Publications
63 results found
Cheng AF, Agrusa HF, Barbee BW, et al., 2023, Momentum transfer from the DART mission kinetic impact on asteroid dimorphos, Nature, Vol: 616, Pages: 457-460, ISSN: 0028-0836
The NASA Double Asteroid Redirection Test (DART) mission performed a kinetic impact on asteroid Dimorphos, the satellite of the binary asteroid (65803) Didymos, at 23:14 UTC on September 26, 2022 as a planetary defense test1. DART was the first hypervelocity impact experiment on an asteroid at size and velocity scales relevant to planetary defense, intended to validate kinetic impact as a means of asteroid deflection. Here we report the first determination of the momentum transferred to an asteroid by kinetic impact. Based on the change in the binary orbit period2, we find an instantaneous reduction in Dimorphos's along-track orbital velocity component of 2.70 ± 0.10 mm s-1, indicating enhanced momentum transfer due to recoil from ejecta streams produced by the impact3,4. For a Dimorphos bulk density range of 1,500 to 3,300 kg m-3, we find that the expected value of the momentum enhancement factor, [Formula: see text], ranges between 2.2 and 4.9, depending on the mass of Dimorphos. If Dimorphos and Didymos are assumed to have equal densities of 2,400 kg m-3, [Formula: see text]. These [Formula: see text] values indicate that significantly more momentum was transferred to Dimorphos from the escaping impact ejecta than was incident with DART. Therefore, the DART kinetic impact was highly effective in deflecting the asteroid Dimorphos.
Steele SC, Fu R, Volk MWR, et al., 2023, Paleomagnetic evidence for a long-lived, potentially reversing martian dynamo at ~3.9 Ga, Science Advances, ISSN: 2375-2548
North TL, Collins G, Davison T, et al., 2023, The heterogeneous response of Martian meteorite Allan Hills 84001 to planar shock, Icarus, Vol: 390, ISSN: 0019-1035
Impact-generated shock waves can change the physical properties of meteorites and their constituent minerals. Accounting for these effects is key to recovering information about the early solar system from meteorite observations. ALH 84001 is a rare ancient sample from the Martian crust, providing a unique window into the thermal and metamorphic evolution of Mars. A well-studied meteorite, past geochemical and petrologic investigations have attempted to deduce its thermal and impact history with some contradictory results. By simulating the passage of a planar shock wave through a synthetic analog for samples of ALH 84001 using the iSALE-2D shock physics code we have determined the meteorite’s likely thermodynamic and physical response during an impact. Our simulations show that heterogeneous shear heating, induced by the planar shock wave, can produce strong thermal gradients on the sub-millimeter ‘mesoscale’ throughout the meteorite, even in relatively weak shock waves (5 GPa). We are able to place new constraints on deformation events experienced by the meteorite during its time on the parent body, including the maximum pressure ALH 84001 has experienced since it acquired its remanent magnetization and its subsequent ejection from Mars.
Davison TM, Collins GS, 2022, Complex crater formation by oblique impacts on the Earth and Moon, Geophysical Research Letters, Vol: 49, Pages: 1-9, ISSN: 0094-8276
Almost all meteorite impacts occur at oblique incidence angles, but the effect of impact angle on crater size is not well understood, especially for large craters. To improve oblique impact crater scaling, we present a suite of simulations of complex crater formation on Earth and the Moon over a range of impact angles, velocities and impactor sizes. We show that crater diameter is larger than predicted by existing scaling relationships for oblique impacts; there is little dependence on obliquity for impacts steeper than 45° from the horizontal. Crater depth, volume and diameter depend on impact angle in different ways—relatively shallower craters are formed by more oblique impacts. Our simulation results have implications for how crater populations are determined from impactor populations and vice-versa. They suggest that existing approaches to account for impact obliquity may underestimate the number of complex craters larger than a given size by as much as one-third.
Stickle AM, DeCoster ME, Burger C, et al., 2022, Effects of impact and target parameters on the results of a kinetic impactor: predictions for the double asteroid redirection test (DART) mission, The Planetary Science Journal, Vol: 3, Pages: 248-248, ISSN: 2632-3338
The Double Asteroid Redirection Test (DART) spacecraft will impact into the asteroid Dimorphos on 2022 September 26 as a test of the kinetic impactor technique for planetary defense. The efficiency of the deflection following a kinetic impactor can be represented using the momentum enhancement factor, β, which is dependent on factors such as impact geometry and the specific target material properties. Currently, very little is known about Dimorphos and its material properties, which introduces uncertainty in the results of the deflection efficiency observables, including crater formation, ejecta distribution, and β. The DART Impact Modeling Working Group (IWG) is responsible for using impact simulations to better understand the results of the DART impact. Pre-impact simulation studies also provide considerable insight into how different properties and impact scenarios affect momentum enhancement following a kinetic impact. This insight provides a basis for predicting the effects of the DART impact and the first understanding of how to interpret results following the encounter. Following the DART impact, the knowledge gained from these studies will inform the initial simulations that will recreate the impact conditions, including providing estimates for potential material properties of Dimorphos and β resulting from DART's impact. This paper summarizes, at a high level, what has been learned from the IWG simulations and experiments in preparation for the DART impact. While unknown, estimates for reasonable potential material properties of Dimorphos provide predictions for β of 1–5, depending on end-member cases in the strength regime.
Luther R, Raducan S, Burger C, et al., 2022, Momentum enhancement during kinetic impacts in the low-intermediate-strength regime: benchmarking & validation of impact shock physics codes, The Planetary Science Journal, Vol: 3, Pages: 1-14, ISSN: 2632-3338
In September 2022, the DART spacecraft (NASA’s contribution to the Asteroid Impact & Deflection Assessment collaboration, AIDA) will impact the asteroid Dimorphos, the secondary in the Didymos system. The crater formation and material ejection will affect the orbital period. In 2027, Hera (ESA’s contribution to AIDA) will investigate the system, observe the crater caused by DART, and characterise Dimorphos. Before Hera’s arrival, the target properties are not well constrained. The relationships between observed orbital change and specific target properties are not unique, but Hera’s observations will add additional constraints for the analysis of the impact event, which will narrow the range of feasible target properties. In this study, we use three different shock physics codes to simulate momentum transfer from impactor to target and investigate the agreement between the results from the codes for well defined target materials. In contrast to previous studies, care is taken to use consistent crushing behaviour (e.g., distension as a function of pressure) for a given porosity for all codes. First, we validate the codes against impact experiments into a regolith simulant. Second, webenchmark the codes at the DART impact scale for a range of target material parameters (10-50% porosity, 1.4 - 100 kPa cohesion). Aligning the crushing behaviour improves theconsistency of the derived momentum enhancement between the three codes to within +/- 5%for most materials used. Based on the derived mass-velocity distributions from all three codes, we derive scaling parameters that can be used for studies of the ejecta curtain.
Raducan SD, Jutzi M, Davison TM, et al., 2022, IMPACT FORMATION MODELS OF METAL-RICH BODIES AND IMPLICATIONS FOR ASTEROID (16) PSYCHE, 85th Annual Meeting of the Meteoritical-Society, Publisher: WILEY, ISSN: 1086-9379
Davison TM, Baijal N, Collins GS, 2022, HIGH-RESOLUTION OBLIQUE IMPACT SIMULATIONS OF THE FORMATION OF THE SOUTH POLE-AITKEN, 85th Annual Meeting of the Meteoritical-Society, Publisher: WILEY, ISSN: 1086-9379
North TL, Muxworthy AR, Collins GS, et al., 2022, THERMOREMANENT MAGNETISATION RECORDED DURING IMPACT-INDUCED COMPACTION EXPERIMENTS ON SYNTHETIC CHONDRITIC METEORITES, LSPC, Publisher: WILEY, ISSN: 1086-9379
Wakita S, Genda H, Kurosawa K, et al., 2022, Effect of Impact Velocity and Angle on Deformational Heating and Postimpact Temperature, JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS, Vol: 127, ISSN: 2169-9097
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Cashion MD, Johnson BC, Krot AN, et al., 2022, Chondrule formation via impact jetting in the icy outer solar system, ICARUS, Vol: 384, ISSN: 0019-1035
Raducan SD, Jutzi M, Davison TM, et al., 2022, Influence of the projectile geometry on the momentum transfer from a kinetic impactor and implications for the DART mission, International Journal of Impact Engineering, Vol: 162, Pages: 104147-104147, ISSN: 0734-743X
The DART spacecraft will impact Didymos’s secondary, Dimorphos, at the end of 2022 and cause a change in the orbital period of the secondary. For simplicity, most previous numerical simulations of the impact used a spherical projectile geometry to model the DART spacecraft. To investigate the effects of alternative, simple projectile geometries on the DART impact outcome we used the iSALE shock physics code in two and thee-dimensions to model vertical impacts of projectiles with a mass and speed equivalent to the nominal DART impact, into porous basalt targets. We found that the simple projectile geometries investigated here have minimal effects on the crater morphology and momentum enhancement. Projectile geometries modelled in two-dimensions that have similar surface areas at the point of impact, affect the crater radius and the crater volume by less than 5%. In the case of a more extreme projectile geometry (i.e., a rod, modelled in three-dimensions), the crater was elliptical and 50% shallower compared to the crater produced by a spherical projectile of the same momentum. The momentum enhancement factor in these test cases, commonly referred to as , was within 7% for the 2D simulations and within 10% for the 3D simulations, of the value obtained for a uniform spherical projectile. The most prominent effects of projectile geometry are seen in the ejection velocity as a function of launch position and ejection angle of the fast ejecta that resides in the so-called ‘coupling zone’. These results will inform the LICIACube ejecta cone analysis.
Raducan SD, Davison TM, Collins GS, 2022, Ejecta distribution and momentum transfer from oblique impacts on asteroid surfaces, Icarus, Vol: 374, Pages: 1-16, ISSN: 0019-1035
NASA’s Double Asteroid Redirection Test (DART) mission will impact its target asteroid, Dimorphos, at anoblique angle that will not be known prior to the impact. We computed iSALE-3D simulations of DARTlike impacts on asteroid surfaces at different impact angles and found that the vertical momentum transferefficiency, 𝛽, is similar for different impact angles, however, the imparted momentum is reduced as the impactangle decreases. It is expected that the momentum imparted from a 45◦impact is reduced by up to 50%compared to a vertical impact. The direction of the ejected momentum is not normal to the surface, howeverit is observed to ‘straighten up’ with crater growth. iSALE-2D simulations of vertical impacts provide contextfor the iSALE-3D simulation results and show that the ejection angle varies with both target properties andwith crater growth. While the ejection angle is relatively insensitive to the target porosity, it varies by upto 30◦ with target coefficient of internal friction. The simulation results presented in this paper can helpconstrain target properties from the DART crater ejecta cone, which will be imaged by the LICIACube. Theresults presented here represent the basis for an empirical scaling relationship for oblique impacts and canbe used as a framework to determine an analytical approximation of the vertical component of the ejectamomentum, 𝛽 − 1, given known target properties.
Wakita S, Johnson BC, Garrick-Bethell I, et al., 2021, Impactor material records the ancient lunar magnetic field in antipodal anomalies, NATURE COMMUNICATIONS, Vol: 12
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Wakita S, Johnson BC, Denton CA, et al., 2021, Jetting during oblique impacts of spherical impactors, ICARUS, Vol: 360, ISSN: 0019-1035
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Halim SH, Crawford IA, Collins GS, et al., 2021, Assessing the survivability of biomarkers within terrestrial material impacting the lunar surface, Icarus, Vol: 354, Pages: 1-15, ISSN: 0019-1035
The history of organic and biological markers (biomarkers) on the Earth is effectively non-existent in the geological record >3.8 Ga ago. Here, we investigate the potential for terrestrial material (i.e., terrestrial meteorites) to be transferred to the Moon by a large impact on Earth and subsequently survive impact with the lunar surface, using the iSALE shock physics code. Three-dimensional impact simulations show that a typical basin-forming impact on Earth can eject solid fragments equivalent to ~10−3 of an impactor mass at speeds sufficient to transfer from Earth to the Moon. Previous modelling of meteorite survivability has relied heavily upon the assumption that peak-shock pressures can be used as a proxy for gauging survival of projectiles and their possible biomarker constituents. Here, we show the importance of considering both pressure and temperature within the projectile, and the inclusion of both shock and shear heating, in assessing biomarker survival. Assuming that they survive launch from Earth, we show that some biomarker molecules within terrestrial meteorites are likely to survive impact with the Moon, especially at the lower end of the range of typical impact velocities for terrestrial meteorites (2.5 km s−1). The survival of larger biomarkers (e.g., microfossils) is also assessed, and we find limited, but significant, survival for low impact velocity and high target porosity scenarios. Thermal degradation of biomarkers shortly after impact depends heavily upon where the projectile material lands, whether it is buried or remains on the surface, and the related cooling timescales. Comparing sandstone and limestone projectiles shows similar temperature and pressure profiles for the same impact velocities, with limestone providing slightly more favourable conditions for biomarker survival.
Raducan S, 2020, Impact ejecta and crater formation on asteroid surfaces
Asteroids in the Solar System are numerous and have varied composition. Analysis of impact crater sizes and morphologies on asteroids can provide a direct diagnosis of the surface material properties and near-surface structures. This thesis describes numerical simulations of impacts into low-gravity asteroid surfaces using the iSALE shock physics code to inform this diagnosis. Asteroids may pose a future catastrophic threat to Earth and to avoid it, the incoming asteroid can be deflected by a spacecraft impact. However, the efficiency of the deflection is determined by target properties. This work considered different target scenarios to determine the sensitivity of crater morphology, ejecta mass-velocity distribution and momentum transferred, to asteroid surface properties and shallow structures. For homogeneous targets, the surface cohesion, initial porosity, and internal friction were found to greatly influence ejecta mass/velocity distributions and the amount an asteroid can be deflected. In a two-layer target scenario, the presence of a less porous, stronger lower layer can cause both amplification and reduction of ejected mass and momentum relative to the homogeneous case. Impacts into targets with decreasing porosity with depth only produced an enhancement in the ejected momentum for sharp exponential decreases in porosity. Using reasonable estimates for the material properties of the Double Asteroid Redirection Test (DART) asteroid target, the simulations show that the ejecta produced from the impact can enhance the deflection 2 to 4 times. Simulations of impacts into possible target structures on Psyche show large diversity in possible crater morphologies that the ‘Psyche’ mission could encounter. If Psyche’s interior is homogeneous, then the mission will find simple bowl-shaped craters, with a depth-diameter ratio diagnostic of rock or iron. If Psyche has a layered structure, the spacecraft could find craters with more complex morphologie
Raducan SD, Davison TM, Collins GS, 2020, Morphological diversity of impact craters on asteroid (16) Psyche: insight from numerical models, Journal of Geophysical Research: Planets, Vol: 125, Pages: 1-19, ISSN: 2169-9097
The asteroid (16) Psyche, target of NASA's “Psyche” mission, is thought to be one of the most massive exposed iron cores in the solar system. Earth‐based observations suggest that Psyche has a metal‐rich surface; however, its internal structure cannot be determined from ground‐based observations. Here we simulate impacts into a variety of possible target structures on Psyche and show the possible diversity in crater morphologies that the “Psyche” mission could encounter. If Psyche's interior is homogeneous, then the mission will find simple bowl‐shaped craters, with a depth‐diameter ratio diagnostic of rock or iron. Craters will be much deeper than those on other visited asteroids and possess much more spectacular rims if the surface is dominated by metallic iron. On the other hand, if Psyche has a layered structure, the spacecraft could find craters with more complex morphologies, such as concentric or flat‐floored craters. Furthermore, if ferrovolcanism occurred on Psyche, then the morphology of craters less than 2 km in diameter could be even more exotic. Based on three to four proposed large craters on Psyche's surface, model size‐frequency distributions suggest that if Psyche is indeed an exposed iron core, then the spacecraft will encounter a very old and evolved surface, that would be 4.5 Gyr old. For a rocky surface, then Psyche could be at least 3 Gyr old.
Collins G, Patel N, Davison T, et al., 2020, A steeply-inclined trajectory for the Chicxulub impact, Nature Communications, Vol: 11, Pages: 1-10, ISSN: 2041-1723
The environmental severity of large impacts on Earth is influenced by their impact trajectory. Impact direction and angle to the target plane affect the volume and depth of origin of vaporized target, as well as the trajectories of ejected material. The asteroid impact that formed the 66 Ma Chicxulub crater had a profound and catastrophic effect on Earth’s environment,but the impact trajectory is debated. Here we show that impact angle and direction can be diagnosed by asymmetries in the subsurface structure of the Chicxulub crater. Comparison of 3D numerical simulations of Chicxulub-scale impacts with geophysical observations suggests that the Chicxulub crater was formed by a steeply-inclined (45 -60° to horizontal) impact from the northeast; several lines of evidence rule out a low angle (< 30°) impact. Asteeply-inclined impact produces a nearly symmetric distribution of ejected rock and releases more climate-changing gases per impactor mass than either a very shallow or near-vertical impact.
Stickle AM, Bruck Syal M, Cheng AF, et al., 2020, Benchmarking impact hydrocodes in the strength regime: Implications for modeling deflection by a kinetic impactor, Icarus, Vol: 338, Pages: 1-24, ISSN: 0019-1035
The Double Asteroid Redirection Test (DART) is a NASA-sponsored mission that will be the first direct test of the kinetic impactor technique for planetary defense. The DART spacecraft will impact into Didymos-B, the moon of the binary system 65803 Didymos, and the resulting period change will be measured from Earth. Impact simulations will be used to predict the crater size and momentum enhancement expected from the DART impact. Because the specific material properties (strength, porosity, internal structure) of the Didymos-B target are unknown, a wide variety of numerical simulations must be performed to better understand possible impact outcomes. This simulation campaign will involve a large parameter space being simulated using multiple different shock physics hydrocodes. In order to understand better the behaviors and properties of numerical simulation codes applicable to the DART impact, a benchmarking and validation program using different numerical codes to solve a set of standard problems was designed and implemented. The problems were designed to test the effects of material strength, porosity, damage models, and target geometry on the ejecta following an impact and thus the momentum transfer efficiency. Several important results were identified from comparing simulations across codes, including the effects of model resolution and porosity and strength model choice: 1) momentum transfer predictions almost uniformly exhibit a larger variation than predictions of crater size; 2) the choice of strength model, and the values used for material strength, are significantly more important in the prediction of crater size and momentum enhancement than variation between codes; 3) predictions for crater size and momentum enhancement tend to be similar (within 15‐20%) when similar strength models are used in different codes. These results will be used to better design a modeling plan for the DART mission as well as to better understand the potential results that may be
Erickson TM, Kirkland CL, Timms NE, et al., 2020, Precise radiometric age establishes Yarrabubba, Western Australia, as Earth’s oldest recognised meteorite impact structure, Nature Communications, Vol: 11, Pages: 1-8, ISSN: 2041-1723
The ~70 km-diameter Yarrabubba impact structure in Western Australia is regarded as among Earth’s oldest, but has hitherto lacked precise age constraints. Here we present U–Pb ages for impact-driven shock-recrystallised accessory minerals. Shock-recrystallised monazite yields a precise impact age of 2229 ± 5 Ma, coeval with shock-reset zircon. This result establishes Yarrabubba as the oldest recognised meteorite impact structure on Earth, extending the terrestrial cratering record back >200 million years. The age of Yarrabubba coincides, within uncertainty, with temporal constraint for the youngest Palaeoproterozoic glacial deposits, the Rietfontein diamictite in South Africa. Numerical impact simulations indicate that a 70 km-diameter crater into a continental glacier could release between 8.7 × 1013 to 5.0 × 1015 kg of H2O vapour instantaneously into the atmosphere. These results provide new estimates of impact-produced H2O vapour abundances for models investigating termination of the Paleoproterozoic glaciations, and highlight the possible role of impact cratering in modifying Earth’s climate.
Raducan SD, Davison TM, Collins GS, 2020, The effects of asteroid layering on ejecta mass-velocity distribution and implications for impact momentum transfer, Planetary and Space Science, Vol: 180, ISSN: 0032-0633
Most bodies in the Solar System do not have a homogeneous structure. Understanding the outcome of an impact into regolith layers of different properties is especially important for NASA’s Double Asteroid Redirection Test (DART) and ESA’s Hera missions. Here we used the iSALE shock physics code to simulate the DART impact into three different target scenarios in the strength regime: a homogeneous porous half-space; layered targets with a porous weak layer overlying a stronger bedrock; and targets with exponentially decreasing porosity with depth. For each scenario we determined the sensitivity of crater morphology, ejecta mass-velocity distribution and momentum transferred from the impact for deflection, , to target properties and structure. We found that for a homogeneous porous half-space, cohesion and porosity play a significant role and the DART impact is expected to produce a between 1 and 3. In a two-layer target scenario, the presence of a less porous, stronger lower layer close to the surface can cause both amplification and reduction of ejected mass and momentum relative to the homogeneous upper-layer case. For the case of DART, the momentum enhancement can change by up to 90%. Impacts into targets with an exponentially decreasing porosity with depth only produced an enhancement in the ejected mass and momentum for sharp decreases in porosity that occur within 6 m of the asteroid surface. Together with measurements of the DART crater by the Hera mission, these results can be used to test the predictive capabilities of numerical models of asteroid deflection.
Wakita S, Genda H, Kurosawa K, et al., 2019, Enhancement of impact heating in pressure-strengthened rocks in oblique impacts, Geophysical Research Letters, Vol: 46, Pages: 13678-13686, ISSN: 0094-8276
Shock‐induced metamorphism in meteorites informs us about the collisional environment and history of our solar system. Recently, the importance of material strength in impact heating was reported from head‐on impact simulations. Here, we perform three‐dimensional oblique impact simulations and confirm the additional heating due to material strength for oblique impacts. Despite a large difference in the peak pressure at the impact point at a given impact velocity, we find that the heated mass for an oblique impact is nearly the same as that for a head‐on impact. Thus, our results differ from the previous finding that the heated mass decreases as the impact becomes more oblique and show that the additional shear heating is more effective for oblique impacts than for head‐on impacts. This also indicates that material ejected during oblique impact tends to experience lower shock pressures but higher temperatures.
Raducan SD, Davison TM, Luther R, et al., 2019, The role of asteroid strength, porosity and internal friction in impact momentum transfer, Icarus, Vol: 329, Pages: 282-295, ISSN: 0019-1035
Earth is continually impacted by very small asteroids and debris, and a larger object, though uncommon, could produce a severe natural hazard. During impact crater formation the ballistic ejection of material out of the crater is a major process, which holds significance for an impact study into the deflection of asteroids. In this study we numerically simulate impacts into low-gravity, strength dominated asteroid surfaces using the iSALE shock physics code, and consider the Double Asteroid Redirection Test (DART) mission as a case study. We find that target cohesion, initial porosity, and internal friction coefficient greatly influence ejecta mass/velocity/launch-position distributions and hence the amount by which an asteroid can be deflected. Our results show that as the cohesion is decreased the ratio of ejected momentum to impactor momentum, β − 1, increases; β − 1 also increases as the initial porosity and internal friction coefficient of the asteroid surface decrease. Using nominal impactor parameters and reasonable estimates for the material properties of the Didymos binary asteroid, the DART target, our simulations show that the ejecta produced from the impact can enhance the deflection by a factor of 2 to 4. We use numerical impact simulations that replicate conditions in several laboratory experiments to demonstrate that our approach to quantify ejecta properties is consistent with impact experiments in analogous materials. Finally, we investigate the self-consistency between the crater size and ejection speed scaling relationships previously derived from the point-source approximation for impacts into the same target material.
McMullan S, Collins GS, Davison TM, 2019, ASTEROID TO AIRBURST; COMPARING SEMI-ANALYTICAL AIRBURST MODELS TO HYDROCODES, 82nd Annual Meeting of the Meteoritical-Society (MetSoc), Publisher: WILEY, ISSN: 1086-9379
Lyons RJ, Bowling TJ, Ciesla FJ, et al., 2019, The effects of impacts on the cooling rates of iron meteorites, Meteoritics and Planetary Science, Vol: 54, Pages: 1604-1618, ISSN: 1086-9379
Iron meteorites provide a record of the thermal evolution of their parent bodies, with cooling rates inferred from the structures observed in the Widmanstätten pattern. Traditional planetesimal thermal models suggest that meteorite samples derived from the same iron core would have identical cooling rates, possibly providing constraints on the sizes and structures of their parent bodies. However, some meteorite groups exhibit a range of cooling rates or point to uncomfortably small parent bodies whose survival is difficult to reconcile with dynamical models. Together, these suggest that some meteorites are indicating a more complicated origin. To date, thermal models have largely ignored the effects that impacts would have on the thermal evolution of the iron meteorite parent bodies. Here we report numerical simulations investigating the effects that impacts at different times have on cooling rates of cores of differentiated planetesimals. We find that impacts that occur when the core is near or above its solidus, but the mantle has largely crystallized can expose iron near the surface of the body, leading to rapid and nonuniform cooling. The time period when a planetesimal can be affected in this way can range between 20 and 70 Myr after formation for a typical 100 km radius planetesimal. Collisions during this time would have been common, and thus played an important role in shaping the properties of iron meteorites.
Derrick JG, Rutherford ME, Chapman DJ, et al., 2019, Investigating shock processes in bimodal powder compaction through modelling and experiment at the mesoscale, International Journal of Solids and Structures, Vol: 163, Pages: 211-219, ISSN: 0020-7683
Impact-driven compaction is a proposed mechanism for the lithification of primordial bimodal granular mixtures from which many meteorites derive. We present a numerical-experimental mesoscale study that investigates the fundamental processes in shock compaction of this heterogeneous matter, using analog materials. Experiments were performed at the European Synchrotron Radiation Facility generating real-time, in-situ, X-ray radiographs of the shock's passage in representative granular systems. Mesoscale simulations were performed using a shock physics code and set-ups that were geometrically identical to the experiments. We considered two scenarios: pure matrix, and matrix with a single chondrule. Good agreement was found between experiments and models in terms of shock position and post-shock compaction in the pure powder setup. When considering a single grain embedded in matrix we observed a spatial porosity anisotropy in its vicinity; the compaction was greater in the region immediately shockward of the grain, and less in its lee. We introduced the porosity vector, C, which points in the direction of lowest compaction across a chondrule. This direction-dependent observation may present a new way to decode the magnitude, and direction, of a single shock wave experienced by a meteorite in the past.
Bowling TJ, Ciesla FJ, Davison TM, et al., 2019, Post-impact thermal structure and cooling timescales of Occator Crater on Asteroid 1 Ceres, Icarus, Vol: 320, Pages: 110-118, ISSN: 0019-1035
Occator crater is perhaps the most distinct surface feature observed by NASA's Dawn spacecraft on the Cerean surface. Contained within the crater are the highest albedo features on the planet, Cerealia Facula and Vinalia Faculae, and relatively smooth lobate flow deposits. We present hydrocode simulations of the formation of Occator crater, varying the water to rock ratio of our pre-impact Cerean surface. We find that at water to rock mass ratios up to 0.3, sufficient volumes of Occator's post-impact subsurface would be above the melting point of water to allow for the deposition of Faculae like deposits via impact-heat driven hydrothermal effusion of brines. This reservoir of hydrothermally viable material beneath the crater is composed of a mixture of impactor material and material uplifted from 10’s of kilometers beneath the pre-impact surface, potentially sampling a deep subsurface volatile reservoir. Using a conductive cooling model, we estimate that the lifetime of hydrothermal activity within such a system, depending on choice of material constants, is between 0.4 and 4 Myr. Our results suggest that impact heating from the Occator forming impact provides a viable mechanism for the creation of observed faculae, with the proviso that the faculae formed within a relatively short time window after the crater itself formed.
Rae A, Collins G, Poelchau M, et al., 2019, Stress-strain evolution during peak-ring formation: a case study of the Chicxulub impact structure, Journal of Geophysical Research: Planets, Vol: 124, Pages: 396-417, ISSN: 2169-9097
Deformation is a ubiquitous process that occurs to rocks during impact cratering; thus, quantifying the deformation of those rocks can provide first‐order constraints on the process of impact cratering. Until now, specific quantification of the conditions of stress and strain within models of impact cratering has not been compared to structural observations. This paper describes a methodology to analyze stress and strain within numerical impact models. This method is then used to predict deformation and its cause during peak‐ring formation: a complex process that is not fully understood, requiring remarkable transient weakening and causing a significant redistribution of crustal rocks. The presented results are timely due to the recent Joint International Ocean Discovery Program and International Continental Scientific Drilling Program drilling of the peak ring within the Chicxulub crater, permitting direct comparison between the deformation history within numerical models and the structural history of rocks from a peak ring. The modeled results are remarkably consistent with observed deformation within the Chicxulub peak ring, constraining the following: (1) the orientation of rocks relative to their preimpact orientation; (2) total strain, strain rates, and the type of shear during each stage of cratering; and (3) the orientation and magnitude of principal stresses during each stage of cratering. The methodology and analysis used to generate these predictions is general and, therefore, allows numerical impact models to be constrained by structural observations of impact craters and for those models to produce quantitative predictions.
Derrick J, LaJeunesse J, Davison T, et al., 2018, Mesoscale simulations of shock compaction of a granular ceramic: effects of mesostructure and mixed-cell strength treatment, Modelling and Simulation in Materials Science and Engineering, Vol: 26, ISSN: 0965-0393
The shock response of granular materials is important in a variety of contexts but the precise dynamics of grains during compaction is poorly understood. Here we use 2D mesoscale numerical simulations of the shock compaction of granular tungsten carbide to investigate the effect of internal structure within the particle bed and ’stiction’ between grains on the shock response. An increase in the average number of contactswith other particles, per particle, tends to shift the Hugoniot to higher shock velocities, lower particle velocities and lower densities. This shift is sensitive to inter-particle shear resistance. Eulerian shock physics codes approximate friction between, and interlocking of, grains with their treatment of mixed cell strength (stiction) and here we show thatthis has a significant effect on the shock response. When studying the compaction of particle beds it is not common to quantify the pre-compaction internal structure, yet our results suggest that such differences should be taken into account, either by usingidentical beds or by averaging results over multiple experiments.
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