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On February 28, 2021, a fireball dropped ∼0.6 kg of recovered CM2 carbonaceous chondrite meteorites in South-West England near the town of Winchcombe. We reconstruct the fireball's atmospheric trajectory, light curve, fragmentation behavior, and pre-atmospheric orbit from optical records contributed by five networks. The progenitor meteoroid was three orders of magnitude less massive (∼13 kg) than any previously observed carbonaceous fall. The Winchcombe meteorite survived entry because it was exposed to a very low peak atmospheric dynamic pressure (∼0.6 MPa) due to a fortuitous combination of entry parameters, notably low velocity (13.9 km s−1). A near-catastrophic fragmentation at ∼0.07 MPa points to the body's fragility. Low entry speeds which cause low peak dynamic pressures are likely necessary conditions for a small carbonaceous meteoroid to survive atmospheric entry, strongly constraining the radiant direction to the general antapex direction. Orbital integrations show that the meteoroid was injected into the near-Earth region ∼0.08 Myr ago and it never had a perihelion distance smaller than ∼0.7 AU, while other CM2 meteorites with known orbits approached the Sun closer (∼0.5 AU) and were heated to at least 100 K higher temperatures.

Reduced surface environments are required for many prebiotic chemical pathways. Large impacts onto Hadean Earth have been suggested as scenarios able to generate such environments on a global scale. Physical and chemical processes that occur shortly after impact can, however, limit the reducing power that is available at planet surface. Here, we present impact simulations and chemical calculations that demonstrate the efficient loss of reducing power from the post-impact planetary surface. This loss occurs via the dynamic escape of impactor core material, or the sinking of this material to the planet core, during impact, and through interactions between the atmosphere and the impact-generated melt phase. We suggest that post-impact surface environments are sufficiently reduced that species important to prebiotic chemistry can form (e.g., HCN, HCCCN). Additionally, the formation of a reduced impact-generated melt phase allows for reducing power to be stored in the planet mantle, where it can degas to the surface on geologic timescales., {"references":["Wünnemann, K., Collins, G. S., & Osinski, G. R. (2008). Numerical modelling of impact melt production in porous rocks. Earth and Planetary Science Letters, 269(3-4), 530-539.","Bottke, W. F., Walker, R. J., Day, J. M., Nesvorny, D., & Elkins-Tanton, L. (2010). Stochastic late accretion to Earth, the Moon, and Mars. science, 330(6010), 1527-1530.","Day, J. M., Brandon, A. D., & Walker, R. J. (2016). Highly siderophile elements in Earth, Mars, the Moon, and asteroids. Reviews in Mineralogy and Geochemistry, 81(1), 161-238.","Genda, H., Brasser, R., & Mojzsis, S. J. (2017). The terrestrial late veneer from core disruption of a lunar-sized impactor. Earth and Planetary Science Letters, 480, 25-32.","Marchi, S., Canup, R. M., & Walker, R. J. (2018). Heterogeneous delivery of silicate and metal to the Earth by large planetesimals. Nature geoscience, 11(1), 77-81.","Benner, S. A., Bell, E. A., Biondi, E., Brasser, R., Carell, T., Kim, H. J., ... & Trail, D. (2020). When did life likely emerge on Earth in an RNA‐first process?. ChemSystemsChem, 2(2), e1900035.","Zahnle, K. J., Lupu, R., Catling, D. C., & Wogan, N. (2020). Creation and evolution of impact-generated reduced atmospheres of early Earth. The Planetary Science Journal, 1(1), 11.","Citron, R. I., & Stewart, S. T. (2022). Large impacts onto the early Earth: planetary sterilization and iron delivery. The Planetary Science Journal, 3(5), 116.","Itcovitz, J. P., Rae, A. S., Citron, R. I., Stewart, S. T., Sinclair, C. A., Rimmer, P. B., & Shorttle, O. (2022). Reduced atmospheres of post-impact worlds: The early Earth. The Planetary Science Journal, 3(5), 115."]}

Fusion crusts form during the atmospheric entry heating of meteorites and preserve a record of the conditions that occurred during deceleration in the atmosphere. The fusion crust of the Winchcombe meteorite closely resembles that of other stony meteorites, and in particular CM2 chondrites, since it is dominated by olivine phenocrysts set in a glassy mesostasis with magnetite, and is highly vesicular. Dehydration cracks are unusually abundant in Winchcombe. Failure of this weak layer is an additional ablation mechanism to produce large numbers of particles during deceleration, consistent with the observation of pulses of plasma in videos of the Winchcombe fireball. Calving events might provide an observable phenomenon related to meteorites that are particularly susceptible to dehydration. Oscillatory zoning is observed within olivine phenocrysts in the fusion crust, in contrast to other meteorites, perhaps owing to temperature fluctuations resulting from calving events. Magnetite monolayers are found in the crust, and have also not been previously reported, and form discontinuous strata. These features grade into magnetite rims formed on the external surface of the crust and suggest the trapping of surface magnetite by collapse of melt. Magnetite monolayers may be a feature of meteorites that undergo significant degassing. Silicate warts with dendritic textures were observed and are suggested to be droplets ablated from another stone in the shower. They, therefore, represent the first evidence for intershower transfer of ablation materials and are consistent with the other evidence in the Winchcombe meteorite for unusually intense gas loss and ablation, despite its low entry velocity. Science and Technology Facilities Council (STFC): ST/V000799/1

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 test. 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 period, we find an instantaneous reduction in Dimorphos's along-track orbital velocity component of 2.70 +/- 0.10 mm/s, indicating enhanced momentum transfer due to recoil from ejecta streams produced by the impact. 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, $\beta$, 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$^3$, $\beta$= 3.61 +0.19/-0.25 (1 $\sigma$). These $\beta$ 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., Comment: accepted by Nature

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.

Introduction: During impact cratering, target materials are subjected to extreme deformation conditions. Brittle deformation under these conditions, where strain rates can exceed 101 to 102 s­­-1, is rate-sensitive. Typically, rocks are stronger when deformed at high strain-rate conditions [1]. This occurs because fracture propagation has a limited velocity; at high loading rates, the weakest flaws in a material are not able to cause failure before other, increasingly strong flaws are activated. This results in significant changes to mechanical properties and causes fragmentation of the target material [2, 3]. Dynamic compressive strength and fragmentation in brittle materials is not currently implemented in numerical impact simulations. In this study, we present results of high strain rate mechanical tests to determine the characteristic strain rate for rate-dependent brittle failure and dynamic strength increase, and the fragment size and shape distributions that result from failure at these conditions. We investigated a variety of rock types and considered whether the fragment characteristics can be used as diagnostic indicators of loading conditions during brittle failure. In addition, we use numerical impact simulations to assess the significance of dynamic strength increase and compressive fragmentation during impact cratering at a variety of scales. Methods: Mechanical data and samples were obtained using a hydraulic loading frame and a Split-Hopkinson Pressure Bar (SHPB). The hydraulic loading frame achieves strain rates from 10-6 s-1 to 10-4 s-1, while the SHPB achieves strain rates from 101 s-1 to 103 s-1. As sample materials, we chose a variety of igneous, metamorphic, and sedimentary rocks in order to investigate differences between target types and material properties. From the mechanical experiments, the strain-rate dependency of strength was calculated and samples were generated for microstructural analysis. We focussed our microanalysis on the distributions of fragment size and fragment shape as functions of strain rate. Numerical impact simulations in this study were conducted using the iSALE shock physics code [4 and refs. therein]. Results: We find that the characteristic strain rates of rocks, where the dynamic strength is twice the quasi-static strength, ranges between ~150 and ~350 s-1 depending on lithology (Figure 1). Fragment size analysis demonstrates an inverse power-law relationship between fragment size and strain rate for dynamic failure under uniaxial compression. Unlike fragment size, we find that fragment shape is independent of strain rate under dynamic uniaxial loading (Figure 3). Figure 1. Rock strengths from quasi-static and dynamic testing. Each lithology follows the universal scaling relation (inset) of [1], where each material possesses a unique characteristic strength and characteristic strain rate. In all tested lithologies, strength dramatically increases at strain rates > ~101 s-1. Data from [2, 3]. Figure 2. Scaled empirical fragmentation relationship for uniaxial compressive failure. The exponent of the relationship remains the same, while the constant varies between porous and non-porous rocks. From [3]. Figure 3. Fragment shape distributions for fragments > 2 mm as a function of strain rate. Distributions of fragment shape (circularity and axial ratio) for each sample are shown as kernel density estimates. Each point shows the mean shape parameter plotted against the strain rate, where the x-error bar shows the uncertainty in the strain rate. From [3]. Discussion: Numerical impact simulations demonstrate that strain rates are sufficiently high to produce rate-dependent effects in planetary impacts (vi > ~5 km/s) where impactor diameters are ~100 m or smaller (Figure 4). Thus, rate-dependent strength and fragmentation plays an important role in small planetary impacts and laboratory impact experiments. Based on the results of this study, we are developing a semi-empirical approach to account for rate-dependent shear strength in numerical impact simulations (Figure 5); an important development due to the need to accurately ground-truth numerical impact models against laboratory scale experiments. Additionally, this study demonstrates that fragment size may be used as a diagnostic indicator of the strain rate at failure during impact loading while fragment shape cannot be used. However, our experiments only used uniaxial compressive loads, it is uncertain how truly triaxial stress states may affect fragmentation behaviour. Figure 4. Crater excavation in an iSALE simulation of the impact of a 1 m diameter sphere at 15 km s-1. Left) Coloured regions indicate pressures between 0 and 500 MPa, regions of tension are coloured light grey while regions of high confining pressure (e.g. within the shock wave) are coloured dark grey. Right) Coloured regions indicate strain rates greater than 22.5 s-1, regions with lower strain rates are coloured light grey. Regions that are coloured in both panels are undergoing deformation in a regime where strength and fragmentation should be strongly controlled by strain rate. From [2]. Figure 5. Implementation of dynamic strength as a modification of the ROCK strength model [5] (black). Uniaxial compressive and tensile strength follows the scaling law of [1]. In the high-pressure ductile regime (dark grey background shading), rock strength is not controlled by the behaviour of fractures, providing an upper limit for dynamic strength at ~105 s-1 [6] (grey). Acknowledgments: H. Ickler and G. Mette are thanked for sample preparation. L. Müller, M. Dörffler, and T. Kern are thanked for lab assistance. The developers of iSALE are gratefully acknowledged. This work was funded by DFG Project KE 732/27-1. References: [1] Kimberley, J. et al. (2013) Acta Materialia, 61:9, 3509-3521. [2] Rae, A. S. P. et al. (2020) JGR: Planets, 125:10. [3] Rae, A. S. P. et al. (2022) Tectonophysics, 824, 229221. [4] Wünnemann, K. et al. (2006) Icarus, 180:2, 514-527. [5] Collins, G. S. et al. (2004) MAPS, 39:2, 217-231. [6] Grady, D. E. (1998) Mechanics of Materials, 29:3-4, 181-203.

International audience; Meteoroid impacts shape planetary surfaces by forming new craters and alter atmospheric composition. During atmospheric entry and impact on the ground, meteoroids excite transient acoustic and seismic waves. However, new crater formation and the associated impact-induced mechanical waves have yet to be observed jointly beyond Earth. Here we report observations of seismic and acoustic waves from the NASA InSight lander’s seismometer that we link to four meteoroid impact events on Mars observed in spacecraft imagery. We analysed arrival times and polarization of seismic and acoustic waves to estimate impact locations, which were subsequently confirmed by orbital imaging of the associated craters. Crater dimensions and estimates of meteoroid trajectories are consistent with waveform modelling of the recorded seismograms. With identified seismic sources, the seismic waves can be used to constrain the structure of the Martian interior, corroborating previous crustal structure models, and constrain scaling relationships between the distance and amplitude of impact-generated seismic waves on Mars, supporting a link between the seismic moment of impacts and the vertical impactor momentum. Our findings demonstrate the capability of planetary seismology to identify impact-generated seismic sources and constrain both impact processes and planetary interiors.

NASA's InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission has operated a sophisticated suite of seismology and geophysics instruments on the surface of Mars since its arrival in 2018. On 18 February 2021, we attempted to detect the seismic and acoustic waves produced by the entry, descent and landing of the Perseverance rover using the sensors onboard the InSight lander. Similar observations have been made on Earth using data from both crewed(1,2) and uncrewed(3,4) spacecraft, and on the Moon during the Apollo eras(5), but never before on Mars or another planet. This was the only seismic event to occur on Mars since InSight began operations that had an a priori known and independently constrained timing and location. It therefore had the potential to be used as a calibration for other marsquakes recorded by InSight. Here we report that no signal from Perseverance's entry, descent and landing is identifiable in the InSight data. Nonetheless, measurements made during the landing window enable us to place constraints on the distance-amplitude relationships used to predict the amplitude of seismic waves produced by planetary impacts and place in situ constraints on Martian impact seismic efficiency (the fraction of the impactor kinetic energy converted into seismic energy)., Nature Astronomy, 6 (1), ISSN:2397-3366

The third stage of the Saturn IV rocket used in the five Apollo missions made craters on the Moon ∼30 m in diameter. Their initial impact conditions were known, so they can be considered controlled impacts. Here, we used the iSALE‐2D shock physics code to numerically simulate the formation of these craters, and to calculate the vertical component of seismic moment (∼4 × 1010 Nm) and seismic efficiency (∼10−6) associated with these impacts. The irregular booster shape likely caused the irregular crater morphology observed. To investigate this, we modeled six projectile geometries, with footprint area between 3 and 105 m2, keeping the mass and velocity of the impactor constant. We showed that the crater depth and diameter decreased as the footprint area increased. The central mound observed in lunar impact sites could be a result of layering of the target and/or low density of the projectile. Understanding seismic signatures from impact events is important for planetary seismology. Calculating seismic parameters and validating them against controlled experiments in a planetary setting will help us understand the seismic data received, not only from the Moon, but also from the InSight Mission on Mars and future seismic missions.

An increasing number of newly formed impact craters on Mars have been detected in the last 15 years. These small craters are normally identified via dark spots in lower resolution images that formed during the impact process, presumably through the removal or disturbance of bright surface material [1]. Later higher resolution images revealed single craters or crater clusters, which form when impactors fragment in the atmosphere, within those halos [1,2]. Due to this detection method, most of the new impact sites found are in dusty regions, which imposes an observational bias [3]. Newly formed clusters consist of two to thousands of individual craters and can be tightly clustered or spread out over hundreds of meters [2]. Since the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed on Mars in 2018 [4], the search for newly formed impact craters has become even more important, because identifying impacts in seismic signals could provide further constraints on both the atmospheric and solid-body effects of impact cratering process on Mars, as well as help place further constraints on the properties of the uppermost layer of the crust. As one of InSight’s mission goals is to estimate the current impact rate on Mars, the seismic detection of impacts is also crucial [4]. The aim of this new study is to describe the properties of the complete catalog of known newly formed craters on Mars and examine correlations between different crater cluster properties. We investigated 559 crater clusters and 493 single craters detected between 2008 and 2020 using 25 cm/px HiRISE images. The locations and diameters were noted for each single crater, as well as for every individual crater within a cluster down to 1 m diameter. This was done using ArcMap (ArcGIS) software with the three-point method of the CraterTools add-in [5]. We describe the cluster characteristics, such as the number of craters within a cluster, largest crater in a cluster, cluster effective diameter, cluster dispersion, elevation of the impact sites, and the variation in sizes of craters within a cluster. More than half of the new impact sites form as clusters. We did not find any differences between the spatial distribution of single and crater clusters across Mars. The mapped crater clusters from this study consist of 2 to 2334 individual craters. More than half of all clusters (58%) consist of 10 craters or less. Crater clusters containing more than 100 craters are rare. With regard to the sizes of craters within crater clusters, we found that for highly populated clusters, the majority of craters are very small, and clusters with few craters have a tendency for craters that are more equal in size. Clusters having large effective diameters contain more equally sized craters. Our results show the full range of parameter spaces that are possible for cluster properties, which can help validate theoretical atmospheric fragmentation models. References: [1] Malin M. C. et al. (2006) Science, 314, 1573-1577. [2] Daubar I. J. et al. (2019) JGR, 124, 958-969. [3] Daubar I. J. et al. (2013) Icarus, 225, 506-516. [4] Banerdt B. W. et al. (2020) Nature, 13, 183-189. [5] Kneissl T. et al. (2011) Planet. Space Sci., 59, 1243-1254.

Introduction: NASA’s Double Asteroid Redirection Test (DART) will impact the smaller component of the 65803 Didymos asteroid system, Dimorphos, and alter its orbital period around the primary, thus demonstrating the controlled deflection capabilities of near-Earth asteroids by a kinetic impactor [1, 2]. ESA’s Hera mission [2] will arrive at Dimorphos several years after the DART impact and provide a detailed characterization of the impact outcome, including the morphometry and morphology of the crater. Recent impact experiments and numerical studies [3–5] have shown that the kinetic impact efficiency depends strongly on the target properties and structure, and is non-unique (i.e., a number of target property configurations, such as different cohesion-porosity combinations, can result in the same deflection and a measure of the deflection alone can be interpreted in different ways depending on the target and impact properties). Therefore, for a successful interpretation of the DART impact outcome it is important to understand the influence of asteroid properties on the cratering process. Moreover, the DART impact outcome analysis will be based on numerical models, which require extensive and accurate prior validation. Most previous impact experiments and the subsequent validation work of numerical models have focused on homogeneous targets [e.g., 6, 7]. However, it is unlikely that Dimorphos is homogeneous at the scale of DART impact. Here we present preliminary results from impact experiments and numerical simulations specifically designed to mimic asteroid surface materials and structures (e.g., layered targets, rubble piles). The experiments are performed at the Experimental Projectile Impact Chamber (EPIC) at Centro de Astrobiología CSIC-INTA, Spain. Methods: The EPIC utilizes a 20 mm calibre compressed N2 (300 bar) cannon that launches projectiles at velocities up to ≈420 m/s and at angles of 20– 90◦ from horizontal [8]. The experiments, half- or quarter-space, are recorded with high-speed cameras and the resulting crater profiles are scanned in 3D with 0.5 mm resolution. Here we use the iSALE-2D [9] and the Bern SPH [10] shock physics codes to simulate the laboratory experiments. Both iSALE-2D and SPH include material models suitable for geological materials, various equations of state and porosity compaction models. We compare numerical models with impact experiments into targets with three different structures: homogeneous, layered and heterogeneous. Results and discussion: Vertical impacts into homogeneous targets. These impact experiments were performed into dry beach sand ("slow pour" maximum compaction) and iron grit targets and the results are used to validate the ejecta curtain formation and crater size from 2D numerical models into homogeneous targets. This setup is also used as reference for comparison with more complex target settings. Layered targets. Recently visited asteroids (e.g. Itokawa, Ryugu) have been observed to have a layered structure (i.e., a layer of regolith overlying a much stronger and denser substrate). The effects of target layering on impact cratering in the strength regime have been studied extensively through laboratory experiments [e.g., 11] and simulations [e.g., 12]. However, it is possible that Dimorphos has a granular cohesionless upper layer and that cratering in this layer is gravity-controlled, much like Hayabusa2’s SCI experiment [13]. Little is known of the effects of layering in the gravity regime, however it is likely that the momentum transferred from the DART spacecraft can be both amplified or reduced, depending on Dimorphos’ layering configuration [12]. We carry out experiments with layers of different density (i.e., beach sand over iron grit) to simulate gravity-controlled cratering in layered targets. While much of the kinetic energy of the projectile is released in the weaker upper target, this material also requires relatively less energy to be cratered resulting in a concentric crater morphology. This crater morphology is well reproduced in our iSALE-2D numerical models. Complex target structures/"rubble piles". Variations in strength and density also occur in very heterogeneous objects such as rubble-pile asteroids. An irregular crater growth and, consequently, irregular ejection of material affect the asteroid deflection (e.g., unpredictable motions). Such irregularities are difficult to model numerically and require full 3D geometry, which is computationally expensive. Nevertheless, efforts to model rubble pile geometries in the context of DART are undertaken [14] and laboratory experiments are essential to validate such models. We have performed shots into targets with porous ceramic balls (simulating “boulders”) of approximately equal size and weight as the projectile embedded in a matrix of beach sand (“slow pour” maximum compaction) (Fig. 1). Experiment 1 had balls distributed uniformly and at regular intervals within the sand. Experiment 2 had the same configuration, but only in half of the target while the other half was beach sand only. Comparisons were then made with the reference shot in pure beach sand. There is now a very good fit between the reference crater (Exp3) and the part of the Experiment 2 crater developed in the pure sand target, and there is an equally good fit between the half of the crater developed in the “rubble-pile” part of the target and the crater from Experiment 1 (Fig. 2). The “boulders” in the target seem to affect the excavation flow lines, resulting in steeper ejection near the limit of the excavation cavity. This results in a more flat-floored and steep-walled crater (Fig. 2). Numerical simulation of Experiment 1 replicated the experimental result very well (Fig. 3). Balls placed in immediate proximity to the impact point were ejected and ‘rays’ were created in the ejecta blanket. Balls placed at about 3 ball diameters away from the impact point were displaced towards the crater rim, as also noted in the numerical simulation (Fig. 4). Conclusions: The impact experiments represent a first step in our work towards understanding the importance of target heterogeneities in the cratering process and impact momentum transfer. SPH simulations to reproduce these impact experiments are underway and preliminary results from the early cratering process show good agreement with the experiment.

1. Introduction The AIDA international collaboration, which includes the DART (NASA) and Hera (ESA) missions, aims to test the technology of deflection by a kinetic impactor and to enhance our understanding of small bodies in general. In the context of these missions, a spacecraft, DART, will impact the secondary of the 65803 Didymos system, Dimorphos, at the end of October 2022 [1]. The impact will eject asteroid material from the target surface, leading to a measurable change in the orbital period of the binary. A second, follow-up spacecraft, Hera, will arrive at the system several years after the impact to characterize the system and the impact consequences [2]. The objective of this study, which is conducted in the context of the NEO-MAPP project, is to model the collision of the kinetic impactor with Dimorphos and to predict the outcome of the impact with respect to parameters that are measurable by spaceborne and in-situ instrumentation provided by the Hera mission. We model the impact using two different numerical schemes: a continuum approach using iSALE-2D/-3D & a smooth particle approach using Bern SPH. Although all codes solve similar forms of conservation equations and use similar constitutive models, different numerical schemes can produce systematically different results. Hence, as a first step, accurate validation tests against laboratory experiments are conducted to improve the reliability of results from numerical modelling.2. MethodiSALE-2D/-3D [3, 4] is a grid-based arbitrary Eulerian Lagrangian (ALE) code and is best suited to study crater formation and the propagation of shock waves from a high velocity impact into targets with a variety of different properties. On the other hand, Bern's grid-free Smooth Particle Hydrodynamics (SPH) [5, 6] is most appropriate to study the ejection of material and processes where the entire target body is involved. Both codes include the simulation of material compaction (iSALE: ε-α model; SPH: P-α model) and different strength models. In this study, we employ the Drucker-Prager and the Lundborg rheology models to describe the strength of the material. For both codes, the ejection behaviour is analysed as described in [7,8,9], and the ejection data is used to determine the momentum transferred to the target for deflection (momentum enhancement factor β = ejecta + impactor momentum / impactor momentum). This approach was used previously for systematic parameter studies [9] and benchmarking studies [10]. To validate our shock physics codes, we compare our results against observations from a recent laboratory study [11], where PVC projectiles with a mass of ~25 mg were accelerated to velocities of 1-2 km/s, impacting glass beads, sand and regolith simulant targets. In a second step, we continue the benchmarking work done by the Hera impact working group [12] to detect, assess and remove deviations between two different numerical schemes, iSALE (in 2D and 3D) and Bern SPH at the scale of the DART impact. 3. Validation We have simulated the crater formation in glass beads and regolith simulant with iSALE-2D at impact speeds of 2.4 km/s and 2.2 km/s, respectively. The glass beads target was modelled using a Drucker-Prager criterion with a coefficient of internal friction, f = 0.5 and an initial porosity of 35%. The regolith simulant used f ~ 0.8 and an initial porosity of 42%. We determine β-values of 2.9 and 1.4, which agree well with experimentally determined values of 2.7 and 1.3, respectively. In the glass beads target laboratory experiment, at ~0.5-1 ms after the impact, the ejecta curtain made an angle of 50-60°. In the case of the regolith simulant, at ~0.5 ms after the impact, the ejecta curtain angle was 30-40°. From our numerical models, we determined ejecta curtain angles 0.5 ms after the impact as 48° (glass beads) and 30° (regolith, Fig. 1). Both results agree with the lower bound of the experimental constraints. 4. Benchmark The first benchmark study focuses on the influence of target properties on the efficiency of the momentum transfer from the DART impact, β, for materials similar to the regolith simulant. The 600 kg projectile impacts targets with varying porosities, between 10 and 50%, and cohesions from 1 to 100 kPa, at a velocity of 6 km/s. The second benchmark study focuses on the influence of the impact angle on β for the 20% porosity case with 10 kPa cohesion at an impact velocity of 7 km/s. In previous studies, we found generally good agreement between the results derived with iSALE and SPH [12, 13, 14]. However, for certain porosities the deviations between the results were larger (e.g., 20% porosity and 1 kPa cohesion), which may have been the result of discrepancies in the details of the crush curve used in each model. As a result, here we conducted new models with more consistent crush curves (Fig. 2). Varying the impact angle for a specific impact scenario and set of target properties shows a remarkably good agreement between the two codes (Fig. 3). 5. SummaryOur joint modelling and experimental approach to study the efficiency of asteroid deflection by a kinetic impactor shows that there is generally a good agreement between different numerical approaches and experimental work. The benchmark studies show that the results from the grid based iSALE (-2D/-3D) and the meshless SPH can deviate for similar initial porosities when the crush curves are not consistent, but they produce similar results when the same impact conditions are considered. AcknowledgementsWe gratefully acknowledge the developers of iSALE (www.isale-code.de). This work has received funding from the European Union’s Horizon 2020 research and innovation programme, NEO-MAPP, grant agreement No. 870377.References[1]A.F.Cheng et al.,P&SS157(2018)104-115.[2]P.Michel et al.,AdvSpaceRes.62(2018)2261-2272.[3]K.Wünnemann et al.,Icarus180(2006)514–527.[4]D.Elbeshausen et al.,Icarus204(2009)716–731.[5]M.Jutzi et al.,Icarus198(2008)242–255.[6]M.Jutzi,P&SS107(2015)3–9.[7]M.Jutzi&P.Michel,Icarus229(2014)247-253.[8]R.Luther et al.,M&PS53(2018)1705–1732.[9]S.D.Raducan et al.,Icarus329(2019)282–295.[10]A.Stickle et al.,Icarus338(2020)113446.[11]S.Chourey et al.,P&SS194(2020)105112.[12]R.Luther et al.,EPSC-DPS2019-1776(2019).[13]S.D.Raducan et al.,52th LPSC(2021)#2548id1908.[14]R.Luther et al.,7th PDC(2021)#122.

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

Meteorite impacts often occur in layered targets, where the strength of the target varies as a function of depth, but this complexity is often not represented in numerical impact simulations because of the high computational cost of resolving thin layers. To address this limitation, we developed a method to approximate the effect of multiple thin weak layers within a sedimentary sequence using a single material layer to represent the entire sequence. Our approach, implemented in the iSALE (impact‐Simplified Arbitrary Lagrangian Eulerian) shock physics code, combines an anisotropic yield criterion with a cell‐based method to track the orientation of layers. To demonstrate the efficacy of the method and constrain parameters of the anisotropic strength model required to replicate the effects of thin, weak layers, we compare results of simulations of a ~20 – 25‐km diameter complex crater on Earth using the new method to those from simulations that explicitly resolve multiple thin weak layers. We show that our approach allows for a reduction in computational cost, negating the need for an increase in spatial resolution to resolve thin layers in the target, while replicating crater formation and final morphology from the high‐resolution models. In keeping with field observations, we also find that anisotropic layers may be responsible for a lack of central uplift expression observed at many craters formed in targets with thick sedimentary layers (e.g., the Haughton and Ries impact structures).

International audience; The entry, descent, and landing (EDL) sequence of NASA's Mars 2020 Perseverance Rover will act as a seismic source of known temporal and spatial localization. We evaluate whether the signals produced by this event will be detectable by the InSight lander (3,452 km away), comparing expected signal amplitudes to noise levels at the instrument. Modeling is undertaken to predict the propagation of the acoustic signal (purely in the atmosphere), the seismoacoustic signal (atmosphere-to-ground coupled), and the elastodynamic seismic signal (in the ground only). Our results suggest that the acoustic and seismoacoustic signals, produced by the atmospheric shock wave from the EDL, are unlikely to be detectable due to the pattern of winds in the martian atmosphere and the weak air-to-ground coupling, respectively. However, the elastodynamic seismic signal produced by the impact of the spacecraft's cruise balance masses on the surface may be detected by InSight. The upper and lower bounds on predicted ground velocity at InSight are 2.0 × 10 −14 and 1.3 × 10 −10 m s −1. The upper value is above the noise floor at the time of landing 40% of the time on average. The large range of possible values reflects uncertainties in the current understanding of impact-generated seismic waves and their subsequent propagation and attenuation through Mars. Uncertainty in the detectability also stems from the indeterminate instrument noise level at the time of this future event. A positive detection would be of enormous value in constraining the seismic properties of Mars, and in improving our understanding of impact-generated seismic waves.

The first seismometer operating on the surface of another planet was deployed by the NASA InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission to Mars. It gives us an opportunity to investigate the seismicity of Mars, including any seismic activity caused by small meteorite bombardment. Detectability of impact generated seismic signals is closely related to the seismic efficiency, defined as the fraction of the impactor's kinetic energy transferred into the seismic energy in a target medium. This work investigated the seismic efficiency of the Martian near surface associated with small meteorite impacts on Mars. We used the iSALE‐2D (Impact‐Simplified Arbitrary Lagrangian Eulerian) shock physics code to simulate the formation of the meter‐size impact craters, and we used a recently formed 1.5 m diameter crater as a case study. The Martian crust was simulated as unfractured nonporous bedrock, fractured bedrock with 25% porosity, and highly porous regolith with 44% and 65% porosity. We used appropriate strength and porosity models defined in previous works, and we identified that the seismic efficiency is very sensitive to the speed of sound and elastic threshold in the target medium. We constrained the value of the impact‐related seismic efficiency to be between the order of ∼10‐7 to 10‐6 for the regolith and ∼10‐4 to 10‐3 for the bedrock. For new impacts occurring on Mars, this work can help understand the near‐surface properties of the Martian crust, and it contributes to the understanding of impact detectability via seismic signals as a function of the target media.

NASA’s Double Asteroid Redirection Test (DART) mission will impact its target asteroid, Dimorphos, at an oblique angle that will not be known prior to the impact. We computed iSALE-3D simulations of DART-like impacts on asteroid surfaces at different impact angles and found that the vertical momentum transfer efficiency, β , is similar for different impact angles, however, the imparted momentum is reduced as the impact angle 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, however it is observed to ‘straighten up’ with crater growth. iSALE-2D simulations of vertical impacts provide context for the iSALE-3D simulation results and show that the ejection angle varies with both target properties and with crater growth. While the ejection angle is relatively insensitive to the target porosity, it varies by up to 30 ∘ with target coefficient of internal friction. The simulation results presented in this paper can help constrain target properties from the DART crater ejecta cone, which will be imaged by the LICIACube. The results presented here represent the basis for an empirical scaling relationship for oblique impacts and can be used as a framework to determine an analytical approximation of the vertical component of the ejecta momentum, β − 1 , given known target properties.

The Double Asteroid Redirection Test (DART) plans to impact the smaller component of the 65803 Didymos asteroid system [1], Dimorphos, and alter the binary orbit period of the system [2]. The experiment aims to demonstrate the controlled deflection capabilities of a near-Earth asteroid. ESA’s Hera mission [1,2] will arrive at Dimorphos four years after the DART impact and provides a detailed characterization of the impact. For a successful analysis it is important to know the influence of various parameters known to affect the cratering. Here we present some preliminary results as well as planned experiments at the Experimental Projectile Impact Chamber (EPIC) at Centro de Astrobiología CSIC-INTA, Spain, carried out in concert with observations of natural craters and numerical simulations for mutual verification of the methods and a first analysis of the effects of factors such as heterogeneities in a granular (i.e. ‘rubble-pile’) target. Introduction, aim and methods As shown by previous impact experiments and numerical simulations [3,4,5], the amount by which the target asteroid can be deflected is highly dependent on the target properties and structure, and non-unique. Thus, the knowledge of the final crater as observed by Hera is important to assess the target properties of Dimorphos [6]. However, such an analysis based on numerical models requires accurate validation of the applied numerical codes [7,8]. Previous validation work has focused on impacts into homogeneous targets [e.g. 9,10]. However, it is unlikely that asteroids are as homogeneous at this scale [cf. 11] as on the scale of previous laboratory experiments. Therefore, there is a need for impact experiments specifically designed to mimic asteroid surface materials and structures (e.g. layered targets, rubble piles). The EPIC can be used to provide the impact experiments needed. In addition, field observation of relevant natural craters, both strength-controlled (e.g. Kaali, 110m) and gravity-controlled (e.g. Lockne, 7.5km) may provide additional information. The EPIC utilizes a 20mm calibre compressed N2 (300bar) cannon that can shoot projectiles of various materials and composition (e.g., solid, clustered) at velocities of up to ~420m/s and into various target materials at impact angles from 20-90° over the horizontal. The experiments, half- or quarter-space, are recorded by high speed cameras, and the resulting craters (approx. 30cm in dry sand) can be scanned in 3D with 0.5mm resolution. For the results shown here, we use the iSALE shock physics code [12,13] to simulate the conducted experiments from the laboratory. The strengths of iSALE are the availability of: different material models (brittle/ductile rheology), a damage model, various equations of state, and a porosity compaction model. Planned activities and preliminary results Vertical impacts into homogeneous targets Vertical impact experiments into homogeneous targets can be used to validate ejecta curtain formation and final crater size from 2D numerical models. Then, simulations can be used to determine the influence of specific material properties in systematic parameter studies, which are difficult to be conducted in the laboratory (Fig. 1). Layered targets Most small bodies in the Solar System are not homogeneous, as shown by past missions to asteroids. Several visited asteroids have been observed to be covered by a layer of regolith, which overlies a much stronger or denser substrate [14]. The effects of target layering on impact cratering in the strength regime was studied extensively through laboratory experiments [e.g. 15] and simulations [16]. While much of the kinetic energy of the projectile is released in the weaker upper target, this material also requires relatively less energy to be cratered. Likewise, differing impedances of the cover layer and the substrate (i.e., basement) could result in reflection of the shock, with a reduction of the energy transferred into the basement and, consequently, more energy available for cratering the upper layer. Therefore, the crater in the weak layer may get relatively wider than a corresponding crater in an equivalent homogeneous material. For asteroids like Dimorphos the transition crater diameter will be well above the size of the target, giving that all impacts would be strength dominated. Nevertheless, even in the case of a strength dominated substrate, a layer of granular cohesionless material will still be gravity-dominated. For example, Hayabusa2’s SCI experiment created a concentric crater morphology specific to layered targets, which is believed to have formed in the gravity regime [11]. Layering affects also large gravity-dominated craters [17]. However, little is known about impact cratering into layered targets in the gravity regime. As shown from numerical simulations into layered targets in the strength-regime, the presence of a stronger and/or denser substrate can both amplify and reduce the momentum transferred from a spacecraft for deflecting an asteroid. [6] This can have important implications for NASA’s DART mission. We have already carried out some experiments at the EPIC with layers of different density (i.e., beach sand over iron grit) to simulate gravity-controlled cratering in layered targets (Figs. 2,3). 3. Complex target structures/Rubble piles However, the variations in strength and density can also be compared to the configuration of greatly heterogeneous objects such as rubble-piles. Rubble-pile asteroids may have a highly irregular distribution of massive rock vs rubble (i.e. not in an orderly “layered” fashion), and strength and density vary locally. An irregular crater growth and, consequently, irregular ejection of material affect the asteroid deflection (e.g. unpredictable motions). Such irregularities may be difficult to model numerically and would require full 3-dimensional geometry, which is computationally expensive. Nevertheless, current efforts to model rubble pile geometries in the context of the DART impact are undertaken [18]. Laboratory experiments are highly needed to validate such models. We intend to investigate these effects experimentally for both strength and gravity dominated targets (Fig. 4). Acknowledgements Ormö was supported by grants ESP2017-87676-C5-1-R from the Spanish Ministry of Economy and Competitiveness and Fondo Europeo de Desarrollo Regional. Ormö and Herreros were supported by the Spanish State Research Agency Project No.MDM-2017-0737 Unidad de Excelencia "María de Maeztu"- Centro de Astrobiología (INTA-CSIC), and the CSIC I-LINK project LINKA20203.

Introduction: Earth is continuously impacted by space debris and small asteroids, and, while large asteroid impacts are very rare, they have the potential to cause severe damage. NASA's Double Asteroid Redirection Test (DART) aims to be the first mission to test a controlled deflection of a Near-Earth binary asteroid [1, 2], by impacting the smaller component of the 65803 Didymos asteroid system, Dimorphos. The impact will thereby alter the binary orbit period by an amount detectable from Earth [3]. ESA's Hera mission [3, 4], that will arrive at Dimorphos several years after the DART impact. It will rendezvous with the asteroid system and perform detailed characterisation of Dimorphos's volume and surface properties, as well as measure the DART impact outcome, such as change in the binary system orbit and the volume and morphology of the DART impact crater. In high velocity impacts on an asteroid the change in momentum of the asteroid ΔP can be amplified by the momentum of crater ejecta that exceeds the escape velocity, which is often expressed in terms of the parameter β=ΔP/mU, where mU is the impactor momentum [5]. The amount by which crater ejecta enhances asteroid deflection-that is, the normalised momentum of the crater ejecta that escapes the gravitational attraction of the target body (β-1)-has been found to vary significantly depending on the target asteroid's properties and composition [6]. Previous numerical simulations [7, 8] have quantified the sensitivity of the asteroid deflection to target material properties. To allow for a large variety of material properties to be studied, these simulations employed a two-dimensional shock physics code with an axially-symmetric mesh geometry, which restricted the studies to vertical impacts only. However the DART spacecraft will impact the surface of Didymoon at an oblique angle [3]. Here we investigate the influence of impact angle on the ejecta momentum transfer with the aim of developing an empirical scaling relationship for β as a function of impact angle. Numerical methods: We used the iSALE3D shock physics code [9] to numerically simulate the DART impact in two and three dimensions. The DART spacecraft structure was modelled as a porous aluminium sphere, impacting a 20% porous, homogeneous basaltic regolith target at 7 km/s. The cohesive strength of the damaged material was 10 kPa. Influence of the impact angle on the net momentum: Consistent with previous laboratory-scale oblique impact experiments [10, 11] and DART impact models [12], our simulations show that the ejecta from oblique impacts displays higher speeds and lower ejection angles in the downrange direction, and lower speeds and higher ejection angles in the uprange direction of the impact. Figure 1 compares the surface topography of a vertical DART impact, at a 90o angle of incidence to the target plane, and an oblique DART impact at a 45o angle. The time-frames of the oblique impact Figure 1b show a highly asymmetric ejecta distribution at early times of the cratering process The asymmetry of the ejecta can have important implications for momentum transfer vector. The net momentum of the target after the impact is the vector sum of the impactor momentum and the crater ejecta momentum enhancement vectors. The projectile imparts an its momentum along the impact direction; that is, downrange and into the target. As most of the ejecta momentum is launched in the downrange direction, the net momentum imparted to the target, which acts in the opposite direction, acts downward and slightly uprange. The vector sum of the impactor momentum and the momentum enhancement vectors is in between the individual vectors, implying that the ejecta momentum acts to increase the vertical component of momentum transfer and reduce the horizontal downrange component. Figure 2 shows the direction of the total momentum imparted to the asteroid, as a function of time, measured relative to the downrange horizontal direction, for four different impact angles. As the crater grows towards its final diameter (at about 1 s), the uprange direction of the ejecta momentum becomes more perpendicular to the surface. The direction of the net momentum imparted to the target also changes, from the downrange direction, towards the vertical direction. In the scenarios simulated here, for impacts into a 10 kPa target, the direction of the net momentum at the end of the crater growth is about 83o for the 60o impact, ~77o for the 45o impact and ~66o for the 30o impact. In the simulations presented here, crater growth is halted by the target's strength before the total momentum direction becomes vertical. However, it is expected that with increasing cratering efficiency (e.g. decreasing strength), the ejecta momentum will make a larger contribution towards the total momentum vector. Towards an ejecta scaling relationship for oblique impacts: Ejecta scaling relationships are useful to predict the ejecta distribution and momentum transfer for vertical impacts. However, most planetary impacts are oblique and stationary point-source scaling [6] becomes inadequate. An aim of this work is to develop an ejecta scaling relationship for oblique impacts, based on numerical simulation data. Three-dimensional simulations of the DART impact at vertical, 60o, 45o and 30o impact angles provide information about the ejecta mass-velocity distribution as a function of impact angle and azimuth. The momentum carried away by the ejecta from a vertical impact, β-1, can be found from integrating the mass, dM, within the radial distance range from n1 to n2R/a [1]. Here we apply this semi-analytical equation on a per azimuth basis and then sum each azimuthal ejecta distribution to derive β-1 values from all ejecta in oblique impact scenarios (Fig. 3). Acknowledgements: We gratefully acknowledge the developers of iSALE (www.isale-code.de) and STFC for funding (Grant ST/N000803/1).

In the UK there are five meteor camera networks using four different camera and software systems that are aiming to recover meteorites. Utilising all observations of a fireball event from each network is crucial to constrain a precise orbit and fall position. However, the various camera systems generate a diversity of data outputs that are not compatible with each other. As a result, when a potentially meteorite-dropping fireball event occurs it is currently challenging to exchange calibrated observations between networks, which creates obstacles to response time and rapid meteorite recovery. If recorded by at least two observatories, the fireball’s trajectory, pre-arrival orbit, final mass, and (in combination with a ‘dark flight’ model) the final fall position of any surviving meteorite can also be calculated. For this, the minimum useful data set from each camera consists of (a) the location of the observatory, and (b) a set of timed direction vectors representing each point at which the meteor was observed. While the inclusion of additional data are recommended, this is the minimum required dataset for a fireball observation from a single camera, that can be exchanged between cooperating fireball networks to provide for accurate triangulation. Camera systems currently used in the UK or considered as candidates for adoption as a data exchange standard are: UFOAnalyzer. Used by UK Meteor Observation Network and the NEMETODE network, UFOAnalyzer’s “A.XML” file contains the essential and recommended data in XML format. UFOAnalyzer is widely used by amateurs in the UK, Western Europe, and Japan. Raspberry Meteor System (RMS, or Global Meteor Network). Increasingly deployed in the UK. Observations are recorded in two files, the “CAL” file containing all essential metadata, and the “FTPDetect” file containing data for all meteors observed in any given night. Desert Fireball Network (DFN), UK Fireball Network, Global Fireball Observatory – generates a single file with all essential and recommended data, written in Astropy ECSV table format. FRIPON, SCAMP - produces a file in Pixmet or SExtractor format, but lacking metadata, which needs to be added from a separate list of observatory parameters. Cameras for Allsky Meteor Surveillance (CAMS) – similar to RMS. Used in Benelux countries, which have occasional observational overlap with the UK Virtual Meteor Observatory (VMO)1 – an XML single-meteor format used by some German and Polish networks and by the European Space Agency (ESA). Not yet used in the UK. Three existing fireball data formats (UFOAnalyzer, VMO and DFN) are identified and evaluated as candidates for information exchange between networks. Each is adequate, though the UFOAnalyzer A.XML format would need to be generalised to be unambiguous. Each can be read with standard Python library routines. Currently it would appear that the DFN format is the easiest to write using standard library routines and the easiest to represent internally as a data structure. We are working towards a recommendation for the standard format for fireball data exchange. Agreeing a common data format enables data sharing but does not require it. Whilst the minimum dataset described above can be utilised effectively, additional information to refine the accuracy of the measurement is also highly desirable and should be included. This recommended data set includes additional information regarding the observing system and uncertainties and additional observations of the event observed at each point in time and will be discussed in detail at the meeting. We have also developed and present a new converter script that can convert UFOAnalyzer and Desert Fireball Network files to any agreed standard format. Based on a Jupyter notebook kindly provided by Hadrien Devillepoix of the Desert Fireball Network in Australia, the converter was further developed by SCAMP and is now being developed jointly by the authors. Whilst it is feasible to produce a converter for a single static format such as UFOAnalyzer’s “A.XML”, it is not practicable to write and maintain a reliable converter program for the multiplicity of file formats that exist now or may exist in the future. As such this converter program represents a stopgap while a global fireball data exchange standard is established. Conclusion and Recommendation Meteorite recovery is inhibited in countries where multiple incompatible fireball camera systems are used. Adopting a common file format that could be read by each system would greatly assist in the observation and recovery of new meteorite falls with orbits. Pending further consultation, our draft recommendation is adoption of the DFN data format as a global standard for fireball data exchange. References 1 Ceplecha, Z. (1987) Geometric, Dynamic, Orbital and Photometric Data on Meteoroids From Photographic Fireball Networks. Bulletin of the Astronomical Institutes of Czechoslovakia 38, 222. 2 Devillepoix, H. A. R., Cupák, M., Bland, P. A., Sansom, E. K., Towner, M. C., Howie, R. M., ... & Benedix, G. K. (2020). A Global Fireball Observatory. arXiv preprint arXiv:2004.01069. 3 Sansom, E. K., Rutten, M. G., Bland, P. A. (2017). Analyzing Meteoroid Flights Using Particle Filters. The Astronomical Journal 153, 87-96. 4 SonotaCo (2009). “A meteor shower catalog based on video observations in 2007-2008”. WGN, Journal of the International Meteor Organization, 37, 55–62. 5 Vida, D., Mazur, M. J., Segon, D., Zubovic, D., Kukic, P., Parag, F., & Macon, A. (2018). First results of a Raspberry Pi based meteor camera system. WGN, the Journal of the International Meteor Organisation 46(2), 71-78. 6 Bertin, E., Arnouts, S. (1996.) SExtractor: Software for source extraction. Astronomy and Astrophysics Supplement Series 117, 393-404. 7 Jenniskens, P., Gural, P. S., Dynneson, L., Grigsby, B. J., Newman, K. E., Borden, M., Koop, M., Holman, D. (2011). CAMS: Cameras for Allsky Meteor Surveillance to establish minor meteor showers. Icarus 216(1), 40-61. 8 Barentsen, G., Arlt, R., Koschny, D., … & Zoladek, P. (2010). The VMO file format. I. Reduced camera meteor and orbit data. WGN, the Journal of the International Meteor Organisation 38(1), 10-24.

Introduction: Meteoroid bombardment is one of the sources of seismic activity on planetary bodies. The very first seismometer operating on the surface of another planet was successfully deployed by the NASA InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission to Mars. It gives us an opportunity to investigate the seismicity of Mars, including impact-induced seismic activity. This work investigated the seismic efficiency associated with small meteorite impacts on Mars, using numerical methods in targets analogue to the Martian surface. The Martian crust was simulated as non-porous bedrock (0% porosity) or regolith with different porosities (25%, 44% and 65%) The seismic efficiency, k, is presented as a portion of impact energy that is transferred into seismic energy. It has been suspected that consolidated (bedrock) and non-consolidated (regolith) materials will have different values of seismic efficiency. Estimates of seismic efficiency range from k=10-2 to 10-6 (Schultz and Gault, 1975; Daubar et al., 2018; McGarr et al., 1969; Hoerth et al., 2014; Richardson & Kedar, 2013; Güldemeister & Wünnemann, 2017). High seismic efficiency is typical in bedrock or highly consolidated materials (k>10-3). Low seismic efficiency is typical for sediments or unconsolidated sands and soils (k-5) (e.g., Patton and Walter 1993). In this work, we used a simplified approach (e.g. Güldemeister & Wünnemann, 2017) that defines the seismic efficiency as: ; where x represents distance from the impact point, P is the amplitude of the pressure pulse, t is the duration of the pressure pulse, ρ is the density of the target, Cp is the speed of sound in the target and Ek is the kinetic energy of the impactor. Numerical impact modelling: All simulations were performed with the iSALE-2D shock physics hydrocode (Collins, et al., 2004; Wünnemann et al., 2006). The impact conditions were modelled to replicate recent fresh meter size impact that occurred on Mars since the landing of InSight (Daubar et al., 2020). Impact crater was estimated to be ~1.5 m in diameter. Impactor radius was 4.4 cm and kinetic energy of 1.8x106 J. To simulate bedrock and fractured bedrock (25% porosity) we used the ROCK strength model (Collins et al., 2004). To simulate the regolith (44% and 65% porosity) we used Lundborg strength model (Lundborg, 1968) (Table 1). We used the Tillotsen equation of state for basalt (Tillotson, 1962; Wójcicka et al., 2020). For porous cases, we used the ε-α porosity model (Wünnemann et al., 2006;) (Table 2). Table 1. Strength model parameters for targets with different porosity Parameter 0% 25% 44% 65% Strength model ROCK ROCK LUNDD LUNDD Strength (damaged) (kPa) 10 0 10 0.3 Friction (damaged) 0.6 0.67 0.7 0.7 Limiting strength (damaged) (GPa) 3.5 0.17 0.25 0.25 Strength (intact) (MPa) 10 0.2 Friction (intact) 1.2 1.8 Limiting strength (intact) (GPa) 3.5 0.17 Table 2. ε-α porosity model parameters (Borg et al., 2005; Wünnemann et al., 2006; Wójcicka et al, 2020). Parameter 25% 44% 65% Initial distension, α 1.33 1.8 2.8 Elastic threshold, ε0 -4x10-4 10-4 10-5 Distension at transition αx 1.1 1.15 1.0 The rate change of distension with respect to volumetric strain, k 0.98 0.98 0.98 Ratio of speed of sound in porous over non-porous medium, χ 0.6 0.33 0.21 All variables in the equation for the seismic efficiency were calculated from iSALE outputs. The pressure wave was observed via gauges cells, placed at 45° equidistantly throughout the target. The pressure wave amplitude and pulse duration were calculated at full width half maximum. The sound speed was calculated from assumed bulk modulus of basalt (Wójcicka et al, 2020). Results: Seismic efficiency was calculated for the same impact conditions in all four material models, representing the reference 1.5 m crater recently observed on Mars. There is a clear decrease in seismic efficiency with increasing porosity. It is of the order of 10-5 for porous and highly porous regolith and 10-4 for fractured bedrock. Estimates for the non-porous basalt bedrock are in the order of 10-3 (Figure 1). Porosity of the target affected pressure wave amplitudes, duration of the pressure pulse and speed of sound in the target. These are all parameters used in calculation of seismic efficiency. This implies that if impact occurs on very dusty parts of Mars with thick regolith cover, efficiency would be smaller than efficiency of the impact that occurred on the bedrock, or area with thinner regolith cover. Figure 1. Seismic efficiency calculated in targets with different porosity Conclusions. Impact cratering represents one of the most important geological processes in the Solar System. Defining relationship between target’s properties and seismic efficiency is of interest to the NASA InSight science, since it helps in understanding the properties of the uppermost crust on Mars. Previous approximations of seismic efficiency were of 2x10-5 with an order of magnitude uncertainty for seismic efficiency on Mars (Teanby and Wookey, 2011) and Daubar et al. (2018) adopted the seismic efficiency of 5x10-4 calculated from the seismic moment (Gudkova et al., 2011; 2015; Teanby 2015). In this work, we calculated the seismic efficiency in meter-size impacts on Mars to be different for bedrock (order of 10-3) and porous materials (order of 10-5).

Since landing in late 2018, the InSight lander has been recording seismic signals on the surface of Mars. Despite nominal pre-landing estimates of 1–3 meteorite impacts detected per Earth year, none have yet been identified seismically. To inform revised detectability estimates, we simulated numerically a suite of small impacts onto Martian regolith and characterized their seismic source properties. For the impactor size and velocity range most relevant for InSight, crater diameters are 1-30 m. We found that in this range scalar seismic moment is 106 − 1010 Nm and increases almost linearly with impact momentum. The ratio of horizontal to vertical seismic moment tensor components is ∼1, implying an almost isotropic P-wave source, for vertical impacts. Seismic efficiencies are ∼ 10−6, dependent on the target crushing strength and impact velocity. Our predictions of relatively low seismic efficiency and seismic moment suggest that meteorite impact detectability on Mars is lower than previously assumed. Detection chances are best for impacts forming craters of diameter >10m

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.

A new 1.5 m diameter impact crater was discovered on Mars only ~40 km from the InSight lander. Context camera images constrained its formation between 21 February and 6 April 2019; follow‐up High Resolution Imaging Science Experiment images resolved the crater. During this time period, three seismic events were identified in InSight data. We derive expected seismic signal characteristics and use them to evaluate each of the seismic events. However, none of them can definitively be associated with this source. Atmospheric perturbations are generally expected to be generated during impacts; however, in this case, no signal could be identified as related to the known impact. Using scaling relationships based on the terrestrial and lunar analogs and numerical modeling, we predict the amplitude, peak frequency, and duration of the seismic signal that would have emanated from this impact. The predicted amplitude falls near the lowest levels of the measured seismometer noise for the predicted frequency. Hence, it is not surprising this impact event was not positively identified in the seismic data. Finding this crater was a lucky event as its formation this close to InSight has a probability of only ~0.2, and the odds of capturing it in before and after images are extremely low. We revisit impact‐seismic discriminators in light of real experience with a seismometer on the Martian surface. Using measured noise of the instrument, we revise our previous prediction of seismic impact detections downward, from ~a few to tens, to just ~2 per Earth year, still with an order of magnitude uncertainty. ISSN:0148-0227 ISSN:2169-9097 ISSN:2169-9100

Large tsunamis can be generated by submarine slides, but these events are rare on human timescales and challenging to observe. Experiments and numerical modelling offer methods to understand the mechanisms by which they generate waves and what the potential hazard might be. However, to fully capture the complex waveform generated by a submarine slide, the slide dynamics must also be accurately modelled. It is computationally difficult to model both a three-dimensional submarine slide whilst simultaneously simulating oceanic-scale tsunamis. Past studies have either coupled localised models of the slide generation to oceanic-scale tsunami simulations or simplified the slide dynamics. Here, we present a new methodology of model coupling that generates the wave in the ocean-scale model via boundary-condition coupling of a two-dimensional dynamic slide simulation. We verify our coupling methodology by comparing model results to a previous simulation of a tsunamigenic slide in the Gulf of Mexico. We then examine the effect of slide deformation on the risk posed by hypothetical submarine slides around the UK. We show the deformable submarine slide simulations produce larger waves than the solid slide simulations due to the details of acceleration and velocity of the slide, although lateral spreading is not modelled. This work offers a new methodology for simulating oceanic-scale tsunamis caused by submarine slides using the output of a two-dimensional, multi-material simulation as input into a three-dimensional ocean model. This facilitates future exploration of the tsunami risk posed by tsunamigenic submarine slides that affect coastlines not normally prone to tsunamis.

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 expected due to unknown target properties. The DART impact simulation team will determine a specific desired material parameter set appropriate for the Didymos system that will be standardized (to the extent possible) across the different codes when making predictions for the DART mission. Some variation in predictions will still be expected, but that variation can be bracketed by the results shown in this study.

Simulations of landslide generated waves (LGWs) are prone to high levels of uncertainty. Here we present a probabilistic sensitivity analysis of an LGW model. The LGW model was realised through a smooth particle hydrodynamics (SPH) simulator, which is capable of modelling fluids with complex rheologies and includes flexible boundary conditions. This LGW model has parameters defining the landslide, including its rheology, that contribute to uncertainty in the simulated wave characteristics. Given the computational expense of this simulator, we made use of the extensive uncertainty quantification functionality of the Dakota toolkit to train a Gaussian process emulator (GPE) using a dataset derived from SPH simulations. Using the emulator we conducted a variance-based decomposition to quantify how much each input parameter to the SPH simulation contributed to the uncertainty in the simulated wave characteristics. Our results indicate that the landslide&rsquo, s volume and initial submergence depth contribute the most to uncertainty in the wave characteristics, while the landslide rheological parameters have a much smaller influence. When estimated run-up is used as the indicator for LGW hazard, the slope angle of the shore being inundated is shown to be an additional influential parameter. This study facilitates probabilistic hazard analysis of LGWs, because it reveals which source characteristics contribute most to uncertainty in terms of how hazardous a wave will be, thereby allowing computational resources to be focused on better understanding that uncertainty.

The world's meteorite collections contain a very rich picture of what the early Solar System would have been made of, however the lack of spatial context with respect to their parent population for these samples is an issue. The asteroid population is equally as rich in surface mineralogies, and mapping these two populations (meteorites and asteroids) together is a major challenge for planetary science. Directly probing asteroids achieves this at a high cost. Observing meteorite falls and calculating their pre-atmospheric orbit on the other hand, is a cheaper way to approach the problem. The Global Fireball Observatory (GFO) collaboration was established in 2017 and brings together multiple institutions (from Australia, USA, Canada, Morocco, Saudi Arabia, the UK, and Argentina) to maximise the area for fireball observation time and therefore meteorite recoveries. The members have a choice to operate independently, but they can also choose to work in a fully collaborative manner with other GFO partners. This efficient approach leverages the experience gained from the Desert Fireball Network (DFN) pathfinder project in Australia. The state-of-the art technology (DFN camera systems and data reduction) and experience of the support teams is shared between all partners, freeing up time for science investigations and meteorite searching. With all networks combined together, the GFO collaboration already covers 0.6% of the Earth's surface for meteorite recovery as of mid-2019, and aims to reach 2% in the early 2020s. We estimate that after 5 years of operation, the GFO will have observed a fireball from virtually every meteorite type. This combined effort will bring new, fresh, extra-terrestrial material to the labs, yielding new insights about the formation of the Solar System., Accepted in PSS. 19 pages, 9 figures

Banerdt, William B. et al., NASA’s InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in Elysium Planitia on Mars on 26 November 2018. It aims to determine the interior structure, composition and thermal state of Mars, as well as constrain present-day seismicity and impact cratering rates. Such information is key to understanding the differentiation and subsequent thermal evolution of Mars, and thus the forces that shape the planet’s surface geology and volatile processes. Here we report an overview of the first ten months of geophysical observations by InSight. As of 30 September 2019, 174 seismic events have been recorded by the lander’s seismometer, including over 20 events of moment magnitude M = 3–4. The detections thus far are consistent with tectonic origins, with no impact-induced seismicity yet observed, and indicate a seismically active planet. An assessment of these detections suggests that the frequency of global seismic events below approximately M = 3 is similar to that of terrestrial intraplate seismic activity, but there are fewer larger quakes; no quakes exceeding M = 4 have been observed. The lander’s other instruments—two cameras, atmospheric pressure, temperature and wind sensors, a magnetometer and a radiometer—have yielded much more than the intended supporting data for seismometer noise characterization: magnetic field measurements indicate a local magnetic field that is ten-times stronger than orbital estimates and meteorological measurements reveal a more dynamic atmosphere than expected, hosting baroclinic and gravity waves and convective vortices. With the mission due to last for an entire Martian year or longer, these results will be built on by further measurements by the InSight lander., With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737)

Main text The UK has a long history of meteorite falls (where the meteorite fireball is witnessed, and the stone recovered, dating back to 1623 (MetBull, 2020). But the last meteorite fall in the UK was nearly 30 years ago when the Glatton stone, an L6 ordinary chondrite was recovered in 1991 (Hutchinson et al., 1991). Meteorite falls are important samples as they are usually recovered within days of the fireball event. As such, they have not experienced the deleterious effects terrestrial weathering that can change their extraterrestrial mineralogy, chemistry and isotopic composition (Bland et al., 2006). In exceptional circumstances rapidly recovered falls may have avoided rainfall so that soluble extraterrestrial minerals such as salts may be preserved (Chan et al., 2018). Therefore, meteorite falls represent much more pristine extraterrestrial material than their find counterparts within the same group, and consequently, characterisation of their texture and chemical signatures provides a clearer window into solar system processes. However, even falls are limited in their interpretive power as the geological context of the stone (i.e. where in the solar system it originated from) is unknown. To derive this contextual information requires the imaging of the fireball event from multiple geographical positions (Devillepoix et al., 2020). This data provides two vital pieces of information; the initial orbit of the meteoroid can be calculated and the final fall position can be predicted with increased accuracy (Devillepoix et al., 2020). As such, dedicated fireball camera networks (Bowden, 2006) are entering ‘a golden age’ with improvements in hardware and software capabilities as well as a reduction in production costs (Spurný et al., 2014). Continent-scale observatories have been established (Howie et al., 2017) and global networks are under construction (Devillepoix et al., 2020). In addition, these same developments have enabled the amateur astronomy community to construct their own networks either as groups or individuals with functional data pipelines and observations that can rival funded networks. Consequently, the number of recovered meteorite falls with orbits globally has grown rapidly over recent years (Borovička et al., 2015). The UK is a hotbed for such activity but does not have a recent recovered meteorite fall…yet. There are currently two active academically funded networks in the UK: the UK Fireball Network (UKFN: part of the Global Fireball Observatory built on the hardware and software developed by the Desert Fireball Network in Australia), and the System for Capture of Asteroid and Meteorite Paths (SCAMP; the UK arm of the French-led Fireball Recovery and InterPlanetary Observation Network (FRIPON)) (Figure 1, 2). In addition, there are two major amateur networks the UK Meteor Observation Network (UKMON), and NEMETODE, as well as an emerging presence of the Raspberry-Pi based Global Meteor Network and countless individual citizen scientists also imaging the UK night skies (Figure 1, 2). However, environmental factors in the UK such as light pollution and regular low-lying cloud cover means that a single fireball may not be captured multiple times by one network, preventing them from calculating an orbit and accurate fall position. As such, these networks, together with UK planetary scientists and National museums, have formed a collaborative data-sharing initiative called the UK Fireball Alliance (UKFAll) in order to maximise the chances of capturing a meteorite-dropping fireball event that makes landfall on the UK. Since the initiation of UKFAll in late 2018 many joint fireball observations have been made between UK networks including a fireball on the 16th February 2020 that likely dropped a few 10s of grams of extraterrestrial material into the North Sea (Figure 3). However, the diversity of hardware, software and data processing pipelines for capturing fireball events vary between camera networks. This hinders the speed of detecting and triangulating a meteorite-dropping fireball event as each fireball requires a bespoke solution to translate the data into the other networks’ format. A consistent method for rapidly transferring and converting the diversity of outputs produced by each network into a standard format that can be read and utilised by each network is required and is critical to facilitate a rapid UK response to fireball events and associated recovery effort. Here we describe the first iteration of a new code that will enable rapid conversion of data outputs from both video and still image camera networks. The code provides an effective bridging solution, while the ultimate aim is to agree and implement a globally accepted standardised format for fireball observations that can be readily transferred and utilised between camera networks to facilitate meteorite fall recovery. We also describe the logistical issues encountered by UKFAll and the solutions being implemented, including: recruitment of citizen scientists as searchers; conduct and liability issues; and best practice for collection. References Bland, P.A., et al., (2006). Weathering of chondritic meteorites. Meteorites and the early solar system II, 1, 853-867. Borovička J., et al., (2015). Small near‐Earth asteroids as a source of meteorites. In Asteroids IV, edited by Michel P., DeMeo F.E., and Bottke W.F. Tucson, Arizona: University of Arizona Press. pp. 257–280. Bowden, A.J. 2006. “Meteorite provenance and the asteroid connection”. In The history of meteoritics and key meteorite collections; fireballs, falls and finds, Edited by: G.J.H., McCall, A.J., Bowden and R.J., Howarth. Vol. 256, 379–403. Geological Society, London, Special Publications. Chan, Q.H., et al., (2018). Organic matter in extraterrestrial water-bearing salt crystals. Science advances, 4(1), eaao3521. Devillepoix, H.A., et al., (2018). The dingle dell meteorite: a halloween treat from the main belt. Meteoritics & Planetary Science, 53(10), 2212-2227. Devillepoix, H.A.R., et al., (2020). A Global Fireball Observatory. arXiv preprint arXiv:2004.01069. Howie, R.M., et al., (2017). How to build a continental scale fireball camera network. Experimental Astronomy, 43(3), 237-266. Hutchison, R., et al., (1991). The L6 chondrite fall at Glatton, England, 1991 May 5. Meteoritics, 26, 349. MetBull (2020), The meteoritical bulletin database. URL: https://www.lpi.usra.edu/meteor/metbull.php Spurný P., et al., (2014). Reanalysis of the Benešov bolide and recovery of polymict breccia meteorites—Old mystery solved after 20 years. Astronomy & Astrophysics 570:A39.

Multiring basins dominate the crustal structure, tectonics, and stratigraphy of the Moon. Understanding how these basins form is crucial for understanding the evolution of ancient planetary crusts. To understand how preimpact thermal structure and crustal thickness affect the formation of multiring basins, we simulate the formation of lunar basins and their rings under a range of target and impactor conditions. We find that ring locations, spacing, and offsets are sensitive to lunar thermal gradient (strength of the lithosphere), temperature of the deep lunar mantle (strength of the asthenosphere), and preimpact crustal thickness. We also explore the effect of impactor size on the formation of basin rings and reproduce the observed transition from peak‐ring basins to multiring basins and reproduced many observed aspects of ring spacing and location. Our results are in broad agreement with the ring tectonic theory for the formation of basin rings and also suggest that ring tectonic theory applies to the rim scarp of smaller peak‐ring basins.

Submarine landslides can exhibit complex rheologies, including a finite yield stress and shear thinning, yet are often simulated numerically using a Newtonian fluid rheology and simplistic boundary conditions. Here we present improvements made to a Smoothed Particle Hydrodynamics simulator to allow the accurate simulation of submarine landslide generated waves. The improvements include the addition of Bingham and Herschel‐Bulkley rheologies, which better simulate the behavior of submarine mudflows. The interaction between the base of the slide and the slope is represented more accurately through the use of a viscous stress boundary condition. This condition treats the interface between the seafloor and the slide as a fluid boundary layer with a user‐defined viscosity and length scale. Modifications to the pressure and density calculations are described that improve their stability for landslide generated wave scenarios. An option for pressure decomposition is introduced to prevent particle locking under high pressure. This facilitates the application of this simulator to landslide scenarios beneath significant water depths. Additional modifications to the reaveraging and renormalization routines improve the stability of the free surface and fluid density. We present the mathematical formulations of these improvements alongside commentary on their performance and applicability to landslide generated wave modeling. The modifications are verified against analytical fluid flow solutions and a wave generation experiment.

Primitive solar system solids were accreted as highly porous bimodal mixtures of mm-sized chondrules and sub-μm matrix grains. To understand the compaction and lithification of these materials by shock, it is necessary to investigate the process at the mesoscale; i.e., the scale of individual chondrules. Here we document simulations of hypervelocity compaction of primitive materials using the iSALE shock physics model. We compare the numerical methods employed here with shock compaction experiments involving bimodal mixtures of glass beads and silica powder and find good agreement in bulk material response between the experiments and models. The heterogeneous response to shock of bimodal porous mixtures with a composition more appropriate for primitive solids was subsequently investigated: strong temperature dichotomies between the chondrules and matrix were observed (non-porous chondrules remained largely cold, while the porous matrix saw temperature increases of 100’s K). Matrix compaction was heterogeneous, and post-shock porosity was found to be lower on the lee-side of chondrules. The strain in the matrix was shown to be higher near the chondrule rims, in agreement with observations from meteorites. Chondrule flattening in the direction of the shock increases with increasing impact velocity, with flattened chondrules oriented with their semi-minor axis parallel to the shock direction.

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, β − 1 , 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 β − 1 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.

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.