Insight into the Distribution of High-pressure Shock Metamorphism in Rubble-pile Asteroids

Funding Information: This work was supported by the Academy of Finland, project Nos. 293975 and 335595, the European Regional Development Fund, the Mobilitas Pluss programme (grant No. MOBJD639), and the NASA Solar System Exploration Research Virtual Institute Center for Lunar and Asteroid Surface Science, and it was conducted within institutional support RVO 67985831 of the Institute of Geology of the Czech Academy of Sciences. R.L. appreciates funding from the European Union’s Horizon 2020 research and innovation program, NEO-MAPP, grant agreement No. 870377. Publisher Copyright: © 2022. The Author(s). Published by the American Astronomical Society.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 (∼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−1. 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 (<0.2). Varying the porosity of the boulders from 10% to 30% does not significantly affect the distribution of pressure and fraction of shock-darkened material. The pressure distribution in rubble-pile asteroids is very similar to that of monolithic asteroids with the same porosity. Thus, producing significant volumes of high-degree shocked ordinary chondrites requires strong collision events (impact velocities above 8 km s−1 and/or large sizes of impactors). A large amount of asteroid material escapes during an impact event (up to 90%); however, only a small portion of the escaping material is shock-darkened (6%).Peer reviewe

Pressure-temperature evolution of primordial solar system solids during impact-induced compaction

Prior to becoming chondritic meteorites, primordial solids were a poorly consolidated mix of mm-scale igneous inclusions (chondrules) and high-porosity sub-μm dust (matrix). We used high-resolution numerical simulations to track the effect of impact-induced compaction on these materials. Here we show that impact velocities as low as 1.5 km s−1 were capable of heating the matrix to >1,000 K, with pressure–temperature varying by >10 GPa and >1,000 K over ~100 μm. Chondrules were unaffected, acting as heat-sinks: matrix temperature excursions were brief. As impact-induced compaction was a primary and ubiquitous process, our new understanding of its effects requires that key aspects of the chondrite record be re-evaluated: palaeomagnetism, petrography and variability in shock level across meteorite groups. Our data suggest a lithification mechanism for meteorites, and provide a ‘speed limit’ constraint on major compressive impacts that is inconsistent with recent models of solar system orbital architecture that require an early, rapid phase of main-belt collisional evolution

Assessing the survivability of biomarkers within terrestrial material impacting the lunar surface

The history of organic and biological markers (biomarkers) on the Earth is effectively non-existent in the geological record >3.8 Ga ago. Here, we investigate the potential for terrestrial material (i.e., terrestrial meteorites) to be transferred to the Moon by a large impact on Earth and subsequently survive impact with the lunar surface, using the iSALE shock physics code. Three-dimensional impact simulations show that a typical basin-forming impact on Earth can eject solid fragments equivalent to ~10–3 of an impactor mass at speeds sufficient to transfer from Earth to the Moon. Previous modelling of meteorite survivability has relied heavily upon the assumption that peak-shock pressures can be used as a proxy for gauging survival of projectiles and their possible biomarker constituents. Here, we show the importance of considering both pressure and temperature within the projectile, and the inclusion of both shock and shear heating, in assessing biomarker survival. Assuming that they survive launch from Earth, we show that some biomarker molecules within terrestrial meteorites are likely to survive impact with the Moon, especially at the lower end of the range of typical impact velocities for terrestrial meteorites (2.5 km s-1). The survival of larger biomarkers (e.g., microfossils) is also assessed, and we find limited, but significant, survival for low impact velocity and high target porosity scenarios. Thermal degradation of biomarkers shortly after impact depends heavily upon where the projectile material lands, whether it is buried or remains on the surface, and the related cooling timescales. Comparing sandstone and limestone projectiles shows similar temperature and pressure profiles for the same impact velocities, with limestone providing slightly more favourable conditions for biomarker survival

Numerische Modellierung impaktinduzierter Stoß- und elastischer Wellenausbreitung sowie Kraterbildung in heterogenem Material

Impact processes have shaped the development and evolution of the planetary bodies in our solar system. Despite the importance of this fundamental geological process, the consequences of impact events for targets with varying properties have not been sufficiently quantified. Thus, this thesis investigates the effect of target properties on impact-induced shock and elastic wave propagation and the crater formation using numerical simulations. This approach aims to offer a better understanding of impact processes on heterogeneous targets, which is key for quantitatively assessing of the role of impact and collision processes in the formation of the solar system and the evolution of planetary surfaces. For this study, the iSALE shock physics code has been used to conduct numerical simulations of impact processes within the Multidisciplinary Experimental and Modeling Impact Research Network (MEMIN). The usage of numerical models first requires rigorous validation and calibration of numerical parametrizations of the thermodynamic and mechanical response of material upon impact (so-called material models) against experimental observations. Then, the study focuses on the simulation of laboratory impact experiments in quartzite and in dry and water-saturated sandstone. Finally, the numerical data are applied to impact cratering in nature. To investigate the entire cratering process in detail, the first thing needed is an understanding of the propagation of the shock wave and how the target material responds to shock loading as a function of petrophysical properties is needed. To provide detailed quantitative insights, mesoscale models, where single pores and grains are resolved, have been developed and analyzed in this thesis to gain a detailed understanding of shock wave-induced pore collapse. Pore collapse results in localized pressure amplifications, which can be up to four times greater than the average shock pressure in a porous sample. Mesoscale simulations, therefore, can explain the observed localized high shock pressure phases that appear next to more or less unshocked grains in impactites and meteorites; they can also explain the occurrence of shock effects such as the formation of diaplectic quartz glass in experiments in the low-pressure range. In addition to the investigation of the shock wave, the elastic wave, which eventually evolves from the initial shock wave, has been recorded and analyzed using numerical sensors in iSALE. A systematic modeling study of impacts into targets with varying properties and the analysis of recorded seismic signals resulted in the determination of the so-called seismic efficiency k, which relates the seismic energy to the impact energy. According to our results, k decreases slightly with porosity and is approximately two orders of magnitude lower for water-saturated materials than for dry nonporous material. The seismic quality factor Q, which quantifies how fast the wave attenuates, ranges between 35 and 80 for “dry” materials and is much lower (<10) for “wet” materials. The seismic magnitude of an impact event is about one order of magnitude larger for a solid or porous target than for a water-saturated target, showing that the seismic consequences are significantly dependent on target properties, and less seismic energy is induced if targets contain water. Finally, the numerical results obtained at the laboratory scale were then extrapolated to natural crater dimensions. Therefore, numerical models were used to investigate crater formation beyond the scale of laboratory impact experiments, where crater size is controlled by the yield strength of the target material. It is well known that on the scale of natural impact craters, crater size is primarily controlled by gravity. In the current study, scaling parameters have been determined for cohesive materials, whereby the dynamic strength of the materials was accounted for.Impaktprozesse haben wesentlich zur Entwicklung und Evolution von planetaren Körpern in unserem Sonnensystem beigetragen. Auch wenn die Bedeutung dieses fundamentalen Prozesses allgemein bekannt ist, wurden die Konsequenzen von Impaktereignissen unter der Berücksichtigung von Targeteigenschaften bisher nicht ausreichend quantifiziert. Diese Arbeit beschäftigt sich mit der Untersuchung des Einflusses von Targeteigenschaften auf die impaktinduzierte Stoßwellenausbreitung, die elastische Wellenausbreitung und auf die Kraterbildung unter Verwendung von numerischen Simulationen. Dieser Ansatz hat das Ziel, ein besseres Verständnis von Kraterprozessen in heterogenen Targetmaterialien zu erlangen. Dies ist Voraussetzung für eine quantitative Bewertung, welche Rolle Impakt- und Kollisionsprozesse in der Entwicklung unseres Sonnensystems und der Evolution von planetaren Oberflächen spielen. Der iSALE shock physics code wird benutzt um numerische Simulationen von Kraterprozessen im Rahmen des "Multidisciplinary Experimental and Modelling Impact research Network” (MEMIN) auszuführen. Die Verwendung von numerischen Modellen setzt eine komplexe Validierung und Kalibrierung von numerischen Parametrizierungen des thermodynamischen und mechanischen Verhaltens des Materials (sogenannter Materialmodelle), basierend auf experimentelle Beobachtungen, während eines Impaktes voraus. Die Studie konzentriert sich vorwiegend auf die Modellierung von Impaktexperimenten in Quarzit und in trockenen und wassergesättigten Sandstein. Letztendlich werden die numerischen Daten auf Impaktkrater in der Natur angewendet. Um den gesamten Kraterprozess im Detail zu untersuchen, ist es zunächst nötig ein gutes Verständnis über die Ausbreitung der Stoßwelle und wie das Targetmaterial als Funktion seiner petrophysikalischen Eigenschaften auf die Stoßwelle reagiert, zu erlangen. Mesoskalige Modelle, in denen einzelne Poren und Kornstrukturen aufgelöst werden, wurden entwickelt und analysiert um einem detaillierten Verständnis über stoßwelleninduzierten Porenkollaps gerecht zu werden. Porenkollaps führt zu lokalen Druckerhöhungen, die das Vierfache der gemittelten Stoßwellendrücke in einem porösen Material erreichen können. Mesoskalige Modelle können so das Auftreten von beobachteten lokalen Stoßwelleneffekten direkt neben ungeschockten Körnern in Impaktiten und Meteroriten sowie die Bildung von diaplaktischem Quarzglas in Experimenten im Niedrigdruckbereich erklären. Zusätzlich zu der Untersuchung der Stoßwellen, wurden elastische Wellen mit Hilfe von numerischen Sensoren aufgezeichnet und ausgewertet. Eine systematische Modellierungsstudie von Impakten in Zielgesteinen unterschiedlicher Eigenschaften und die Analyse aufgenommener seismischer Signale führt zur Bestimmung der sogenannten seismischen Effizienz k, welche die seismische Energie mit der Impaktenergie in Relation setzt. Laut der ausgeführten Studie nimmt k mit Zunahme der Porosität leicht ab und ist ungefähr zwei Größenordnungen kleiner für wassergesättigte Materialien als für Festgesteine ohne Wasseranteil. Der sogenannte seismische Qualitätsfaktor Q quantifiziert das Abklingverhalten der elastischen Welle in einem bestimmten Material, und liegt zwischen Werten von 35 und 80 für trockenes Material und ist signifikant kleiner (<10) für nasse Materialien. Die seismische Magnitude eines Impaktereignisses ist ungefähr eine Magnitude größer in einem Festgestein ohne Wasser als in einem Gestein welches Wasser enthält. Dies führt zu dem Schluss, dass seismische Konsequenzen signifikant von den Eigenschaften des Zielgesteins abhängig sind und weniger seismische Energie in wassergesättigte Gesteine induziert wird. Die numerischen Ergebnisse auf der Skala von Laborexperimenten konnten letztendlich auf natürliche Kraterdimensionen hochskaliert werden. Dafür wurden numerische Modelle verwendet um den Kraterbildungsprozess nicht nur auf der Skala von Laborexperimenten, wo die Kratergröße durch die Festigkeit des Zielgesteins dominiert wird, zu untersuchen. Auf der Skala von natürlichen Impaktkratern wird die Kratergröße hauptsächlich durch die Schwerkraft kontrolliert. Beim Hochskalieren der numerischen Ergebnisse wurde die dynamische Festigkeit des Materials berücksichtigt und Skalierungsparameter für Festgesteine bestimmt

Insight into the Distribution of High-pressure Shock Metamorphism in Rubble-pile Asteroids

Funding Information: This work was supported by the Academy of Finland, project Nos. 293975 and 335595, the European Regional Development Fund, the Mobilitas Pluss programme (grant No. MOBJD639), and the NASA Solar System Exploration Research Virtual Institute Center for Lunar and Asteroid Surface Science, and it was conducted within institutional support RVO 67985831 of the Institute of Geology of the Czech Academy of Sciences. R.L. appreciates funding from the European Union’s Horizon 2020 research and innovation program, NEO-MAPP, grant agreement No. 870377. Publisher Copyright: © 2022. The Author(s). Published by the American Astronomical Society.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 (∼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−1. 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 (<0.2). Varying the porosity of the boulders from 10% to 30% does not significantly affect the distribution of pressure and fraction of shock-darkened material. The pressure distribution in rubble-pile asteroids is very similar to that of monolithic asteroids with the same porosity. Thus, producing significant volumes of high-degree shocked ordinary chondrites requires strong collision events (impact velocities above 8 km s−1 and/or large sizes of impactors). A large amount of asteroid material escapes during an impact event (up to 90%); however, only a small portion of the escaping material is shock-darkened (6%).Peer reviewe

Effect of target properties and impact velocity on ejection dynamics and ejecta deposition

Impact craters are formed by the displacement and ejection of target material. Ejection angles and speeds during the excavation process depend on specific target properties. In order to quantify the influence of the constitutive properties of the target and impact velocity on ejection trajectories, we present the results of a systematic numerical parameter study. We have carried out a suite of numerical simulations of impact scenarios with different coefficients of friction (0.0–1.0), porosities (0–42%), and cohesions (0–150 MPa). Furthermore, simulations with varying pairs of impact velocity (1–20 km s−1) and projectile mass yielding craters of approximately equal volume are examined. We record ejection speed, ejection angle, and the mass of ejected material to determine parameters in scaling relationships, and to calculate the thickness of deposited ejecta by assuming analytical parabolic trajectories under Earth gravity. For the resulting deposits, we parameterize the thickness as a function of radial distance by a power law. We find that strength—that is, the coefficient of friction and target cohesion—has the strongest effect on the distribution of ejecta. In contrast, ejecta thickness as a function of distance is very similar for different target porosities and for varying impact velocities larger than ~6 km s−1. We compare the derived ejecta deposits with observations from natural craters and experiments