsolidated, e.g., as a rubble pile. The former set of bodies could have formed by direct aggregation of nebula dust. Whereas, the majority of the rubble pile type small planetary bodies accreted latter by the assemblage of the fragmented debris of the initially existing planetesimals. These bodies cooled over tens of million years. Generations of small planetesimals formed over time in the early solar system evolved from an ensemble of compact consolidated bodies to rubble pile bodies due to collision induced fragmentation and re-accretion.

Investigating the effects of density and spin period on surface slopes of asteroids

The recent encounters with two C-type rubble-pile asteroids, Bennu and Ryugu, confirmed that their internal density was 1.2 gcm−3, lower than the density of the previously explored S-type rubble pile Itokawa, 1.9 gcm−3. As these are the only three rubble piles investigated in detail to date, little is known on how subtle differences in internal density of similarly shaped asteroids could affect surface slopes and thus downslope movement, particularly in near-Earth asteroids whose spin periods are affected by YORP. We modelled surface slopes for a range of internal densities and spin periods for seven small asteroid shape models to understand how differences in internal density of similarly shaped asteroids affect surface slope sensitivity to changes in spin period.We found that for similarly shaped asteroids with spin periods under 6 to 8 h small differences in internal density can affect the slope sensitivity of different asteroids to changes in the spin period. Using the metric of 50% of the surface over 40° slope, as a proxy for catastrophic resurfacing, we found that for 6 of the 7 shape models catastrophic resurfacing would occur at <2.6 h for an internal density of 1.9 gcm−3 compared to <3.2 h for an internal density of 1.2 gcm−3. While this is not a large difference in spin periods, the result that lower density asteroids have surface slopes that are more sensitive to changes in the spin period suggests that low-density rubble piles (e.g. C-type) may have more evidence of downslope motion compared to similar shaped higher density rubble piles (e.g., S-type). The majority of the asteroid shape models investigated were spinning-tops, however, results from the Itokawa shape model hint that irregularly shaped asteroids are more sensitive to spin period changes than top-shaped asteroids, which may help explain why Itokawa has extensive lowlands compared to Ryugu and Bennu’s more rocky surfaces. Our study shows that for similarly shaped asteroids both density and spin periods are critical for determining surface stability and evolution.

On the Stability of Spinning Asteroids

Most asteroids with a diameter larger than sim 300 {mathrm{m}} are rubble piles, i.e., consisting of more than one solid object. All asteroids are rotating but almost all asteroids larger than sim 300 mathrm{m} rotate with a period longer than 2.3 {text{hours}}, which is the critical period where the centrifugal force equals the gravitational force. This indicates that there are nearly no adhesive interaction forces between the asteroid fragments. We show that this is due to the surface roughness of the asteroid particles which reduces the van der Waals interaction between the particles by a factor of 100 for micrometer sized particles and even more for larger particles. We show that surface roughness results in an interaction force which is independent of the size of the particles, in contrast to the linear size dependency expected for particles with smooth surfaces. Thus, two stone fragments of size 100 {mathrm{nm}} attract each other with the same non-gravitational force as two fragments of size 10 {mathrm{m}}.

Interior of top-shaped asteroids with cohesionless surface

Multiple sources of evidence suggest that asteroids ranging from hundreds meters to few kilometers in size are rubble piles, i.e. gravitational aggregates of loosely consolidated material. However, no direct data on their internal structure is available to date. Cohesion between rubble-pile building blocks has been invoked in the past to explain the stability of top-shaped asteroids, which in most cases would not be capable of maintaining their large-scale shape features (low flattening, and a pronounced equatorial ridge) otherwise. However, the physical origin of cohesion is unclear and there is no direct evidence of it. Recent close-range imaging and local sampling of the surfaces of top-shaped Near Earth Asteroids (NEA) suggest the presence of very porous surface structure with minimal strength and nearly no cohesion. This raises new questions about the internal structure of such objects, with important implications on their origin and evolution. Here we show by numerically simulating the dynamics of irregular rocky fragments, that the presence of a rigid core within the asteroid’s rubble-pile structure can explain the top shape and surface features observed recently on Bennu and Ryugu, without the need of cohesion between building blocks. Also, we find that the rigid core model produces more easily equatorial mass shedding, which is thought to be responsible for satellite formation. The presence of a rigid core has never been revealed so far, but is consistent with the accretion history of those objects, and with recent estimates of their internal mass distribution. Our findings will be tested directly by ESA’s Hera mission, which will scan the interior of Dimorphos, the small moon of Didymos binary system, providing for the first time direct data on the interior of a NEA.

Habitat Bennu: Design Concepts for Spinning Habitats Constructed From Rubble Pile Near-Earth Asteroids

The authors explore the possibility that near-earth, rubble pile asteroids might be used as habitats for human settlement by increasing their rotation to produce spin gravity. Using previously published scaling by Maindl et al. and studies of asteroid populations, it is shown that there is no class of hollowed body that would survive the spin-up process on its own without additional reinforcement. Large solid-rock asteroids (diameterD&amp;gt; 10 km) would not have the tensile strength to withstand the required rotation rates and would fracture and break apart. Smaller asteroids, being ‘rubble piles’, have little tensile strength and would quickly disperse. The possibility of containing the asteroid mass using higher-strength materials like carbon nanofiber is instead considered. It is found that a moderate tensile strength container can maintain the integrity of a large spinning cylinder composed of dispersed asteroid regolith. The research extends the range of possible asteroid habitat candidates, since it may become feasible to construct habitats from the more numerous smaller bodies, including NEAs (Near Earth Asteroids). The required tensile strength of the container material scales with habitat radius and thickness and is ∼ 200 MPa for a starting asteroid body of radius 300 m that is spun up to provide 0.3 g⊕while increasing its radius to 3 km and maintaining a rubble and regolith shield thickness of 2 m to protect against cosmic rays. Ambient solar power can be harvested to aid in spin-up and material processing.

Exsolution in alkali feldspar in ordinary chondrites: Ubiquitous evidence for rapid cooling at high temperatures

Thermal metamorphism in undisrupted ordinary chondrite (OC) parent bodies is thought to occur through the radioactive decay of 26Al, producing an onion-shell-like structure with higher peak metamorphic temperatures corresponding to increasing depth. During retrograde metamorphism, the onion-shell model predicts slower cooling rates with increasing petrologic type. However, cooling rates determined by pyroxene diffusion, metallographic, and other methods are inconsistent with onion-shell-like cooling, leading to a model of asteroid disruption and reaccretion into a rubble pile, after peak metamorphism. Potassium-feldspar exsolution in albite, in a perthite texture, has been noted in OCs and can be used as another method for determining cooling rates. We conducted a survey of K-feldspar occurrences and textures, within chondrules, in petrologic type 3.6–6 H, L, and LL OCs. Potassium-feldspar is present as a secondary feature, in primary and secondary albite, as fine-scale exsolution lamellae, 0.1–1.5 μm wide, as well as in larger patches up to 50 μm in size. Exsolution is present in all OC groups and is most common in petrologic type 4.In the H4 chondrite Avanhandava, we estimate the cooling rate from perthite to be 3–17 °C/yr over a temperature interval of 765–670 °C. Peristerite is also present in Avanhandava for which we estimate a cooling rate of 0.2–2.4 × 10−3 °C/yr from 570-540 °C. In general, the relatively fast, high-temperature cooling rate determined by perthite is similar to cooling rates recovered from two-pyroxene speedometry. The peristerite cooling rate is closer to the slow, lower temperature metallographic cooling rates. Because K-feldspar exsolution is present in similar fine-scale lamellae in all OC groups, we suggest that all OC parent bodies experienced the same cooling history at high temperatures. These results are inconsistent with predictions of OC asteroid cooling from undisturbed onion-shell metamorphism but are consistent with models involving disruption after peak metamorphism followed by reassembly.

Xenoliths in ordinary chondrites and ureilites: Implications for early solar system dynamics

AbstractForeign clasts (xenoliths) in meteoritic breccias are a serendipitous source of information about the impact environment in which their hosts formed, including impactor flux and cosmochemical types. These parameters may be related to timing and/or heliocentric distance of xenolith origin and implantation, and thus can be used to test or inform models of early solar system dynamics. We use xenoliths in ordinary chondrites (OCs) and ureilites to do this. We first conducted a petrologic and oxygen isotope study of a new, cm‐sized igneous‐textured clast in L3.7 Northwest Africa (NWA) 092, which highlighted some of the difficulties of identifying xenoliths in meteorites. Results indicate that this clast is not a xenolith but an impact melt of non‐local OC material. We add this result to a literature survey of more than 3000 OCs and find that the fraction of OCs that contain xenoliths is &lt;&lt;1%, and, even in these, the abundance and the diversity of xenoliths are very low. This contrasts markedly with the ureilites, ˜5% of which contain ˜1–10 vol% xenoliths from every major meteorite class, including multiple groups and petrologic types. To investigate reasons for this difference, we compare the histories of OC and ureilite parent bodies. The OC and ureilitic parent bodies accreted in the inner solar system within ˜1 AU of one another. The OC bodies accreted ˜2–3 Myr after calcium‐aluminum‐rich inclusion (CAI) formation and were heated slowly, experiencing thermal metamorphism over ˜50–60 Myr. The ureilite parent body (UPB) accreted &lt;1 Myr after CAIs and was heated rapidly, experiencing partial melting over ˜4 Myr. Both OC parent bodies and the UPB were catastrophically disrupted and reassembled into rubble piles. For ureilites, this occurred ˜5.0–5.4 Ma after CAIs, while for OCs, it did not occur until 50–60 Myr after CAIs. Xenoliths in OC and ureilitic breccias were acquired as fragments of impactors on the rubble piles. The presence in polymict ureilites of xenoliths of all OC groups (H, L, LL) and petrologic types (3–6), and the intimate scale on which these and myriad other xenolith types are mixed, indicate that most xenoliths were acquired within a short time period around ˜50–60 Myr after CAIs when OC (likely also Rumuruti chondrite and enstatite chondrite) parent bodies were disrupted. This timing is consistent with the early instability dynamical model for a period of excitation in the asteroid belt. Outer solar system (CC) xenoliths were also acquired during this period, but were derived indirectly from C‐type bodies that had already been emplaced in orbits in the asteroid belt. The large discrepancy in xenolith abundance between ureilites and OC may be due to different physical properties of their regoliths at 50–60 Myr after CAIs. CC‐like xenoliths in OC may represent a different, more recently acquired, population than those in polymict ureilites.

Simultaneous Localization of Biobotic Insects using Inertial Data and Encounter Information.

Several recent research efforts have shown that the bioelectrical stimulation of their neuro-mechanical system can control the locomotion of Madagascar hissing cockroaches (Gromphadorhina portentosa). This has opened the possibility of using these insects to explore centimeter-scale environments, such as rubble piles in urban disaster areas. We present an inertial navigation system based on machine learning modules that is capable of localizing groups of G. portentosa carrying thorax-mounted inertial measurement units. The proposed navigation system uses the agents' encounters with one another as signals of opportunity to increase tracking accuracy. Results are shown for five agents that are operating on a planar (2D) surface in controlled laboratory conditions. Trajectory reconstruction accuracy is improved by 16% when we use encounter information for the agents, and up to 27% when we add a heuristic that corrects speed estimates via a search for an optimal speed-scaling factor.

Model-scale tests on ice-structure interaction in shallow water: Global ice loads and the ice loading process

Laboratory-scale experiments on ice-structure interaction process in shallow water were performed by pushing a ten-meter-wide ice sheet against an inclined structure of the same width. Seven experiments were performed in three series: In one of the series, the compressive and flexural strengths were both about 50kPa, in the two other test series the ice strength was two and four times higher. The ice thickness was about 50 mm in all experiments. The loading process showed two phases: the ice load on the structure (1) first increased linearly with a rate that was constant for all experiments, after which (2) the loading process reached a steady-state phase with approximately constant load. The magnitude of ice loads was not proportional to ice strength, as the weakest ice yielded higher loads than the ice having twice its strength. The ice rubble grounded in all experiments, but the bottom carried only a small portion of the load. The load records could be normalized by a factor combining the weight and the characteristic length of the intact ice. Based on the normalization, a model explaining the loading process was derived; the weight of the incoming ice has a dominant role during phase (1), while buckling explains the change in the process to phase (2) when the ice is strong enough. The loading process for the weakest ice was different from that for the other two ice types used. For example, instead of forming a rubble pile consisting of distinct ice blocks, weakest ice formed a dense pile of slush. The normalized ice load data highlighted the differences in the loading process.