•Impact-induced seismicity plays an important role on planetary surfaces.•Seismic signals are recorded using numerical modeling.•Seismic parameters are significantly affected by different target ...properties.•Seismic magnitudes resulting from impacts can be predicted using numerical modeling.
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, <10. The obtained values are in the range of literature values. Translating the seismic efficiency into seismic magnitudes we show that the seismic magnitude of an impact event is about one order of magnitude smaller considering a water saturated target in comparison to a solid or porous target. Obtained seismic magnitudes decrease linearly with distance to the point of impact and are consistent with empirical data for distances closer to the point of impact. The seismic magnitude decreases more rapidly with distance for a water saturated material compared to a dry material.
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.
•Melt production on early hot Mars is increased by up to 7× compared to scaling laws.•Decompression can notably contribute to melting in large impacts on early Mars.•Paucity of large carters on Mars may explained by craters drowning in their melt.•Classical scaling laws fail to estimate melt production by large impactors (> 10 km).
•We develop scaling laws for the impact-induced heat distribution and melt shape.•We perform more than 100 smoothed particle hydrodynamic (SPH) simulations.•Our model reproduces the mantle heat ...distribution calculated by SPH generally well.•The pressure at the base of a melt pool is higher than that of a global magma ocean.•Our model is publicly available through GitHub.
Growing protoplanets experience a number of impacts during the accretion stage. A large impactor hits the surface of a protoplanet and produces impact-induced melt, where the impactor's iron emulsifies and experiences metal-silicate equilibration with the mantle of the protoplanet while it descends towards the base of the melt. This process repeatedly occurs and determines the chemical compositions of both mantle and core. The partitioning is controlled by parameters such as the equilibration pressure and temperature, which are often assumed to be proportional to the pressure and temperature at the base of the melt. The pressure and temperature depend on both the depth and shape of the impact-induced melt region. A spatially confined melt region, namely a melt pool, can have a larger equilibrium pressure than a radially uniform (global) magma ocean even if their melt volumes are the same. 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 θ (0∘,30∘,60∘, and 90∘), total mass MT (1MMars−53MMars, where MMars is the mass of Mars), impact velocity vimp (vesc−2vesc, where vesc is the mutual escape velocity), and impactor-to-total mass ratio γ (0.03−0.5). We find that the equilibrium pressure at the base of a melt pool can be higher (up to ≈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 (https://github.com/mikinakajima/MeltScalingLaw).
Elliptical impact craters are rare among the generally symmetric shape of impact structures on planetary surfaces. Nevertheless, a better understanding of the formation of these craters may ...significantly contribute to our overall understanding of hypervelocity impact cratering. The existence of elliptical craters raises a number of questions: Why do some impacts result in a circular crater whereas others form elliptical shapes? What conditions promote the formation of elliptical craters? How does the formation of elliptical craters differ from those of circular craters? Is the formation process comparable to those of elliptical craters formed at subsonic speeds? How does crater formation work at the transition from circular to elliptical craters? By conducting more than 800 three‐dimensional (3‐D) hydrocode simulations, we have investigated these questions in a quantitative manner. We show that the threshold angle for elliptical crater generation depends on cratering efficiency. We have analyzed and quantified the influence of projectile size and material strength (cohesion and coefficient of internal friction) independently from each other. We show that elliptical craters are formed by shock‐induced excavation, the same process that forms circular craters and reveal that the transition from circular to elliptical craters is characterized by the dominance of two processes: A directed and momentum‐controlled energy transfer in the beginning and a subsequent symmetric, nearly instantaneous energy release.
Key Points
Elliptical crater formation is studied by hydrocode simulations
Large‐scale elliptical craters are formed by shock‐induced excavation
Effect of target strength and projectile size on threshold angle is quantified
•Troilite readily melts by shock entropy in olivine at 35–60 GPa of nominal pressure.•Shock melting of iron occurs only in scenarios with strong pressure enhancements.•Peak shock pressures are ...dependent on impedance contrasts (e.g. troilite and iron).•Post-shock temperatures are highly heterogeneous in mixtures of iron and troilite.•Eutectic melting is strongly dependent on post-shock temperature heterogeneities.
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.
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Generation and propagation of shock waves by meteorite impact is significantly affected by material properties such as porosity, water content, and strength. The objective of this work was to ...quantify processes related to the shock‐induced compaction of pore space by numerical modeling, and compare the results with data obtained in the framework of the Multidisciplinary Experimental and Modeling Impact Research Network (MEMIN) impact experiments. We use mesoscale models resolving the collapse of individual pores to validate macroscopic (homogenized) approaches describing the bulk behavior of porous and water‐saturated materials in large‐scale models of crater formation, and to quantify localized shock amplification as a result of pore space crushing. We carried out a suite of numerical models of planar shock wave propagation through a well‐defined area (the “sample”) of porous and/or water‐saturated material. The porous sample is either represented by a homogeneous unit where porosity is treated as a state variable (macroscale model) and water content by an equation of state for mixed material (ANEOS) or by a defined number of individually resolved pores (mesoscale model). We varied porosity and water content and measured thermodynamic parameters such as shock wave velocity and particle velocity on meso‐ and macroscales in separate simulations. The mesoscale models provide additional data on the heterogeneous distribution of peak shock pressures as a consequence of the complex superposition of reflecting rarefaction waves and shock waves originating from the crushing of pores. We quantify the bulk effect of porosity, the reduction in shock pressure, in terms of Hugoniot data as a function of porosity, water content, and strength of a quartzite matrix. We find a good agreement between meso‐, macroscale models and Hugoniot data from shock experiments. We also propose a combination of a porosity compaction model (ε–α model) that was previously only used for porous materials and the ANEOS for water‐saturated quartzite (all pore space is filled with water) to describe the behavior of partially water‐saturated material during shock compression. Localized amplification of shock pressures results from pore collapse and can reach as much as four times the average shock pressure in the porous sample. This may explain the often observed localized high shock pressure phases next to more or less unshocked grains in impactites and meteorites.
Almost every meteorite impact occurs at an oblique angle of incidence, yet the effect of impact angle on crater size or formation mechanism is only poorly understood. This is, in large part, due to ...the difficulty of inferring impactor properties, such as size, velocity and trajectory, from observations of natural craters, and the expense and complexity of simulating oblique impacts using numerical models. Laboratory oblique impact experiments and previous numerical models have shown that the portion of the projectile’s kinetic energy that is involved in crater excavation decreases significantly with impact angle. However, a thorough quantification of planetary-scale oblique impact cratering does not exist and the effect of impact angle on crater size is not considered by current scaling laws. To address this gap in understanding, we developed iSALE-3D, a three-dimensional multi-rheology hydrocode, which is efficient enough to perform a large number of well-resolved oblique impact simulations within a reasonable time. Here we present the results of a comprehensive numerical study containing more than 200 three-dimensional hydrocode-simulations covering a broad range of projectile sizes, impact angles and friction coefficients. We show that existing scaling laws in principle describe oblique planetary-scale impact events at angles greater than 30° measured from horizontal. The displaced mass of a crater decreases with impact angle in a sinusoidal manner. However, our results indicate that the assumption that crater size scales with the vertical component of the impact velocity does not hold for materials with a friction coefficient significantly lower than 0.7 (sand). We found that increasing coefficients of friction result in smaller craters and a formation process more controlled by impactor momentum than by energy.
We investigate how different temperature gradients of the Moon affect the ejection of lithic and molten materials for impact basin several hundred kilometers in diameter to quantify the thickness and ...melt content of ejecta blanket as a function of radial distance. We find, by means of numerical modeling, that the ejecta thickness and melt content, similar to the basin formation, is sensitive to the thermal properties of the target. For two similar impact scenarios, the ejecta thickness with radial distance is proportional to a power law, but for a “warm” target, it declines faster than for a “cold” target. In addition, the impact on the warm target produces more molten ejecta than in the case of the cold target. The thermal effects on the ejecta thickness distribution can be testified by the topographic variations around Imbrium and Orientale basins, which were thought to be formed on a warm and cold Moon, respectively. Our study demonstrates that the thermal effect needs to be taken into account to estimate the ejecta thickness distribution for large‐scale impact basins on airless planetary surfaces.
Key Points
Effect of Moon's thermal states on ejecta blanket profile for large basins is investigated
Similar impacts on warm/cold Moon result in different ejecta blanket profile and different melt content within ejecta deposits
The difference of ejecta blanket profile can be testified by topographic variations around Imbrium and Orientale basins
We report results of an interdisciplinary project devoted to the 26 km‐diameter Ries crater and to the genesis of suevite. Recent laboratory analyses of “crater suevite” occurring within the central ...crater basin and of “outer suevite” on top of the continuous ejecta blanket, as well as data accumulated during the past 50 years, are interpreted within the boundary conditions imposed by a comprehensive new effort to model the crater formation and its ejecta deposits by computer code calculations (Artemieva et al. 2013). The properties of suevite are considered on all scales from megascopic to submicroscopic in the context of its geological setting. In a new approach, we reconstruct the minimum/maximum volumes of all allochthonous impact formations (108/116 km3), of suevite (14/22 km3), and the total volume of impact melt (4.9/8.0 km3) produced by the Ries impact event prior to erosion. These volumes are reasonably compatible with corresponding values obtained by numerical modeling. Taking all data on modal composition, texture, chemistry, and shock metamorphism of suevite, and the results of modeling into account, we arrive at a new empirical model implying five main consecutive phases of crater formation and ejecta emplacement. Numerical modeling indicates that only a very small fraction of suevite can be derived from the “primary ejecta plume,” which is possibly represented by the fine‐grained basal layer of outer suevite. The main mass of suevite was deposited from a “secondary plume” induced by an explosive reaction (“fuel‐coolant interaction”) of impact melt with water and volatile‐rich sedimentary rocks within a clast‐laden temporary melt pool. Both melt pool and plume appear to be heterogeneous in space and time. Outer suevite appears to be derived from an early formed, melt‐rich and clast‐poor plume region rich in strongly shocked components (melt ≫ clasts) and originating from an upper, more marginal zone of the melt pool. Crater suevite is obviously deposited from later formed, clast‐rich and melt‐poor plumes dominated by unshocked and weakly shocked clasts and derived from a deeper, central zone of the melt pool. Genetically, we distinguish between “primary suevite” which includes dike suevite, the lower sublayer of crater suevite, and possibly a basal layer of outer suevite, and “secondary suevite” represented by the massive upper sublayer of crater suevite and the main mass of outer suevite.
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