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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

Abundances of the highly siderophile elements (HSEs) in silicate portions of Earth and the Moon provide constraints on the impact flux to both bodies, but only since ~100 Myr after the beginning of the Solar System (hereafter tCAI). The earlier impact flux to the inner Solar System remains poorly constrained. The former dwarf planet Vesta offers the possibility to probe this early history, because it underwent rapid core formation ~1 Myr after tCAI and its silicate portions possess elevated chondritic HSE abundances. Here we quantify the material accreted into Vesta’s mantle and crust and find that the HSE abundances can only be explained by the bombardments of planetesimals from the terrestrial planet region. The Vestan mantle accreted HSEs within the first 60 Myr; its crust accreted HSEs throughout the Solar System history, with asteroid impacts dominating only since ~4.1 billion years ago. Our results indicate that all major bodies in the inner Solar System accreted planetesimals predominantly from the terrestrial planet region. The asteroid belt was either never significantly more massive than today, or it rapidly lost most of its mass early in the Solar System history. The impact flux in the inner Solar System just after its formation is studied by looking at the highly siderophile element abundance of Vesta. Results show that leftover planetesimals from the terrestrial planet region have been the major impactor source, indicative of a skewed mass distribution in the primordial inner Solar System.

Statler et al., NASA’s Double Asteroid Redirection Test (DART) is the first full-scale test of an asteroid deflection technology. Results from the hypervelocity kinetic impact and Earth-based observations, coupled with LICIACube and the later Hera mission, will result in measurement of the momentum transfer efficiency accurate to ∼10% and characterization of the Didymos binary system. But DART is a single experiment; how could these results be used in a future planetary defense necessity involving a different asteroid? We examine what aspects of Dimorphos’s response to kinetic impact will be constrained by DART results; how these constraints will help refine knowledge of the physical properties of asteroidal materials and predictive power of impact simulations; what information about a potential Earth impactor could be acquired before a deflection effort; and how design of a deflection mission should be informed by this understanding. We generalize the momentum enhancement factor β, showing that a particular direction-specific β will be directly determined by the DART results, and that a related direction-specific β is a figure of merit for a kinetic impact mission. The DART β determination constrains the ejecta momentum vector, which, with hydrodynamic simulations, constrains the physical properties of Dimorphos’s near-surface. In a hypothetical planetary defense exigency, extrapolating these constraints to a newly discovered asteroid will require Earth-based observations and benefit from in situ reconnaissance. We show representative predictions for momentum transfer based on different levels of reconnaissance and discuss strategic targeting to optimize the deflection and reduce the risk of a counterproductive deflection in the wrong direction., This work was supported in part by the DART mission, NASA contract No. 80MSFC20D0004 to JHU/APL. Some of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). S.C. acknowledges funding through the Crosby Fellowship of the Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology. E.D., S.I., A.L., M.P., A.R., and P.T. are grateful to the Italian Space Agency (ASI) for financial support through agreement No. 2019-31-HH.0 in the context of ASI’s LICIACube mission, and agreement No. 2022-8-HH.0 for ESA’s Hera mission. R.L. appreciates the funding from the European Union’s Horizon 2020 research and innovation program, grant agreement No. 870377 (project NEO-MAPP). P.M. acknowledges funding support from the French space agency CNES, ESA and the European Union’s Horizon 2020 research and innovation program under grant agreement No. 870377 (project NEO-MAPP). R.M. acknowledges support from a NASA Space Technology Graduate Research Opportunities (NSTGRO) Award (80NSSC22K1173). R.N. acknowledges support from NASA/FINESST (NNH20ZDA001N). J.O. and I.H. were supported by the Spanish State Research Agency (AEI) project No. MDM-2017-0737 Unidad de Excelencia “María de Maeztu”—Centro de Astrobiología (CSIC-INTA). They are also grateful for all logistical support provided by Instituto Nacional de Técnica Aeroespacial (INTA). S.R.S. acknowledges support from the DART Participating Scientist Program, grant No. 80NSSC22K0318. J.M.T.R. was funded by FEDER/Ministerio de Ciencia e Innovación—Agencia Estatal de Investigación of Spain (grant No. PGC2018-097374-B-I00).

Introduction Mars is a cold dry planet, yet there is ample evidence for fluvial activity on its surface, which paints a different picture for its past climate [1]. If Mars has ever been covered by an ancient ocean or ubiquitous paleolakes, the signatures of the presence of water would have been preserved in the ejecta blankets of some impact sites because the thermodynamics of the excavation flow depends on the target material properties such as its composition and mechanical properties [2]. The main objective of this work is to test whether the difference between impact sites of water-covered, ice-rich, and dry target subsurfaces on Mars is significant enough to be observable. Method To simulate hypervelocity impact processes in solid materials, we use the iSALE-2D shock physics code [3,4,5]. Our simulations make use of its following models: the elasto-plastic constitutive model for multiple materials, the fragmentation and modified strength model, the porosity compaction model, and the acoustic fluidisation block model. Target materials are described by the ANEOS equations of state for basalt, water ice, and water. We consider different impact scenarios with varied target composition: i) 200-400m of liquid water covering basalt, ii) 100-200m basalt surface covering 200-400m of subsurface water ice, iii) dry basaltic surface with no ice or water. The impact parameters are fixed across these scenarios: impacts are vertical and triggered by a single impactor with the impact velocity of 10km/s and the radius of 100m. The projectile is made of intact basalt described by the Lundborg strength model, while the the strength of the target basalt is described by the ROCK model, in which the yield strength is a function of both pressure and damage, with the latter computed by the iSALE damage model. The full trajectories and final distribution of the impact ejecta have to be quantified in post-processing due to the fact that high above the crater, ejecta becomes underresolved in the Eulerian grid. We thus analyse the characteristics (i.e. velocity, launch time, launch position) of tracer particles when they reach an altitude of one projectile radius above the surface [2]. We then calculate their final landing positions, assuming ballistic trajectories, and analyse the ejecta mass distributions. Results Figure 1. Pressure and density maps of different impact scenarios shown at the resolution of 10CPPR (cells per projectile radius). The naming of the scenarios is as follows: ThD1: dry case of pure basalt, ThI200200: 200m of basalt covering 200m of water ice over basalt; ThW200: 200m of water over basalt. In Figure 1 we present different impact scenarios at 3 distinct periods of the excavation stage: 2s, 6s and 20s after the impact. The presence of water or water ice modifies the volume of the cavity during the entire excavation phase. The shapes and angles of the ejecta curtains are also different for varied target scenarios. Figure 2. The evolution of crater volumes and radii over time. In Figure 2 we show the time evolution of the crater volumes and radii quantitatively. The most striking difference is between the volume of the excavated material in dry vs. ice-rich or water-rich targets, which translates into the difference in volumes of the ejecta blanket. Crater radii are also a discriminator between those cases, with water-covered and ice-rich targets being deeper than the “dry” one. However, over time their depth would also be decreased by the sublimation of subsurface ice and evaporation of the surface water. Figure 3. Ejecta properties for the cases with varied ice depth. a) Mass distribution of ejected material. b) Dependency of the median ejection angle of the target material on where it landed. c) Evolution of the crater volume over time. d) Dependency of the ejection velocity of target material on where it landed. Rcrater were computed at t=20s. In Figure 3 we compare the ejection properties for targets with 200m ice at different depths: 0m (ThI0200); 100m (ThI100200) and 200m (ThI200200) depths. The final mass distributions and ejection angles are substantially different between these cases, and those differences would be reflected in the morphologies of the ejecta blankets. The final crater volumes (c) alone are insufficient to discriminate between these cases. Conclusion We conducted numerical experiments of impact cratering into varied targets: ice-rich, water-rich and dry (pure basalt). Our preliminary results show that for the constant projectile and velocity, one can discriminate between those cases based on the crater size and ejecta volume. We also show that the depth of buried ice would leave a characteristic signature in the blanket. Future work We plan to address the effects of the atmosphere and vapour plumes on the trajectories of ejecta. The interaction of ejected material with an atmosphere, accounting for drag forces and momentum exchange, was recently implemented in iSALE [6]. References [1] Zachary I Dickeson, Joel M Davis, Martian oceans, Astronomy & Geophysics, Volume 61, Issue 3, June 2020, Pages 3.11–3.17, https://doi.org/10.1093/astrogeo/ataa038 [2] Luther, R., Zhu, M.-H., Collins, G. and Wünnemann, K. (2018), Effect of target properties and impact velocity on ejection dynamics and ejecta deposition. Meteorit Planet Sci, 53: 1705-1732. https://doi.org/10.1111/maps.13143 [3] Amsden, A., Ruppel, H., and Hirt, C. (1980). SALE: A simplified ALE computer program for fluid flow at all speeds. Los Alamos National Laboratories Report, LA-8095:101p. Los Alamos, New Mexico: LANL. [4] Collins, G. S., Melosh, H. J., and Ivanov, B. A. (2004). Modeling damage and deformation in impact simulations. Meteoritics and Planetary Science, 39:217--231. [5] Wünnemann, K., Collins, G., and Melosh, H. (2006). A strain-based porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets. Icarus, 180:514--527. [6] Robert Luther, Natalia Artemieva, Kai Wünnemann, The effect of atmospheric interaction on impact ejecta dynamics and deposition, Icarus, Volume 333, 2019, Pages 71-86, https://doi.org/10.1016/j.icarus.2019.05.007.

1. Introduction Seismic waves can be applied to study the interior of a body, as was done during the Apollo missions on the Moon [e.g. 1] or during the InSight mission on Mars [e.g. 2]. Recent numerical simulations using the iSALE shock physics code have been applied to study seismic signals in the frame of both missions, and for signals in samples from laboratory experiments [3-6]. In the NEO-MAPP project (Near Earth Object Modelling and Payloads for Protection) by the EC, Horizon 2020, a potential lander with a seismometer is discussed to study the characteristics of the interior of an asteroid (i.e. layering, mechanical properties, size of heterogeneities in the subsurface). Although possible natural sources for seismic waves on an asteroids exist (e.g. tidal forces, meteoroid impacts), we focus on studying an artificial impact of a spacecraft, which is better constrained and plannable. 2. Method We use the iSALE shock physics code [7-9] to conduct a systematic parameter study by simulating different impact scenarios. iSALE enables to describe the resistance of rocks against deformation and the thermodynamic response to compression by different material models. We use a Lundborg rheology with a coefficient of friction of 0.77 and a cohesion of 1.4 kPa. We vary target porosities between 10 - 50%. We use the ANEOS for basalt. These material properties reflect the strength of regolith simulant, which was used for laboratory impact experiments [e.g. 10]. iSALE was validated against these experiments recently [11]. Our study on impact-induced seismicity on asteroids focuses on DART-scale kinetic impactor scenarios and we analyse the seismic signal generated by a 600 kg impactor at 6 km/s. 3. Results We observe a decay of the peak pressure (i.e. P-wave in the elastic regime) with distance for targets with increasing porosity from 10 – 50% (Figure 1). Due to the energy consumption by pore compaction, the decay of pressure occurs closer to the impact point for a more porous target. We quantify the attenuation of the signal with distance r as shown by [3] with the help of the quality factor Q, which describes the decay of the amplitude A of the seismic wave as: A(r) = A0 e -b r , where b is a material parameter, which relates the quality factor to the length λ of the pressure pulse as: b = π / ( Q λ ). Figure 1: Peak pressure decay with distance for targets with a cohesion of 1.4 kPa. We observe a slight increase of the quality factor from 1.8 – 3.8 with increasing porosity (Figure 2). Figure 2: Quality factors for granular homogeneous targets, and for competent rocks [3,12]. 4. Discussion Our results are in line with expectations that seismic waves on asteroidal targets, like the granular regolith materials that we simulated in this study, attenuate faster than more consolidated materials. Comparing our results with simulations assuming competent rock as target [3,12] shows that our determined quality factors are smaller by an order of magnitude than the numerically and experimentally derived values for quartzite, sandstone and tuff, which vary in porosity from 0 – 42%. The dampening of the seismic signal in a granular target is more efficient. However, this also implies that the seismic behaviour on monolithic asteroids is different to the one on rubble pile asteroids. 5. Outlook We aim at analysing further materials. More specifically, we plan to expand our study to heterogeneous targets, where we resolve boulders explicitly. We expect that the presence of boulders causes more complex wave patterns in the interior, depending on the presence or absence of large scale voids and boulders. Acknowledgements We gratefully acknowledge the developers of iSALE (www.isale-code.de). We acknowledge the funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 870377 (NEO-MAPP). References [1] M. Knapmeyer, Planetary Seismology, Landolt-Börnstein 4A, 2009. [2] P. Lognonné et al., Space Sci. Rev. 215, 2019. [3] N. Güldemeister and K. Wünnemann, Icarus 296 , 15-27, 2017. [4] Wójcicka et al., JGR Planets 10.1029/ 2020JE006540, 2020. [5] Rajšić et al., JGR Planets 10.1029/2020JE006662, 2021. [6] Rajšić et al., E&SP 10.1029/2021EA001887, 2021. [7] A. Amsden, H. M. Ruppel and C. W. Hirt, LANL, LA-8095, 101, 1980. [8] G. S. Collins, H. J. Melosh and B. A. Ivanov, M&PS 39, 217-231, 2004. [9] K. Wünnemann, G. S. Collins and H. J. Melosh, Icarus 180, 514-527, 2006. [10] S. Chourey et al., PSS, 194, 2020. [11] R. Luther et al., PSJ, 2022, under review. [12] F. Hoerth et al., JGR: Planets, 119, 2177-2187, 2014.

1. Introduction Impact melt rocks are the only lunar sample material that allows for absolute dating of impact events. Therefore, these samples are key to unveiling the early bombardment history of the Moon. However, due to the mixing of melt products ejected from distant craters, the interpretations of lunar samples are difficult [1]. Here, we use the iSALE2D shock physics code to quantify the production of impact-induced melt and especially its distribution in ejecta blankets for lunar craters of intermediate size (1.5 – 50 km in diameter). 2. Methods We carried out a suite of iSALE simulations with varying projectiles diameter from 100 m to 3 km. We use an analytic equation of state for basalt to describe the thermo-dynamical behavior of the lunar crust (cf. [2]). Material strength, damage, and porosity were accounted for using the models of [3], [4], and [5, 6], respectively. Due to the long-term impact fragmentations, the lunar near-surface lithologies are assumed to be porous. The presence of porosity enhances melt production [7]. We set the porosity profile following the exponential function of [8] that best matches gravity data. To investigate melt production sufficiently high resolution in the models is required. At the same time simulations of the entire ejection process are only possible at coarser resolution. To reach a good compromise for the computational costs in terms of spatial resolution, computation time, and accuracy to quantify melt production and deposition upon ejection, we apply a 3-step approach for simulations: Models to determine impact-induced melts. To quantify melt production, we first determine the critical shock pressure Pc where the post-shock final temperature exceeds the melt temperature as a function of porosity. Impact-induced melts are generated if the material experiences peak shock pressures (Ppeak) in excess of Pc. We use Lagrangian tracers to record Ppeak during the passage of the shock wave. The average impact velocity of 18 km/s on the Moon is kept constant. We use a high resolution of 40 cells per projectile radius (CPPR) to model the early stage of impact processes until the shock wave has attenuated below Ppeak. Models to analyze ejecta deposit. Lagrangian tracer particles are also used to track ejected materials. Tracers are considered to be ejected when they reach the altitude of one projectile radius above the surface. We extrapolate the trajectory back to the surface and record the launch position. The cumulative number of tracers deposited at a given distance allows for determining the thickness of the ejecta blanket. In these models, the normal component of impact speed for the most likely impact on the Moon (45°) is considered (i.e., 13 km/s). Given that 10 CPPR is often sufficient to simulate crater growth [7], 10 CPPR is taken for these models, and they run until the crater reaches the maximum volume, which is assumed to be a good approximation for the transient craters when the ejection process has ceased. Combining data of impact-induced melt and ejection. Typically, each cell in iSALE models is assigned a single tracer. For better analysis, we make the positions of tracers in 10 CPPR-models exactly the same as those in the 40 CPPR-models by putting 16 uniformly distributed tracers in each cell. To estimate melt distribution in ejecta, Ppeak of the ejected tracers derived from 10-CPPR models is revised by replacing it with that from 40-CPPR models. 3. Production of impact-induced melt The volume of impact melt (Vm) increases exponentially with the increasing diameter of transient craters (Dt): Vm = cDtd. Our results (Figure 1) present a more gentle slope (d = 3.09; c = 2.37×10-3) than the analytical value (d = 3.8, [9]) when Dt is directly taken from iSALE models. This is partially caused by the decreasing influence of the porosity gradient with increasing projectile size and is also related to the determination of Dt. After the model-derived Dt is replaced with the scaled Dt. The results present an exponent d = 3.55 (c = 4.41×10-3). 4. Ejection of impact-induced melt A power law expression for the ejecta thickness (T) as a function of landing distance (r) was formulated: T(r)=T0(r/Rt)-B, where Rt is the transient crater radius as derived with iSALE. As shown in Figure 2, T decreases with distance according to a power-law with an exponent of ~ -3.0, which is consistent with the laboratory experiments and the observation of lunar ejecta blankets. We calculate the thickness of a hypothetical “melt blanket” (Tm), which only consists of impact-induced melt material. We use the same way as we calculate the total thickness of the ejecta blanket, but consider only the materials that are molten. The results (Figure 2) show that Tm is also a function of landing distance following a power law: Tm(r)=Tm0(r⁄Rt)-Bm. Tm is decreasing with the distance from the crater center, but the slope of the “melt blanket” is more gentle than that of the total blanket and has an exponent of around -2.3. In addition, even though the thickness of both the ejecta and the melt blanket decreases with distance from the crater center, the concentration of impact melt in the ejecta blanket increases. 5. Conclusions We found the production of impact-induced melt is a function of transient crater size following a power law. But due to the decreasing porosity gradient with depth, the melt production for small craters is more significantly enhanced compared with that of larger craters leading to a gentler slope. In addition, the melt concentration within the blanket is almost linearly increasing. It indicates that impact melt is highly concentrated not only inside the crater but also in the most distal ejecta. References: [1] Kring and Cohen (2002) JGR. [2] Pierazzo+ (2005), Large Meteorite Impacts III. [3] Collins+ (2004) MPS. [4] Ivanov+ (1997) International Journal of Impact Engineering. [5] Wünnemann+ (2006) Icarus. [6] Collins+ (2011) International Journal of Impact Engineering. [7] Wünnemann+ (2008) EPSL. [8] Besserer+ (2014) GRL. [9] Cintala and Grieve (1998) MPS.

Introduction: During the late accretion phase, large differentiated bodies impacted the Earth. The material from these impactors, especially from their cores, could have altered the composition of Earth’s mantle and might offer an explanation [1] for the measured high concentrations of highly siderophile elements (HSE) in Earth’s contemporary mantle [2]. Since the planet was most likely covered by a deep magma ocean during this period due to giant collisions like the moon forming impact [3], a better understanding of impacts into these is essential. Especially important is the fragmentation of the impactor core, because a more significant fragmentation would allow better mixing with the surrounding mantle material. Different experimental and numerical approaches have been used to analyze this in the past [4,5]. Following up on this work we improved the fragmentation process in the Eulerian shock physics code iSALE to analyze the effect of different model parameters like impactor velocity, radius and magma ocean depth. Methods: We use the grid-based Eulerian shock physics code iSALE-2D [6,7] to simulate impacts of differentiated bodies into a magma ocean. iSALE is optimized accurately reproduce large scale features of the impact, such as the resulting crater morphology. Behavior on smaller scales, however, like the detailed fragmentation of impactor material, is not sufficiently resolved. To change this, we developed and implemented a new method that allows for a more accurate simulation of the fragmentation process even on scales close to the grid cell size. The idea of the method is to determine when the size of a fragment as a whole or in part approaches the resolution limit and in such a case use additional petrophysical criteria to determine if fragmentation occurs. In case the criterion is fulfilled, the material is removed from the grid and replaced by a numerical particle representing the fragment in the course of the subsequent material flow. Our simulation setup consists of a differentiated impactor hitting a magma ocean. The radius of the impactor is on the order of hundreds of kilometers and was varied in different simulations. The velocity ranges between 5 and 20 km/s and different possible depths of the magma ocean relative to the impactor size are explored as well. Results: In figure 1 we compare simulation results at different time steps with and without the new method. It is visible that the impactor core fragments significantly in both cases for the shown setup. The relative position of these fragments are comparable as well. Figure 1: Simulations with (Sim 1) and without (Sim 2) the new method at different time steps for a radius of 100 km at a velocity of 11.5 km/s. The impactor core is shown in yellow and the particles used in the new method in red. The resulting fragment size frequency distribution however is very different, as seen in figure 2. The new method shows significantly finer fragmentation, which is in line with the tendency of regular iSALE to artificially clump material together near the resolution limit resulting in artificially too big fragments. Figure 2: Cumulative size frequency distribution of the number of impactor core fragments for the simulations shown in figure 1. The preliminary results generally indicate significant fragmentation of the impactor core, allowing for mixing of the core material with the surrounding mantle. They also show that a higher impact velocity corresponds to stronger disruption, resulting in smaller average fragment sizes. Increasing the impactor radius leads to larger fragment sizes. The effect of the magma ocean depth on the fragmentation is currently being studied. Acknowledgments: We gratefully acknowledge the developers of iSALE-2D, including Gareth Collins, Kai Wünnemann, Dirk Elbeshausen, Tom Davison, Boris Ivanov and Jay Melosh. This work was funded by the Deutsche Forschungsgemeinschaft (SFB-TRR170, subproject C2 and C4). References: [1] Wood B. J. et al. (2006) Nature 441:825-833. [2] Walker R. J. (2009) Geochemistry 69:101-125. [3] Tonks W. B. et al. (1993) Journal of Geophysical Research: Planets 98:5319-5333. [4] Daguen R. et al. (2014) Earth and Planetary Science Letters 391:274-287. [5] Kendall J. D. et al. (2016) Earth and Planetary Science Letters 448:24-33. [6] Collins. G. S. et al. (2004) Meteoritics & Planetary Science 39:217-231. [7] Wünnemann K. et al. (2006) Icarus 180:514-527.

Shock metamorphism in ordinary chondrites allows for reconstructing impact events between asteroids in the main asteroid belt. Shock-darkening of ordinary chondrites occurs at the onset of complete shock melting of the rock (>70 GPa) or injection of sulfide and metal melt into the cracks within solid silicates (\sim 50 GPa). Darkening of ordinary chondrites masks diagnostic silicate features observed in the reflectance spectrum of S-complex asteroids so they appear similar to C/X-complex asteroids. In this work, we investigate the shock pressure and associated metamorphism pattern in rubble-pile asteroids at impact velocities of 4-10 km/s. We use the iSALE shock physics code and implement two-dimensional models with simplified properties in order to quantify the influence of the following parameters on shock-darkening efficiency: impact velocity, porosity within the asteroid, impactor size, and ejection efficiency. We observe that, in rubble-pile asteroids, the velocity and size of the impactor are the constraining parameters in recording high-grade shock metamorphism. Yet, the recorded fraction of higher shock stages remains low (, 12 pages, 9 figures

Introduction: Asteroids smaller than about 50 km in diameter are the result of the break-up of a larger parent body [1]. They are often considered to be rubble-pile objects, aggregates held together only by self-gravity or small cohesive forces [2, 3], and have highly heterogeneous surfaces. Recently, the artificial impact experiment (SCI) of JAXA’s Hayabusa2 mission on the surface of asteroid Ryugu [4] created a relatively large crater (~14 m diameter) despite the presence of large boulders close to the impact location [5]. Post-impact images of the SCI impact site revealed that the boulders had different motion mechanisms depending on their size and initial position relative to the impact point. 1 m-sized boulders were ejected several metres outside of the crater, a 5 m boulder was moved about 3m, while a large, possibly deeply rooted boulder (“Okamoto”) was not moved [4]. Impact cratering on weak, heterogeneous targets is still poorly studied, both by means of laboratory experiments and numerical simulations. For example, it is not yet known how the boulders affect the crater size or how the boulders motion is affected by their mass, size, shape or initial location. This is also important in context of NASA’s Double Asteroid Redirection Test (DART) impact on the surface of Dimorphos (the secondary of the 65803 Didymos asteroid system) on the 26th of September 2022. The impact will demonstrate the controlled deflection capabilities of near-Earth asteroids by a kinetic impactor [6]. ESA’s Hera mission [7] will arrive at Dimorphos several years after the DART impact and provide a detailed characterisation of the impact outcome. Recent impact experiments and numerical studies [8, 9, 10, 11] have shown that the kinetic impact efficiency depends strongly on the target properties and structure. The understanding thereof is imperative for a successful interpretation of the DART impact outcome. Most previous impact experiments and subsequent validation work of numerical models have focused on homogeneous targets [e.g, 12, 13, 14]. However, it is unlikely that Dimorphos is homogeneous at the scale of DART impact. In our previous work we studied impacts into granular targets with embedded porous, spherical boulders and their effect on cratering process, final crater morphology, boulder emplacement, and ejecta distribution [15, 16]. We showed that crater diameters are only slightly affected by heterogeneities as long as the energy required for crushing is small compared to impact energy. However, less mass is ejected at higher velocities compared to homogeneous targets, which reduces the momentum enhancement. In this study we present results from recently conducted experiments at the Experimental Projectile Impact Chamber (EPIC) at Centro de Astrobiología CSIC-INTA, Spain, which focused on the motion mechanism of large boulders placed close to the impact site (Fig. 1). We also study the effect of density and surface roughness of boulders located on the target surface on their mobilisation and ejection, as well as on the excavation flow and ejection of fine-grained matrix. Methods: The EPIC utilises a 20 mm calibre compressed N2 (300 bar) cannon that launches projectiles at velocities up to ≈420 m/s [13]. The experiments, half- or quarter-space, can be recorded with two high-speed cameras. In the three experiments presented here we have kept the impact velocity constant at approximately 400m/s. The projectiles were 20mm massive, spherical delrin balls. All shots were vertical and in half space. We performed three shots into a loosely packed beach sand in which a single, relatively large rectangular boulder was placed (Fig. 1). Coloured sand on the surface allows studies of ejecta distribution. In the first shot (Scenario 1) the boulder had relatively high bulk density (quartzite, ~ 2.8 g/cm3), in the second shot (Scenario 2) the boulder had relatively low density (highly porous pumice, ~ 0.27 g/cm3) and a glossy surface (the boulder was covered in a plastic film). In the last shot (Scenario 3) the same object was used, but now the glossy plastic film had been removed revealing a rough, high-friction surface (Fig. 1A). In all experiments the point of impact was located as shown in Figure 1B. Results and discussion: We observe a significant difference in the ejection behaviour between the cases with boulders of different mass. In Scenario 1, the boulder was moved a few cm by the excavation flow and deposited at the crater rim. The ejecta curtain passes around the boulder, creating a “forbidden zone” behind the boulder (Fig. 2A). Similar ejecta behaviour was seen during the SCI impact around the Okamoto boulder [4]. In Scenarios 2 and 3, the boulder was excavated with the ejecta curtain, rotated around its length axis, and landed about one crater diameter away from the crater. The ejecta curtain was again disrupted by the presence of the boulder (Fig. 2 B, C). The final rim-diameter of the crater produced in Scenario 1 was 27cm and in the other two scenarios 29 cm, suggesting that the presence of a dense (i.e., mainly stationary) boulder close to the impact point does not greatly affect the crater size. However, we can clearly see a significant influence of the boulder on the ejecta curtain behaviour. The dense boulder efficiently block the ejecta while the low-density boulders to great extent travel with the ejecta mainly affecting the distal parts of the continuous ejecta layer. The surface roughness had no obvious effect on boulder movement. Conclusions and future work: Here we continue our study of impacts into heterogeneous targets (The first study was presented at EPSC2021 [15]) and work towards understanding the importance of target heterogeneities in the cratering process and impact momentum transfer. Numerical simulations reproducing these impact experiments are underway and preliminary results from the early cratering process show good agreement with the experiments. These results and further numerical studies are vital for understanding cratering processes on rubble-pile asteroids and will help interpret results from the DART impact and its imaged ejecta plume.

Funding Information: To achieve these objectives, Milani is carrying two scientific payloads, the ASPECT visual and near-infrared (Vis-NIR) imaging spectrometer and the VISTA thermogravimeter aimed at collecting and characterizing volatiles and dust particles below 10 μm. Additionally, navigation payloads include a visible navigation camera and lidar. The Milani consortium is composed of entities and institutions from Italy, the Czech Republic, and Finland. The consortium Prime is Tyvak International, responsible for the whole program management and platform design, development, integration, testing, and final delivery to the customer. Politecnico di Torino is tasked with defining requirements and performing thermal, radiation, and debris analysis. Politecnico di Milano is responsible for mission analysis and GNC. Altec will support the Ground Segment architecture and interface definition. Centro Italiano per la Ricerca Aerospaziale (CIRA) is responsible for the execution of the vehicle environmental campaign. HULD contributes to developing the mission-specific software. VTT is the main payload (ASPECT hyperspectral imager) provider and is supported by the following entities dealing with ASPECT-related development: University of Helsinki (ASPECT calibration); Reaktor Space Lab (ASPECT Data Processing Unit development), Institute of Geology of the Czech Academy of Sciences (ASPECT scientific algorithms requirements and testing); and Brno University of Technology (ASPECT scientific algorithms development). INAF-IAPS is the secondary Payload (VISTA, dust detector) provider. Funding Information: The Mission PI is appointed by ESA and is the primary interface to ESA. The Hera SMB consists of the ESA Hera Project Scientist (ESA PS), the Mission PI, and the Hera Advisory Board, consisting of four mission advisors. The Mission PI chairs the HIT and is supported by the Hera Advisory Board. The tasks of the Hera SMB are 1. advising the Hera mission project team on all aspects related to the Hera mission objectives; 2. ensuring that the WGs’ activities cover the needs of the Hera mission; 3. providing recommendations to ESA concerning the membership in the HIT; and 4. implementing the Publication Policy. Funding Information: Hera is the ESA contribution to the AIDA collaboration. Hera, Juventas, Milani, and their instruments are developed under ESA contract supported by national agencies. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 870377 (project NEO-MAPP), the CNRS through the MITI interdisciplinary programs, ASI, CNES, JAXA, the Academy of Finland project no. 335595, and was conducted with institutional support RVO 67985831 of the Institute of Geology of the Czech Academy of Sciences. M.L., E.P., P.T .and E.D. are grateful to the Italian Space Agency (ASI) for financial support through Agreement No. 2022-8-HH.0 in the context of ESA’s Hera mission. We are grateful to the whole Hera team, including Working Group core members and other contributors for their continuous efforts and support. Their names can be found on the following website: https:// www.heramission.space/team. Publisher Copyright: © 2022. The Author(s). Published by the American Astronomical Society. Hera is a planetary defense mission under development in the Space Safety and Security Program of the European Space Agency for launch in 2024 October. It will rendezvous in late 2026 December with the binary asteroid (65803) Didymos and in particular its moon, Dimorphos, which will be impacted by NASA’s DART spacecraft on 2022 September 26 as the first asteroid deflection test. The main goals of Hera are the detailed characterization of the physical properties of Didymos and Dimorphos and of the crater made by the DART mission, as well as measurement of the momentum transfer efficiency resulting from DART’s impact. The data from the Hera spacecraft and its two CubeSats will also provide significant insights into asteroid science and the evolutionary history of our solar system. Hera will perform the first rendezvous with a binary asteroid and provide new measurements, such as radar sounding of an asteroid interior, which will allow models in planetary science to be tested. Hera will thus provide a crucial element in the global effort to avert future asteroid impacts at the same time as providing world-leading science.

Mars is a cold dry planet, yet there is ample evidence for fluvial activity on its past surface, including sediments suggestive of shallow lakes, which paints a different picture for the Martian past climate. Mars is also heavily cratered, and some of those craters may have resulted from impact cratering into water-covered targets. Distinguishing between water-overed and dry surface at the time of the impact is the topic of this project. We approach this problem from the theoretical point of view and use a shock physics code iSALE capable of simulating different materials with various strength and damage models. This hydrocode is widely used in impact physics and has been extensively tested against laboratory experiments. We realise several impact scenarios with varied rheology, as well as sizes of projectiles and impact angles, in particular water-covered (simulated paleolake), water ice-rich and dry targets. We discuss the theoretical effects of the presence of surface water on the morphology and dynamics of impact sites (both craters and ejecta). Distinguishing between these scenarios can aid the interpretation of remote sensing observations, and open a possibility of using a new independent observable to study the past climate of Mars.

Melting and vaporization of rocks in impact cratering is mostly attributed to be a consequence of shock compression. However, other mechanism such as plastic work and decompression by structural uplift also contribute to melt production. In this study we expand the commonly used method to determine shock-induced melting in numerical models from the peak shock pressure by a new approach to account for additional heating due plastic work and internal friction. We compare our new approach with the straight-forward method to simply quantify melting from the temperature relative to the solidus temperature at any arbitrary point in time in the course of crater formation. This much simpler method does account for plastic work but suffers from reduced accuracy due to numerical diffusion inherent to ongoing advection in impact crater formation models. We demonstrate that our new approach is more accurate than previous methods in particular for quantitative determination of impact melt distribution in final crater structures. In addition, we assess the contribution of plastic work to the overall melt volume and find, that melting is dominated by plastic work for impacts at velocities smaller than 7.5–12.5 km/s in rocks, depending on the material strength. At higher impact velocities shock compression is the dominating mechanism for melting. Here, the conventional peak shock pressure method provides similar results compared with our new model. Our method serves as a powerful tool to accurately determine impact-induced heating in particular at relatively low-velocity impacts.

The early impact bombardment extensively fractured the lunar crust resulting in the formation of the so-called megaregolith. Previous estimates of megaregolith distribution vary significantly with respect to the vertical extent and the size-frequency distribution of fragments was rarely studied. We built a spatially resolved numerical model to simulate the process of cumulative impact fragmentation, aiming to backtrack the megaregolith evolution history and to constrain its fragment distribution. The results highlight the pivotal role of basin-forming events on the megaregolith formation. Especially the South-Pole Aitken (SPA) impact established the initial megaregolith structure which remained distinct after 0.5 Ga subsequent fragmentation. At 3.8 Ga, the megaregolith displays substantial lateral variation and layering: the highly fractured upper layer of ∼2.5 km is dominated by meter-scale fragments; the disturbed lower layer deeper than tens of kilometers is mainly consisting of kilometer-scale fragments; the transition zone >5 km contains fragments of various size scales.

We constructed a system of parameterized or semi-analytical representations of impact-related processes such as crater formation, atmospheric erosion, and melt production due to the direct effects of the impact as well as the long-term thermal effects triggered in the mantle by very large impacts in order to model how impactors of different types and a large range of sizes affect the CO2-H2O atmosphere and the interior of a terrestrial planet similar to Mars, Venus, or the early Earth. Mass fluxes of carbon dioxide and water between the impactor/outer space, the atmosphere, and the interior of the target planet are calculated in order to assess under which conditions atmospheres and interiors are depleted or enriched by processes related to impacts, melting/magmatism on different time scales, or weathering. The impactor flux onto a planetary target is a stochastic process in terms of time, magnitude and location, which we describe with a size-frequency distribution of impactors and a cratering chronology. The temporal aspect, in combination with the complexities of the various effects of a single impact, cause the evolution to depend on its own history. In the absence of unique information about the impact sequence of a target, the evolution should therefore not be uniquely determined by cratering statistics, and the possible paths are expected to vary within a certain range. Thus, we aim to deduce a range of evolutionary paths for the volatile content of the atmosphere and, within limits, of the interior. We consider rocky S-type and icy-rocky C-type asteroids as well as comets, covering a range of impactor-target density contrasts from about 1/6 to about 4/5 as well as a range of (absolute) impact velocities from a little less than 10 to almost 65 km/s. Impactor size ranges from 1 m to half the planetary radius. Atmospheric (surface) densities cover almost five orders of magnitude, ranging from a few millibars (modern Mars) to 95 bar (modern Venus). With regard to atmospheric effects, there is a fundamental distinction to be made between blast-producing and crater-forming impacts; the boundary that separates these two regimes is mostly defined by the deceleration of the impactor and its resistance to breakup under the ram pressure during its traversal of the atmosphere. The direct effects of the former leave the interior essentially unaffected and interact only with the atmosphere. We use the formalism by Svetsov (2007) to assess the bulk mass transfer and balance resulting from mechanical erosion of the atmosphere and the disintegration of the impactor and estimate the balance for the individual volatiles from estimates of the impactor composition. In crater-forming impacts, there are additional effects that need to be included. Ejecta can contribute to the mechanical erosion of the atmosphere (e.g., Shuvalov et al., 2014) and also produce layers of porous material with a large, reactive surface that can absorb CO2 from the atmosphere by weathering in the long-term aftermath of an impact. Moreover, they produce a crater that opens up the interior to mass exchange with the atmosphere. A key process in this context is the production of impact melt, which can serve as a vehicle for volatiles between the atmosphere and the interior by either releasing or removing (by dissolution) CO2 and water, depending mostly on the pressure conditions at the interface; generally outgassing is expected to be more common, but still these two volatiles may behave quite differently. We find that CO2 is expelled from the melt much more easily than water and will therefore enter the atmosphere under all conditions considered, whereas water may be retained in the melt at high atmospheric pressures. In addition to these common effects, very large impacts cause perturbations at sublithospheric depths and implant local thermal anomalies into the mantle that subsequently turn into upwellings and can cause longer-term magmatism and the local degassing from the deep interior.

Planetary collisions play an important role in the compositional and thermal evolution of planetary systems and such collisions are caracteristics of the final stage of planetary formation. The Moon-forming impact event is thought to (re)set the conditions for the subsequent thermochemical evolution of Earth and Moon. Large parts of proto-Earth are thought to melt as a consequence of the impact [e.g.1] and the extent of melting affects the evolution of the Earth’s interior and atmosphere. It is then critical to address the initial conditions of the proto-Earth and the volume and shape of a possible magma ocean after the impact. Previously, the Moon-forming giant impact was modeled with mesh-free so-called smoothed particle hydrodynamics (SPH [1, 2, 3]). In this study, we, in contrast, carried out numerical simulations of the Moon-forming impact event considering different impact scenarios with the three-dimensional (3D) iSALE code [4, 5], that tends to be more accurate in the description of thermodynamics and shock waves than SPH simulations. We also compare simulation results from our iSALE code with SPH models for benchmarking ([1]) because SPH uses self-gravity, whereas iSALE uses central gravity. We vary the impact angle (15° to 90°) and impact velocities (12 to 20 km/s). In order to quantify the volume of impact-induced melt, we use the so-called peak-shock pressure approach (‘Tracer method’) that has been used in several modeling studies [6,7] and is described in more detail by [8]. The benchmark study shows a good agreement of the two different numerical approaches with respect to pressure evolution. However the production of a magma ocean show some differences that need to be further explored, with notably the effects of considering central gravity instead of self-gravity into iSALE 3D simulations. Acknowledgments: We gratefully thank the iSALE developers, including Gareth Collins, Kai Wünnemann, Dirk Elbeshausen, Boris Ivanov and Jay Melosh and Thomas Davison for the development of the pysaleplot tool. We also thank the Deutsche Forschungsgemeinschaft (SFB-TRR 170, subproject C2 and C4) for funding. References:[1] Nakajima M. and Stevenson D. J. (2015) EPSL, 427, 286-295. [2] Canup R. M. et al. (2013) ICARUS 222, 200-219. [3] Canup R, M. (2004) Science 338, 1052-1054. [4] Collins G. S. et al. (2004) Meteoritics & Planet. Sci., 39, 217-231. [5] Wünnemann K. (2006) ICARUS 180, 514-527. [6] Wünnemann K. et al. (2008) EPSL 269, 529-538. [7] Pierazzo et al. (1997) ICARUS 127, 408-423. [8] Manske L. et al. (2018) 49th LPSC, abstract# 2269.[11] Pierazzo and Melosh (1999) EPSL 165, 163-176

Introduction: The origin of the relatively high concentration of highly siderophile elements in Earth’s mantle [1] is still debated. One possible explanation is the addition of iron rich cores of differentiated impactors during the late accretion phase [2]. Since Earth has most likely had a magma ocean during this period [3], a quantitative understanding of impacts into such targets is of pivotal interest. In particular the answer to the question whether the impactor core material breaks into droplets or remains mostly in one piece is key, since this greatly affects the metal-silicate equilibration and mixing in the magma ocean [4]. This topic has been examined by several experimental as well as numerical studies [5,6,7]. The latter include mesh-free methods like smoothed particle hydrodynamics (SPH [8,9]), which may suffer from insufficient resolution, as well as grid based approaches. To expand on previous studies with such grid based algorithms [7], we implemented a method to improve the resolution and tracking of small material fragments in the simulation. This approach lays the groundwork for detailed studies of the size-frequency distribution of impactor cores in dependency to the impact parameters (core size, impact velocity and angle), as well as target properties (depth, temperature, and viscosity of the magma ocean). In a next step our results can then be used as input for further models studying material mixing in a convecting magma ocean [10]. Methods: We performed simulations of asteroid impacts using the iSALE-2D shock physics code [11,12], utilizing an Euler grid. By adding Lagrangian tracers at defined position at the start of the simulation, more detailed tracking of the material flow is possible. However, the exact fate of the material has to be reconstructed from these tracers in a post processing step, like in the stretching ratio model in [7]. In addition, this approach neglects the fact that small fragments tend to be underresolved and, thus, their motion in the model may suffer from numerical artifacts. To address these limitations, we developed a method to reduce numerical artifacts as well as improve the tracking of smaller material chunks in iSALE. To this end, we identify and analyze fragments of a chosen material type, in this case the material of the impactor core, in the whole numerical grid during each simulation step. Based on predefined criteria, for example looking at the shape of a fragment or the strain in its individual cells, we determine if each individual fragment is still sufficiently well resolved. If this is not the case, the fragment will be broken up or completely removed by replacing the impactor core material in its cells with that of the surrounding magma ocean matter. The removed mass and volume will be saved in the nearest tracer, which approximates the movement of the fragment based on the surrounding velocities. By effectively freezing mass and volume of a fragment in this way, artificial distortion caused by insufficient numerical resolution for such small fragments is prevented. The setup of our simulations consists of a 200 km diameter dunite projectile with a 100 km iron core, impacting a dunite half space. The upper 900 km of this target behave purely hydrodynamically without strength or viscosity, approximating a magma ocean. The resolution is varied between 20 and 80 cells per projectile radius (cppr). Results: Figure 1 shows a comparison between simulations with and without the new method for a cppr of 80 at different time steps. It shows the impactor core material (yellow) as well as the tracers used to save information in the method (red dots). The left side (negative x) of each image shows the results with the new method. It is clearly visible that, as impactor material penetrates deeper into the magma ocean, its core fragments into more and more pieces, the smallest of which are removed into tracers when using the method. Discussion and Conclusion: The results obtained so far show that the impactor core breaks into many fragments during and after the impact, similar to observations in other studies [7]. The current criteria, based on the concentration in individual cells and their neighbors, as well as on a simplified approximation of the fragment shape, are not entirely sufficient to find all cases in which numerical artifacts occur. To achieve this, physical parameters like the strain inside the fragments have to be considered as well. It is also important to note that the current use of tracers to save the mass and volume of small fragments means that these cannot fragment further and that their interaction with other matter is strongly simplified - tracers follow the velocity field of the surrounding material. Some additional interaction will need to be implemented to evaluate how valid this approximation is. Acknowledgements: We gratefully acknowledge the developers of iSALE-2D, including Gareth Collins, Kai Wünnemann, Dirk Elbeshausen, Tom Davison, Boris Ivanov and Jay Melosh. This work was funded by the Deutsche Forschungsgemeinschaft (SFB-TRR170, subproject C2 and C4). References: [1] Walker R. J. (2009) Chem. Erde-Geochem. 69, 101-125. [2] Wood B. J. et al. (2006) Nature 441, 825-833. [3] Tonks W. B. et al. (1993) J. Geophys. Res. 98, 5319-5333. [4] Rubie D. C. et al. (2003) Earth Planet Sc. Lett. 205, 239-255. [5] Daguen R. et al. (2014) Earth Planet Sc. Lett. 391, 274-287. [6] Landeau M. et al. (2016) Nat. Geosci. 9, 786-789. [7] Kendall J. D. et al. (2016) Earth Planet Sc. Lett. 448 24-33. [8] Marchi S. et al. (2020) Sci. Adv. 6(7) eaay2338. [9] Monaghan J.J. (1992) Annu. Rev. Astron. Astrophys. 30, 543-574. [10] Maas C. et al. (2021) Earth Planet Sc. Lett. 554. [11] Collins. G. S. et al. (2004) Meteorics & Planet. Sci. 39, 217-231. [12] Wünnemann K. et al. (2006) Icarus 180, 514-527.

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.

Introduction: The morphology of impact basins formed during the first 700 Ma after the Moon-forming event (e.g., [1-2]) is affected by the impactor's size and velocity, and the thermal state of the crust and upper mantle, which is related to the cooling history of the Moon (e.g., [3-5]). Due to alteration and erosion of structural surface features by subsequent impact flux, the size of given basins cannot be determined unequivocally. High resolution Bouguer gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) mission [6-7] show a strong positive anomaly in the center, surrounded by a gravity low for impact basins located at the lunar farside. The relationship between the size of these gravity patterns and basin diameter may allow for estimating the impactor's size and thermal state of the Moon at the time of impact (e.g., [3, 5, 8]). Previous studies (e.g., [7, 9]) showed that basin formation processes are followed by isostatic adjustment and cooling processes. As a result, basin structures undergo relaxation processes, coupled with modifications in gravity signature. Therefore, the direct comparison of GRAIL data with gravity data from numerical models of basin formation may be questionable. We summarize results of our systematic numerical modeling study on the influence of the thermal state and impactor size on basin formation processes [5]. Using observed gravity data as constraints for our numerical basin formation models allows for estimating the thermal conditions and the size of the impactor at the time of impact for observable basin structures [5]. Based on this work we now aim at considering isostatic compensation processes and show preliminary results on how to investigate the problem. Methods: In our previous work, we account for different impactor sizes, crustal thicknesses, and thermal states of the Moon. Different depth-temperature profiles represent the lunar thermal evolution at 4.5 Ga ("warm"), 4.1 Ga ("intermediate") and 3.8 Ga ("cold"), which correspond to approximate basin ages. We correlate modeled Bouguer gravity from our models of basin formation with observed gravity signatures of 16 lunar farside basins. To evaluate the state of isostatic equilibrium in our best-fit models, we use as a first attempt the isostatic model assuming the Airy concept. Here, different topographic heights are accommodated by changes in crustal thickness. With this approach, we can estimate the location of the crust-mantle boundary for our best-fit models to evaluate the state of isostatic compensation. Results: Figure 1 shows the transient crater (Dt) , the diameter of the largest crustal thickness (DLCT), and the diameter of the Bouguer anomaly from basin formation models (DBA-mod) as a function of impactor size (Limp). Apparently, Dt, DLCT, DBA-mod depend on the target's thermal state (indicated by the colors). For impactors larger than 40 km in diameter the thermal state affects the size of the Bouguer anomaly [5]. As examples, we discuss here the best-fit models for Korolev crater and Orientale basin: Figure 2 shows the model for the Korolev crater, assuming an impactor size of 50 km and a temperature profile at ca. 4.1 Ga. In Figure 3, the best-fit model for the Orientale basin formed by an impactor of 80 km in a cold lunar environment (3.8 Ga) is shown. The models are fitted to the observed gravity signal (Fig. 2a, 3a, dashed line). The green line (Fig. 2a, 3a) corresponds to the gravity anomaly derived from the basin formation model assuming constant densities in crust (ρc) and mantle (ρm) (Fig. 2b, 3b, left panel). The latter distinctly deviates from the density distribution in our basin formation models (Fig. 2b, 3b); however, we consider the usage of constant densities as a simple estimate of how Bouguer anomalies of our models may look like after cooling. The gravity signal (Fig. 2a, 3a; black line) based on the inhomogeneous density distribution due to the thermal expansion right after impact shows a much lower amplitude and a "plateau" in the basin center. A closer look to the temperature field (Fig. 2c, 3c) reveals that the "plateau" is directly related to the hot area of the temperature field. In order to assess the isostatic equilibrium of the basin formation models with the inhomogeneous density distribution, we determine the position of the crust-mantle-boundary according to the Airy concept (Fig. 2b, 3b, orange line). Our preliminary results show that the Korolev impact structure is almost in isostatic equilibrium, whereas large changes in the crust-mantle-boundary have to be assumed for the larger Orientale basin. Conclusion: By assuming constant densities in the target, we are able to fit the observed and modeled gravity signatures. But this method of gravity fitting is questionable because the observed gravity data are based on the current subsurface density field, whereas the basin formation models show the density field directly after impact. We expect that cooling of impact structures between the last 4.4 Ga and 3.8 Ga directly affects the density distribution concomitant with isostatic compensation processes changing the position of the crust-mantle-boundary. Thus, the position of the gravity signal will also change. Acknowledgements: We gratefully thank the developers of iSALE and the pySALEPlot tool. This work is funded by the Deutsche Forschungsgemeinschaft (Project-ID 263649064-TRR 170). References: [1] Wilhelms, D.E. (1987) USGS Professional Paper 1348. [2] Morbidelli, A. et al. (2018) Icarus, 305, 262-276. [3] Miljkovic, K. et al. (2016) J. Geophys. Res. Planets, 121, 1695-1712. [4] Potter, R.W.K. et al. (2015) GSA Special Papers, 518, SPE518-506. [5] Lompa, T. et al. (2021) LPSC, Abstract #1254. [6] Zuber, M.T. et al. (2013) Science, 339, 668-671. [7] Melosh, H.J. et al., (2013) Science, 340, 1552-1555. [8] Neumann, G.A., et al. (2015) Sci. Adv., 1, e1500852. [9] Freed, A.M. et al. (2014) J. Geophys. Res. Planets, 119.

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.

1. IntroductionNumerous large impacts during the early bombardment period resulted in a globally fragmented crust known as lunar megaregolith (Hartmann 2019). Understanding how megaregolith evolved from an initially intact crust is important for the thermal and magmatic evolution of the Moon (Jaeger et al., 2007; Melosh, 2011; Warren & Rasmussen, 1987) and interpretations of lunar samples. In this study, we developed a spatial-resolved numerical model of the formation of the lunar megaregolith characterized by a size-frequency distribution of fragments generated by a continuous impactor flux. 2. Model2.1 Crater distributionWe use the typical “accretion tail” model of early intense bombardment to calculate the crater distribution (Neukum 1989). The considered minimum crater diameter is assumed to be the diameter of saturated craters of the lunar highlands (>20 km; Head et al 2010) where 100% of the surface is covered by craters larger than this size. Thirty basin-forming events with a model age after Orgel et al. (2018) are included in our simulations. 2.2 Fragment size distribution of individual impacts2.2.1 Fragment size in ejectaThe size distribution of fragments in ejected materials is simulated by assuming that the distal ejecta deposits contain smaller fragments while ensuring the ejecta thickness, H(r), follows the power-law according to Pike (1974), H(r)=0.033Rt(r/Rt)-3, where r is the distance from the crater center, and Rt is the radius of transient crater diameter.Crater ejecta deposits are composed of rock fragments of various sizes. The cumulative number N(m) of fragments of mass equal to or greater than mass m is (Melosh 1989): N(m) = C1m-b, where C1 is a constant. The value of b depends upon the fragmentation process, and for multiple fragmentations, b near 1.0 (Melosh 1989; Buhl et al. 2014). The largest fragments are usually near the crater rim. The relation between maximum block size lE and crater diameter (D) (Moore 1971) is: lE = C2 D2/3, where C2 is constant with a value ranging from 0.1 to 0.3.2.2.2 Fragment size below the crater Melosh (1979) proposed the so-called Acoustic Fluidisation model to explain the temporary weakening of rocks during crater formation. The simplified variant, the Block model (Melosh & Ivonov 1999) describes the rheology of temporarily weakened rocks by a Bingham viscosity which depends on the average block size (lB0), the viscosity η, the target density ρ, and the average period of the oscillations T. Acoustic wave can only induce shaking of blocks, resulting in acoustically fluidized behavior, if the oscillation wavelength λ (λ=T×cs, where cs is the speed of sound) is comparable to the lB0. For simplicity we assume lB0 = λ. η=2πρlB02/T (1)Since the impact of a large projectile leads to larger blocks and higher viscosity than smaller impactors, Wünnemann et al. (2003) proposed a scale dependency of the viscosity  according to linear relationship:η= γη (cs ρ DP⁄2) (2)where γ_ηis dimensionless scaling parameter and is assumed to be 0.005 (Martellato et al. 2018). Combining eq. 1 & 2 results in:lB0=(γη DP)⁄4π In the Block model the block or fragment size (lB0) is usually taken to be constant through space, and the variations of block sizes concerning the distance from the point of impact are neglected. Based on the well-exposed Upheaval Dome crater, Kenkmann et al. (2006) suggested a linear function of the block size (lB) with the distance from the crater center (rB), where the function slope, C3, is ~0.4. Assuming lB0 to be representative for the fragment size close to the crater floor, the block size distribution beneath the crater with the distance from crater center can then be calculated: lB=lB0+C3 rB3. Results3.1 Formation of megaregolithFigure 1 presents the globally volume-weighted fragment size distribution with depth at different times. Basin-forming events are the dominant factor generating a global megaregolith layer. After the formation of the youngest basin (3.8 Ga), the big picture of the megaregolith was established. In the later period (e.g., 3.7 Ga; black dots in Figure 1), the smaller-scale impacts were predominant and could not significantly alter the megaregolith structure anymore. The generated megaregolith displays two distinct layers: an Upper Megaregolith that mainly consists of ejected fragments with smaller size and a thickness of ~2.5 km; a Lower Megaregolith that mainly consists of autochthonous fragments with a thickness of more than 10s of kilometers. The obtained structure of the megaregolith is consistent with both the lunar seismic and gravity observations. Figure 1 Formation and structure of the megaregolith.3.2 Presence of SPA ejecta bouldersWhen the South Pole-Aitken (SPA) was formed, its excavated fragments distributed over the global surface. These fragments suffered further fragmentation altering their size and spatial distribution. When the structure of the megaregolith was formed (3.8 Ga), the majority of the SPA ejecta fragments became very small. Due to subsequent space weathering, almost no boulders < 50 m can survive for 3.8 Ga (Basilevsky et al. 2013). We estimated the distribution of the >50 m SPA ejected fragments in the near-surface shown in Figure 2. Less than 5% of SPA ejected boulders could have survived until the present day and the majority of the survival is distributed near the rim of young and large impact craters. Figure 2 Distribution of SPA ejecta boulders when the megaregolith was formed.4. ConclusionsA spatial-resolved numerical model was developed to investigate the cumulative fragmentation process on the moon aiming at a quantitative description of the formation of megaregolith. We found that after the cessation time of basin-forming events, the structure of megaregolith was established. The megaregolith possesses a distinct two-layer structure: a more fragmented Upper megaregolith ~2.5 km in thickness that mainly consists of the ejected fragments and a Lower megaregolith >10 km in thickness that mainly consists of autochthonous fragments. The simulation results also suggest that most of the ancient impact fragments, such as boulders ejected upon formation of SPA, cannot survive until the present day.

Shock-darkening, the melting of metals and iron sulfides into a network of veins within silicate grains, altering reflectance spectra of meteorites, was previously studied using shock physics mesoscale modeling. Melting of iron sulfides embedded in olivine was observed at pressures of 40-50 GPa. This pressure range is at the transition between shock stage 5 (C-S5) and 6 (C-S6) of the shock metamorphism classification in ordinary and enstatite chondrites. To better characterize C-S5 and C-S6 with a mesoscale modeling approach and assess post-shock heating and melting, we used multi-phase (i.e. olivine/enstatite, troilite, iron, pores, and plagioclase) meshes with realistic configurations of grains. We carried out a systematic study of shock compression in ordinary and enstatite chondrites at pressures between 30 and 70 GPa. To setup mesoscale sample meshes with realistic silicate, metal, iron sulfide, and open pore shapes, we converted backscattered electron microscope images of three chondrites. The resolved macroporosity in meshes was 3-6%. Transition from shock C-S5 to C-S6 was observed through (1) the melting of troilite above 40 GPa with melt fractions of similar to 0.7-0.9 at 70 GPa, (2) the melting of olivine and iron above 50 GPa with melt fraction of similar to 0.001 and 0.012, respectively, at 70 GPa, and (3) the melting of plagioclase above 30 GPa (melt fraction of 1, at 55 GPa). Post-shock temperatures varied from similar to 540 K at 30 GPa to similar to 1300 K at 70 GPa. We also constructed models with increased porosity up to 15% porosity, producing higher post-shock temperatures (similar to 800 K increase) and melt fractions (similar to 0.12 increase) in olivine. Additionally we constructed a pre-heated model to observe post-shock heating and melting during thermal metamorphism. This model presented similar results (melting) at pressures 10-15 GPa lower compared to the room temperature models. Finally, we demonstrated dependence of post-shock heating and melting on the orientation of open cracks relative to the shock wave front. In conclusion, the modeled melting and post-shock heating of individual phases were mostly consistent with the current shock classification scheme (Stoffler et al., 1991, 2018).

Fine dust from impact ejecta settles slowly in an atmosphere, whereas large chunks of ejecta easily traverse the surrounding gas on nearly parabolic trajectories. In order to study the effects of the interaction of impact ejecta with an atmosphere, we implemented a “representative particle” approach into the iSALE-2D shock physics code. We verify the modelling approach using analytical equilibrium sedimentation velocities, and successfully benchmark the results against simulations with the shock physics code SOVA. We study vertical dust settling scenarios of different initial mass and dust size (100 μm – 10 cm), and find three different regimes of settling – sedimentation, formation of density currents, and the free-fall. Based on this, we model natural ejecta curtains for craters from 200 m – 4 km in radius, and small craters in the laboratory for different atmospheric pressures from 1 kPa – 6 MPa. We assess the range of ballistic ejecta deposition within an atmosphere in terms of ejecta thickness and deposition velocity.

The Moon exhibits striking geological asymmetries in elevation, crustal thickness, and composition between its nearside and farside. Although several scenarios have been proposed to explain these asymmetries, their origin remains debated. Recent remote sensing observations suggest that (1) the crust on the farside highlands consists of two layers: a primary anorthositic layer with thickness of ~30-50 km and on top a more mafic-rich layer ~10 km thick; and (2) the nearside exhibits a large area of low-Ca pyroxene that has been interpreted to have an impact origin. These observations support the idea that the lunar nearside-farside asymmetries may be the result of a giant impact. Here, using quantitative numerical modeling, we test the hypothesis that a giant impact on the early Moon can explain the striking differences in elevation, crustal thickness, and composition between the nearside and farside of the Moon. We find that a large impactor, impacting the current nearside with a low velocity, can form a mega-basin and reproduce the characteristics of the crustal asymmetry and structures comparable to those observed on the current Moon, including the nearside lowlands and the farside's mafic-rich layer on top of a primordial anorthositic crust. Our model shows that the excavated deep-seated KREEP (potassium, rare-earth elements, and phosphorus) material, deposited close to the basin rim, slumps back into the basin and covers the entire basin floor; subsequent large impacts can transport the shallow KREEP material to the surface, resulting in its observed distribution. In addition, our model suggests that prior to the asymmetry-forming impact, the Moon may have had an 182W anomaly compared to the immediate post-giant impact Earth's mantle, as predicted if the Moon was created through a giant collision with the proto-Earth., 45 pages, 12 figures

Impact cratering has been the primary process to alter the distribution of lunar highland material since the formation of a crust. This impact history is recorded in the radiogenic clocks of impact melts which are accessible for study on lunar samples and meteorites. However, primary impact melt is exposed to a long-time gardening process (i.e. re-melting, excavation, burial, and re-excavation) by subsequent impacts resulting in a complex spatial distribution of materials representing specific impact events. To investigate the diffusion behavior of impact melt, a model tracing the evolving distribution of melt laterally and with depth was built using a Monte Carlo approach. Given scaling laws concerning melt production and ejecta distribution, the size-frequency distribution of impact craters, and the rate function for crater formation, we examine the evolution of melt component occurrence of different ages. Three mid- to late-forming basins (Serenitatis, Crisium, and Imbrium) are chosen as a case study for the diffusion of melt from major basin-forming events. The survival probability of basin melt occurrence at the Apollo and Luna sampling spots is derived. It is expected to find abundant Imbrium and Crisium melt at the Apollo and Luna sampling sites, consistent with the K-Ar radiometric dates of highland samples; whereas the older Serenitatis melt was subjected to the later long-term gardening, strongly influenced by later local impacts, and thus is less abundant. Understanding the diffusion of impact melt is helpful for interpretation of radiometric ages of lunar samples and can be used to predict the distribution of differently-aged melts at future landing/sampling sites such as the Chinese Chang'E-4 (CE-4).

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.

China's Chang’e-5 (CE-5) has returned 1.731 kg lunar samples from a young mare unit on December 2020. Here, the authors used a spatially resolved numerical model, which considers both impact mixing and volcanic effusion, to estimate the evolving distribution of components over the landing region. Using this model, the age composition of surface scooped and sub-surface drilled CE-5 samples and their sources were traced back. The proposed model predicts that local mare material predominates in both surface and sub-surface samples and the nonlocal mare component is ∼1%. There is ∼40% of nonmare component. The composition of both nonmare components in the surface and sub-surface samples are similar: Abundant components are basin-sourced melts, where the Imbrium melt are ∼ 25% among all the melt. The ejecta of Sharp B, Harding, Copernicus, and Aristarchus crater that possess different compositions of basin-sourced melt may have significantly altered the material composition of the landing surface.

The impact melt that records the formation time of basins is essential for the understanding of the lunar bombardment history. To better understand melt distribution on the Moon, this study investigates mixing of melt by small impacts using a Monte Carlo numerical model. The obtained mixing behavior is then integrated into a larger scale model developed in previous work. While large impacts produce most of the melt volume in both the regolith and megaregolith, we find that the dominant source of melt near the surface is small impacts. Material in the top meter is affected mainly by impacts that form craters 100 m craters do not contribute abundant spherules. The distribution of the datable melt with depth is also analyzed, which is essential for future sampling missions. Excavated materials of young and large craters (>100 m on highlands; >10 km on maria) appear to be the most fruitful targets., Plain Language Summary: Hypervelocity impact events on the Moon generate great energy that melts materials in the near���surface. The generated melt products record the age of impact craters. The abundance of impact melts of different ages is therefore essential for our understanding of the lunar bombardment history. Most of the returned samples are derived from the near���surface, where the material composition has been significantly affected by the frequent gardening of small impacts. Improving our understanding of how impact processes change the material composition is helpful for sample interpretations. Here, we build a numerical model to investigate this issue. The simulation results show that craters 100 m on highlands; >10 km on maria) would be the optimal targets., Key Points: A numerical model is developed to investigate the effect of small impact gardening on ancient melt in the lunar near���surface. Gardening of craters, Deutche Forschungsgemeinshaft (DFG, German Research Foundation)

The processes leading to the formation of planetary systems leave behind a significant mass of small bodies - up to 35 Earth masses depending on the model [1] - orbiting at large heliocentric distance, and observed around 20% of Sun-like stars [2]. It is established that those bodies play an important role in the migration of gas giants away from their stars and may be necessary for life to develop on the smaller planets. Yet, the conditions within these primitive populations are not well understood, especially their collisional environment. In the last decade, space missions have brought fascinating new data which challenge our concepts of impacts in the Early Solar System. The mission Rosetta at comet 67P, for instance, has revealed a complex cometary world where collisions, from small to catastrophic, played a significant role. Recent work [3,4] suggests that the topography of cometary nuclei and potential layering are shaped by processes which are primarily ancient. Current erosion does not appear sufficient to create features such as deep pits and tall cliffs over the lifetime of a comet in the Inner Solar System. Could they be due to impacts ? Or early activity in the Centaur phase ? On a larger scale, dynamical simulations argue that objects like the Jupiter Family Comets may have been totally disrupted by catastrophic collisions. While models show that the high porosity and volatile content of cometary nuclei would survive such impacts, it is not clear whether the deeper structural features like layers can be preserved. From the same data set, different authors come to different conclusions with respect to collisions in the early outer planetary system (summarized in [5]). Furthermore, different modeling approaches [6,7,8] lead to distinct results. Overall, there is quite some confusion as to how to interpret the cratering record on very porous, icy objects, in the Outer Solar System. This is further complicated by the fact that we may not be able to properly recognize impact features on such bodies. Here we report on the results obtained by the OCEOSS (Outcome of Collisions in the Early Outer Solar System) ISSI group. The concept of this working group is to bring together experts on collisions and cometary morphology. Over the last few years, modelers have developed new numerical simulations to account for cometary-like material. In parallel, thanks to the Rosetta and New Horizon missions, small bodies morphologists have a much better understanding of the type of landform that can exist on comets, and new measurements of the material physical properties. Our group investigates how morphological features observed today on comets and KBOs could relate to the early collisional environment, beyond the typical crater counting studies. We will summarize our results along three main topics: Large scale collisions, small scale collisions, and populations. We find that the outcome of merger collisions, such as the one which may have created comet 67P or KBO Arrokoth can be very different depending on the internal structure of the bodies. If layers are already present before the collision, the impact may lead to shedding and creation of cliffs/terraces resembling what we see on comets. More energetic impacts would lead to catastrophic disruption, although the fragments may come back together by gravitational interaction, leading to cometary rubble piles. Recent work on this process for asteroids shows that this could lead to the high porosity and potential density variation we observe across bilobate-comets. On a local scale, we simulated a km size projectile impacting a 30km KBO, which is the expected order of magnitude for creating features such as the largest crater observed on Arrokoth [9]. We find a large diversity of depth-to-diameter ratio (d/D) for craters, depending on the impact and material parameters. We caution against the simple asteroid-like approximation d/D=10 for all cases [10], as it can lead to erroneous interpretation of the projectile population when extrapolating from crater shapes only. We will discuss what our models mean for the interpretation of current spacecraft data, and how this work can support future missions to comets/KBOs, like ESA's Comet Interceptor [11]. References: [1] Tsiganis et al., Origin of the orbital architecture of the giant planets of the Solar System, Nature 2005 [2] Wyatt, M., Extrasolar Kuiper belts, in book The Trans-Neptunian Solar System, 2019 [3] Vincent et al, Constraints on cometary surface evolution derived from a statistical analysis of 67P's topography, MNRAS 2017 [4] Birch et al, Geomorphology of comet 67P/Churyumov-Gerasimenko, MNRAS 2017 [5] Weissmann et al, Origin and Evolution of Cometary Nuclei, SSR 2020 [6] de Niem et al, Low velocity collisions of porous planetesimals in the early solar system. Icarus 2018 [7] Jutzi et al, Formation of bi-lobed shapes by sub-catastrophic collisions. A late origin of comet 67P's structure, A&A 2017 [8] Schwarz et al, Catastrophic disruptions as the origin of bilobate comets, Nature Astronomy 2018 [9] Spencer et al, The geology and geophysics of Kuiper Belt object (486958) Arrokoth, Science 2020 [10] Bottke et al, Interpreting the Cratering History of Bennu, Ryugu, and Other Spacecraft-Explored Asteroids, EPSC 2019 [11] Snodgrass, C. & Jones, G., The European Space Agency's Comet Interceptor lies in wait, Nature Communications 2019 Acknowledgements The OCEOSS team thanks the International Space Science Institute (ISSI Bern), which made this collaboration possible.

Introduction In recent years, interest in predicting the consequences of the encounter of a NEO with Earth has increased. Most studies focus on rocky or cometary objects [1-3]. However, in the most frequent size range of the smallest objects, the effects of iron objects are more severe than for equal-sized rocky objects. Indeed, most small craters on Earth have been formed by iron objects, which penetrate the atmosphere more efficiently than stony asteroids. Intact meteoroids form single craters (e.g. Kamil), fragmented meteoroids form crater/meteorite strewn fields (e.g. Morasko), and larger asteroids fragment and form a single crater (e.g., Barringer). The atmospheric entry and cratering generate shock waves, which can cause damage on the ground. In this project, we combine different approaches in a knowledge-base to predict the outcome of such events with sufficient accuracy in a fast way. Such a tool is highly desirable for ESA's Planetary Defence Office to release impact warnings and inform emergency response agencies. Method We describe the behaviour of iron objects of few metres in size up to ~50 m in diameter. We assume that their strength follows a typical Weibull power law. In order to simulate the atmospheric entry of an iron object, we follow different approaches: 1) small (i.e. ~1 m sized) meteoroids that are strong enough to avoid fragmentation in the atmosphere are subject to ablation and deceleration, 2) slightly larger asteroids (i.e. few metres) are described by a separate fragment model [e.g. 4,5], and 3) the largest asteroids are described by the pancake model [e.g. 6]. The effect of atmospheric shock waves (i.e. overpressure and wind speeds on the ground) are calculated based on fits to nuclear explosion data [1,7], which yield circular-symmetric results. For cases that are not accurately described by simple parameterisation, we use the shock physics code SOVA [8] to simulate the evolution of shock waves based on pre-calculated energy release curves. Tracer gauges measure the pressure and velocity distribution over time in atmosphere and store the maximum values. Results Distinction between individual fragments and a homogenous cloud of fragments: The fragmentation of an iron meteoroid/asteroid and the altitude at which such an event occurs depends on the initial parameters (velocity, entry angle, size). Large objects typically have a lower strength than smaller ones and fragment earlier in the atmosphere. Once fragmented, the individual pieces separate from each other with a lateral dispersion velocity, which depends on the absolute velocity of the object [4]. To decide which parameterisation to use, we compare the total cross- sectional area of all fragments with their separation distance (similar approach has been used in [9]). As long as the cross-sectional area of all fragments is larger than the corresponding size of the expanding fragment cloud (estimated from the dispersion velocity), the pancake model can be applied. This condition is expressed by the following equation (assuming equal-sized fragments): with the initial size R, separation velocity constant C, air density ρair, asteroid density ρasteroid, Weibull exponent αW, atmospheric scale height H, number of fragments N and entry angle ϑ. The altitude of fragmentation (and, thus, the atmospheric density at fragmentation) depends on the initial parameters, which is why the equation cannot be solved analytically. The dependence is shown in Figure 1. A meteoroid of the size of the Morasko event (~3 m radius, [5]) falls into the regime where the separate fragment model is applied. An asteroid like the one that formed the Barringer crater (~25 m radius) can be modelled with the pancake model. We also apply this approach to fragments with exponential size-frequency distribution, and the results are essentially the same. Figure 1: Expansion radii (colour lines) versus initial radius of a meteoroid (X-axis and grey line). Iron density is 7800 kg/m³, the reference strength for a 1kg sample is 440 MPa, αW=0.2. If the coloured lines are below the grey line, the pancake model is valid (separation distance is smaller than the maximum radius of the compact body). If the coloured lines are above the grey line, the separate fragment model is applicable (fragments are well-separated when the second fragmentation occurs). Colours represent different number of fragments: black – 10; blue – 100; green – 1000; and red – 106. Overpressure: We show results for two size classes of objects, which represent the size of the Kamil and the Barringer cratering events (Figure 2). For the Kamil event (initial radius of ~ 1.5 m), the outcome depends on modelling parameters (e.g. strength) and can vary between the formation of a single crater or a strewn field. However, overpressure distributions show little variation with and without a fragmentation event. For larger objects, the strength is less important. Overpressures are caused mostly by the crater formation and to some part by the atmopsheric entry. Figure 2: Overpressure caused by the entry of two different sized objects. Top: 1.5m radius, 20km/s, 45° entry angle, different strength parameters, Bottom: 23m radius, 18km/s, 30°- 45° entry angle (green, and red & blue, respectively). The red line represents a ten times weaker reference strength. Results based on nuclear explosion data [7] are shown in black. Discussion and Conclusion Shock physics codes give most reliable results for the prediction of overpressures. However, for a range of input parameters, the usage of parameterisation in combination with nuclear explosion data produces reliable estimates much faster (cf. Figure 2). In this project, we use a combination of both approaches. For small iron objects, overpressures are low apart from a small range near the crater, and damage is localised within a few crater radii. Acknowledgements We acknowledge the funding by ESA SSA-NEO, contract code P3-NEO-VIII. References [1]Collins G.S., Melosh H.J. and Marcus R.A. (2005)M&PS,40(6),817-840. [2]Wheeler et al. (2018)Icarus,315,79-91. [3] Artemieva N. and Shuvalov V. (2019)M&PS,54(3),592-608. [4] Artemieva N. and Shuvalov V. (2001)JGR,106,3297-3309. [5] Bronikowska et al. (2017)M&PS,52(8),1704-1721. [6] Chyba C.F., Thomas P.J. and Zahnle K.J. (1993)Nature,361,40-44. [7] Collins et al.(2017)M&PS,52(8),1542-1560. [8] Shuvalov V.V. (1999)ShockWaves,9,381-390. [9] Svetsov V.V., Nemtchinov I.V., and Teterev A.S. (1995)Icarus,116,131-153.

Planetary collisions play an important role in the compositional and thermal evolution of the planetary system. The Moon-forming impact event is thought to be Earth’s last giant collision event, marking the end of the main accretion phase of the Earth. This large event (re)set the conditions for the subsequent thermochemical evolution of both bodies, Earth and Moon. Large parts of proto-Earth are thought to melt as a consequence of the impact and the extent of melting affects the evolution of the Earth’s interior and atmosphere. It is critical to address the initial conditions of the proto-Earth and the volume and shape of a possible magma ocean after the impact to understand the Earth’s subsequent evolution. To address these questions, we consider two different impact scenarios and present a benchmark study by comparing two different numerical codes. Further, we investigate the effect of strength on melt production for the two different impact scenarios. We compared results using the shock physics code iSALE [1,2], an Eulerian code with a fixed grid in space and a Langrangian mesh-free, so-called smoothed particle hydrodynamics code, (SPH [3]). Therefore, we consider two different impact scenarios that are described in Nakajima and Stevenson (2015); the canonical model (i) (e.g., [4]) and a fast-spinning Earth model (ii) [5]. The former (i) assumes a Mars-size impactor colliding with proto-Earth at an impact angle of 41° (90° corresponds to head-on collision) at a velocity corresponding to the escape velocity of Earth. The colliding bodies are differentiated into a mantle and core. The latter (ii) case is characterized by a smaller impactor colliding with a rapidly rotating Earth at a steeper impact angle of 73° and a much higher impact velocity of 20 km/s. In iSALE the impactor and target mantle consist of dunite, the core of iron; both materials are represented by an Analytical Equation of state (ANEOS [6]), the SPH code uses an MgSiO3 liquid and ANEOS for forsterite and iron. In iSALE we neglect the core of the impactor as well as the initial spin of Earth. The pre-impact temperature of the colliding bodies is 2000 K and near the solidus. Nakajima and Stevenson (2015) only consider hydrodynamic behavior of the colliding bodies. In the iSALE models a constitutive model accounting for strength is included. Thus, in addition to a comparison of the two different codes, we investigate the effect of strength on melt production. The simulations of the two codes are compared in terms of thermodynamic parameters (pressure evolution, Figure 1) as well as melt production (Figure 2) after the moon-forming impact. In Figure 1, a comparison of pressure evolution of both impact scenarios and the two numerical approaches is shown. In general, a good agreement in terms of pressure ranges for the two codes is observed. In the fast-spinning model a large fraction of mantle undergoes an extensive expansion and becomes subject to low pressure before some part falls back towards the core where it gains high pressures (Nakajima and Stevenson 2015). This process is not as prominent in the standard model. In the iSALE simulation we do not observe the fall back of the material although the post-impact state of Earth looks similar in both simulations. The SPH simulations show that the majority of mantle experiences melting (between 80 and 100% mantle melting) pointing to the existence of a global magma ocean with a base close to the core-mantle boundary in both impact scenarios (Nakajima and Stevenson 2015). The melt production obtained by the iSALE simulation is at least for the standard impact scenario smaller. For the fast-spinning Earth model about 60% of mantle material are molten. Thus, for the standard model, we only observe partial mantle melting whereas for the fast-spinning model we can assume that a global magma ocean exists. However, we have to note that the iSALE simulations have not run as long as the SPH simulations yet and that with a longer run time melt production most likely increases. Neglecting an impactor core in iSALE will also underestimate melt production. The accretion of the core would increase melting. Regarding the effect of strength, we observe a slight increase of melting when the colliding bodies exhibit some initial strength. However, the effect of the chosen impact scenario is much more significant as seen in Figure 2. The melt efficiency (melt volume normalized by the impactor volume) is over three times larger for the fast-spinning Earth model (2.66 corresponding to 60% of mantle volume) than for the standard model (0.73 corresponding to 20 % of mantle volume). The presented benchmark study shows an overall good agreement of two numerical codes and indicates the significance of the chosen impact scenario with respect to melt production on Earth during the Moon-forming impact. However, the differences of the two numerical methods may also be caused by some differences in the setup (e.g. different EoS, spin of Earth, differentiated impactor). In addition, in an ongoing study we systematically carry out simulations of the moon-forming impact event using iSALE. We vary the impact angle (15° to 60°) and impact velocities (12 to 20 km/s) and use different initial temperature profiles. First results show that with increasing impact angle as well as with increasing impact velocity, melt production increases. The effect of the pre-impact temperature becomes more prominent for increased impact angles.

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: 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).

Introduction: The lunar surface is characterized by large impact basins that formed in the early history of the Moon. To better understand how basin size and their corresponding gravity signature are related to the impactor size and the thermal history of the Moon, we investigated how properties of impactor and target could affect this formation process. Impactor properties are defined by its size, density, and impactor velocity. The target is characterized by the crustal thickness and the thermal state. Previous studies (e.g.[1],[2],[3],[4]) revealed that thermal conditions in crust and mantle rocks affected the crater formation process, the final basin structure, and the gravity signature. Here, we present numerical simulations for 16 basins located on the lunar farside. The simulations are constrained by gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) mission, and differ in impactor size, crustal thickness and target temperature as a function of depth. We established a relationship between the observed gravity, the geometry of the basin and the pre-impact thermal conditions of the target. These observations lead to a better understanding on how the thermal evolution of the Moon is related to changes in the formation of basins. Methods: We used the iSALE2D shock physics code to simulate the basin formation process ([5],[6],[7],[8]). We used impactor sizes of 20km to 100km in diameter. The impactor speed was set to 13km/s. The resolution of all models was set to 25 cells per projectile radius (grid cell sizes from 400m to 2000m). The target was composed of a basaltic crust on top of a dunitic mantle. We changed the target properties by using two different crustal thicknesses (40km, 60km) and three temperature profiles which represent the Moon’s thermal states at 4.4, 4.1, and 3.8 billion years ago [9]. Gravity data from GRAIL mission [10] provides detailed information about the deep structure of lunar basins. By combining the most recent GRAIL gravity model GL1500E [11] with LOLA derived topography [12], we obtained a highly resolved Bouguer gravity field. We assumed constant densities in both crust and mantle to calculate Bouguer gravity anomalies from the basin formation models. This approach can be understood as a simple way to mimic the density distribution after a long-term cooling process in the basin. The shape of the gravity signal preserves information about the position of the crust-mantle boundary and, therefore, assuming constant densities is useful in order to verify the geometry of the model. Results: For all 54 models we determined the diameter of the largest crustal thickness (DLCT) as a measure of the size of the basins. Figure 1 shows the relationship between impactor sizes and the DLCT for different temperature profiles and initial crustal thicknesses. The results show similar basin diameters for small impactors independently from the thermal profile. For impactor sizes larger than 40km (Fig.1A) or 30km (Fig.1B), the diameters of the hotter profile are significantly higher than those of the cold and intermediate profiles. We present Hertzsprung and Orientale basin as two examples for gravity modeling. In the formation model of Hertzsprung (Fig. 2A), we used the thermal profile corresponding to an age of 4.1Ga [13]. Figure 2 shows our best-fit model assuming an impactor size of 60km, and a crustal density of 2950kg/m3. The results for Orientale are shown in Figure 2B. In the best-fit model the impactor is 80km in diameter, crustal density is 2750kg/m3 and we assume a cold thermal profile corresponding to an age of 3.8Ga [13]. Gravity profiles from the formation models show that the position of Bouguerminima correlate with the DLCT. The ratio of Bouguerdiameter and DLCT evaluates the fit of the models. A ratio of one indicates that the Bouguerminimum is close to the DLCT. Conclusion: Our results agree with previous studies [4] in terms of the importance of the thermal state of the target material. Final crater morphologies are sensitive to the thermal conditions in the target material. In our study, temperature effects are visible in simulations with impactors larger than 40km and the change from intermediate to hot thermal states. Isostatic relaxation processes might further modify the initial gravity signature over time. The correlation between basin size and the position of the minimum in observed gravity is a powerful tool to predict impactor diameters without extensive modeling studies. Acknowledgments: We gratefully acknowledge the developers of iSALE. Topography and gravity data are available at NASA’s PDS. This work is funded by the DFG-grant SFB-TRR 170 (A4). Katarina Miljković research is funded by the Australian Research Council. References: [1]Ivanov, B., Melosh, H., and Pierazzo, E. (2010) Large Meteorite Impacts and Planet. Evol. IV,29-49. [2]Potter, R. W. K. et al. (2013) JGR:Planets,118,963-979. [3]Zhu, M.-H., Wünnemann, K. and Potter, R. W. K. (2015) JGR:Planets,120,2118-2134. [4]Miljković, K. et al. (2016) JGR:Planets,121,1695-1712. [5]Collins, G. S., Melosh, H. J. and Ivanov, B. A., (2004) Meteoritics & Planet.Sci.,39,217-231. [6]Wünnemann, K., Collins, G. and Melosh, H. (2006) Icarus,180,514- 527. [7]Thompson, S. L. (1990) Sandia National Laboratories Albuquerque, SAND89-2951. [8]Melosh, H. J. (2007) Meteoritics & Planet.Sci.,42,2079–2098. [9]Padovan, S. et al. (2018) EPSC,Abstract#755. [10]Zuber, M. T. et al. (2013) Science,339,668-671. [11]Park, R. S. et al. (2015) AGU Fall Meeting,Abstract#G41-B01. [12]Smith, D. E. et al. (2017) Icarus,283,70-9. [13]Orgel, C. et al. (2017) JGR:Planets,123,1-15.

Here, we develop scaling laws for (1) the distribution of impact-induced heat within the mantle and (2) shape of the impact-induced melt based on more than 100 smoothed particle hydrodynamic (SPH) simulations. We use Legendre polynomials to describe these scaling laws and determine their coefficients by linear regression, minimizing the error between our model and SPH simulations. The input parameters are the impact angle $\theta$ ($0^{\circ}, 30^{\circ}, 60^{\circ}$, and $90^{\circ}$), total mass $M_T$ ($1M_{\rm Mars}-53M_{\rm Mars}$, where $M_{\rm Mars}$ is the mass of Mars), impact velocity $v_{\rm imp}$ ($v_{\rm esc} - 2v_{\rm esc}$, where $v_{\rm esc}$ is the mutual escape velocity), and impactor-to-total mass ratio $\gamma$ ($0.03-0.5$). We find that the equilibrium pressure at the base of a melt pool can be higher (up to $\approx 80 \%$) than those of radially-uniform global magma ocean models. This could have a significant impact on element partitioning. These melt scaling laws are publicly available on GitHub ($\href{https://github.com/mikinakajima/MeltScalingLaw}{https://github.com/mikinakajima/MeltScalingLaw}$)., Comment: Published in Earth and Planetary Science Letters

Lateral variations in bulk density and porosity of the upper lunar highland crust are mapped using a high‐resolution Gravity Recovery and Interior Laboratory (GRAIL) gravity field model and Lunar Reconnaissance Orbiter (LRO) derived topography. With a higher spatial resolution gravity model than previous studies, we focus on individual impact basins with diameters greater than 200 km. The bulk density of the upper few kilometers of the lunar crust is estimated by minimizing the correlation between the topography and Bouguer gravity at short wavelengths that are unaffected by lithospheric flexure. Porosity is then derived using estimates of the grain density obtained from remote sensing data of the surface composition. The near surface crust in proximity to many large basins is found to exhibit distinct radial porosity signatures. Low porosities are found in the basin centers within the peak ring, whereas high porosities are identified near and just exterior to the main rim. The larger basins exhibit a more pronounced porosity signature than the smaller basins. Though the number of basins investigated in this study is limited, younger basins appear to be associated with the largest amplitude variations in porosity. For basins with increasing age the magnitude of the porosity variations decreases., Plain Language Summary: The gravity field surrounding an impact basin allows us to investigate the properties of the underlying crust and to better understand how craters form and evolve. In the center of the largest lunar craters, the impact basins, positive mass anomalies can be found that are caused by the excavation of crustal materials and the uplift of the mantle during formation. When using only the short wavelength portion of the gravity field, signals from these deeper regions are masked, and this allows us to estimate both the density and porosity of the upper crust. We investigated the crustal porosity of impact basins located in the lunar highlands, with diameters larger than 200 km. Many of these basins reveal similar porosity signatures, having low porosities in their center and high porosities near the crater rim. For the most pristine basins, the magnitude of the porosity variations increases with increasing basin size. Furthermore, the crustal porosity is influenced by the formation age of a basin, where the older basins have more muted signatures than younger basins., Key Points: The bulk density and porosity of the upper highland crust are revisited using a high‐resolution GRAIL gravity field model in combination with LOLA topography and independently estimated grain densities. For many impact basins, porosity is reduced within their peak ring and increased near and just exterior to the main rim. Large impact basins show a stronger pronounced porosity signature than smaller basins, and old impact basins reveal a muted porosity signature compared to younger basins.

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.

The early bombardment history of Mars may have drastically shaped its crustal evolution. Impact-induced melting of crustal and mantle materials leads to the formation of local and regional melt ponds, and the cumulative effects of the impact flux could result in widespread melting of the crust. To quantify impact-melt production, its provenance and final distribution as a function of impact conditions, we carried out a systematic parameter study using the iSALE shock physics code. In contrast to simplified scaling laws for estimating the amount of melt generated by shock compression, we take the planet's thermal state at the time of impact into account. In addition, we consider decompression melting as a consequence of lithostatic uplift of initially deep-seated material. We find that the geothermal profile has a strong effect on melt production, and that melt volumes are significantly increased by up to a factor of seven in comparison to existing analytical estimates. Enhanced melting occurs at impactor sizes (and velocities) that deposit most of their energy at a depth close to the base of the lithosphere. Impactors larger than 10 km penetrate through the lithosphere and can generate a significant amount of melt by decompression due to lithostatic uplift, which can make up to 40% of the total melt volume. In some cases, the total melt volume exceeds the volume of the transient (and final) crater and the surface expression of these collisions may resemble large igneous provinces rather than typical craters.

The importance of highly siderophile elements (HSEs) to track planetary late accretion has long been recognized. However, the precise nature of the Moon's accretional history remains enigmatic. There exists a significant mismatch of HSE budgets between the Earth and Moon, with the Earth disproportionally accreted far more HSEs than the Moon did. Several scenarios have been proposed to explain this conundrum, including the delivery of HSEs to Earth by a few big impactors, the accretion of pebble-sized objects on dynamically cold orbits that enhanced the Earth's gravitational focusing factor, and the "sawtooth model" with much reduced impact flux before ~4.10 Gyr. However, most of these models assume a high impactor retention ratio f (fraction of impactor mass retained on the target) for the Moon. Here, we performed a series of impact simulations to quantify the f-value, followed by a Monte Carlo procedure enacting a monotonically decaying impact flux, to compute the mass accreted into lunar crust and mantle over their histories. We found that the average f-value for the Moon's entire impact history is about 3 times lower than previously estimated. Our results indicate that, to match the HSE budget of lunar crust and mantle, the retention of HSEs should have started ~ 4.35 Gyr ago, when most of lunar magma ocean was solidified. Mass accreted prior to 4.35 Gyr must have lost its HSE to the lunar core, presumably during the lunar mantle crystallization. The combination of a low impactor retention ratio and a late retention of HSEs in the lunar mantle provide a realistic explanation for the apparent deficit of Moon's late accreted mass relative to the Earth., Comment: 31 pages, 3 figures

We studied shock-darkening in ordinary chondrites by observing the propagation of shock waves and melting through mixtures of silicates, metals and iron sulfides. We used the shock physics code iSALE at the mesoscale to simulate shock compression of modeled ordinary chondrites (using olivine, iron and troilite). We introduced FeS-FeNi eutectic properties and partial melting in a series of chosen configurations of iron and troilite grains mixtures in a sample plate. We observed, at a nominal pressure of 45 GPa, partial melting of troilite in all models. Only few of the models showed partial melting of iron (a phase difficult to melt in shock heating) due to the eutectic properties of the mixtures. Iron melting only occurred in models presenting either strong shock wave concentration effects or effects of heating by pore crushing, for which we provided more details. Further effects are discussed such as the frictional heating between iron and troilite and the heat diffusion in scenarios with strongly heated troilite. We also characterized troilite melting in the 32–60 GPa nominal pressure range. We concluded that specific dispositions of iron and troilite grains in mixtures allow for melting of iron and explain why it is possible to find a wide textural variety of melted and unmelted metal and iron sulfide grains in shock-darkened ordinary chondrites. We finally observe shock-melting of albite within a few iron and troilite grain models, and investigate the effects of higher porosity within the olivine matrix in the single iron and troilite grain models.

We quantify the seismicity of impact events using a combined numerical and experimental approach. The objectives of this work are (1) the calibration of the numerical model by utilizing real-time measurements of the elastic wave velocity and pressure amplitudes in laboratory impact experiments; (2) the determination of seismic parameters, such as quality factor Q and seismic efficiency k, for materials of different porosity and water saturation by a systematic parameter study employing the calibrated numerical model. By means of “numerical experiments” we found that the seismic efficiency k decreases slightly with porosity from k = 3.4 × 10−3 for nonporous quartzite to k = 2.6 × 10−3 for 25% porous sandstone. If pores are completely or partly filled with water, we determined a seismic efficiency of k = 8.2 × 10−5, which is approximately two orders of magnitude lower than in the nonporous case. By measuring the attenuation of the seismic wave with distance in our numerical experiments we determined the seismic quality factor Q to range between ∼35 for the solid quartzite and 80 for the porous dry targets. For water saturated target materials, Q is much lower