New simulation of Phobos Stickney crater

In this work we model the Phobos Stickney impact crater using the iSALE hydrocode and considering different scenarios that could form the well-studied crater

Numerical modelling of heat generation in porous planetesimal collisions

An important unanswered question in planetary science is how planetesimals, the ~1–100 km solid precursors to asteroids and planets, were heated in the early Solar System. This thesis quantifies one possible heat source: planetesimal collisions. Recent work has predicted that collision velocities and planetesimal porosities were likely to have been higher than previously thought; this is likely to have significant implications on collision heating. The approach adopted in this research was to numerically model shock heating during planetesimal collisions. Simulations showed that an increase in porosity can significantly increase heating: in a 5 km s-1 collision between equal sized, non-porous planetesimals, no material was heated to the solidus, compared to two thirds of the mass of 50% porous planetesimals. Velocity also strongly influences heating: at 4 km s-1, an eighth of the mass of 50% porous planetesimals was heated to the solidus, compared to the entire mass at 6 km s-1. Further simulations quantified the influence on heating of the impactor-to-target mass ratio, the initial planetesimal temperature and the impact angle. A Monte Carlo model was developed to examine the cumulative heating caused by a population of impactors striking a parent body. In the majority of collisions the impactor was much smaller than the parent body, and only minor heating was possible. However, some larger or faster impactors were capable of causing significant heating without disrupting the parent body; these collisions could have heated up to 10% of the parent body to the solidus. To cause global heating, the collision must have catastrophically disrupted the parent body. The increase in specific internal energy from collisions was compared with the decay of short-lived radionuclides. In the first ~6 Ma, radioactive decay was the most important heat source. After ~10 Ma, the energy caused by collisions was likely to have overtaken radioactive decay as the dominant source

Shocked Quartz in Polymict Impact Breccia from the Upper Cretaceous Yallalie Impact Structure in Western Australia

Yallalie is a ~12 km diameter circular structure located ~200 km north of Perth, Australia. Previous studies have proposed that the buried structure is a complex impact crater based on geophysical data. Allochthonous breccia exposed near the structure has previously been interpreted as proximal impact ejecta; however, no diagnostic indicators of shock metamorphism have been found. Here we report multiple (27) shocked quartz grains containing planar fractures (PFs) and planar deformation features (PDFs) in the breccia. The PFs occur in up to five sets per grain, while the PDFs occur in up to four sets per grain. Universal stage measurements of all 27 shocked quartz grains confirms that the planar microstructures occur in known crystallographic orientations in quartz corresponding to shock compression from 5 to 20 GPa. Proximity to the buried structure (~4 km) and occurrence of shocked quartz indicates that the breccia represents either primary or reworked ejecta. Ejecta distribution simulated using iSALE hydrocode predicts the same distribution of shock levels at the site as those found in the breccia, which supports a primary ejecta interpretation, although local reworking cannot be excluded. The Yallalie impact event is stratigraphically constrained to have occurred in the interval from 89.8 to 83.6 Ma based on the occurrence of Coniacian clasts in the breccia and undisturbed overlying Santonian to Campanian sedimentary rocks. Yallalie is thus the first confirmed Upper Cretaceous impact structure in Australia

Numerical modelling of basin-scale impact crater formation

Understanding of basin-scale crater formation is limited; only a few examples of basin-scale craters exist and these are difficult to access. The approach adopted in this research was to numerically model basin-scale impacts with the aim of understanding the basin-forming process and basin structure. Research was divided into: (1) investigating early stage formation processes (impactor survivability), (2) investigating later stage formation processes (excavation and modification) and basin structure, and (3) constraining an impact scenario for the largest lunar crater, the South Pole-Aitken Basin. Various impact parameters were investigated, quantifying their effect on the basin-forming process. Simulations showed impactor survivability, the fraction of impactor remaining solid during the impact process, greatly increased if the impactor was prolate in shape (vertical length > horizontal length) rather than spherical. Low (≲15 km/s) impact velocities and low impact angles (≲30 ) also noticeably increased survivability. Lunar basin-scale simulations removed a significant volume of crustal material during impact, producing thinner post-impact crustal layers than those suggested by gravity-derived basin data. Most simulations formed large, predominantly mantle, melt pools; inclusion of a steep target thermal gradient and high internal temperatures greatly influenced melt volume production. Differences in crustal thickness between simulations and gravity-derived data could be accounted for by differentiation of the voluminous impact-generated melt pools, predicted by the simulations, into new crustal layers. Assuming differentiation occurs, simulation results were used to predict features such as transient crater size for a suite of lunar basins and tentatively suggest lunar thermal conditions during the basin-forming epoch. Additional simulations concerned the formation of the South Pole-Aitken Basin. By constraining simulation results to geochemical and gravity-derived basin data, a best-fit impact scenario for the South Pole-Aitken Basin was found

Simulation of Europa's water plume and structures related to energetic activities on solar system bodies from satellite images

This PhD thesis focuses on the analysis of different high energetic processes that affect the surface of planets, satellites and minor bodies as well as modify their surrounding environment. Specifically, this work concerns three main topics: (i) the simulation and analysis of one of the most geological energetic process, i.e. impact cratering; (ii) the investigation of the fragmentation processes that could have generated boulders on comet Churyumov-Gerasimenko 67P; (iii) the analysis of a possible transient plume originating from cryovolcanic events on Europa, the Jovian icy satellite, combined with an accurate characterisation of its exospheric background. The first topic addresses the investigation of the impact formation process through numerical modelling. Shocks code represent the most feasible method for studying impact craters, as they can simulate a large span of conditions beyond the reach of experiments (e.g., velocity, size). The iSALE hydrocode was used to simulate two different impact structures located on Mercury and Mars. On Mercury, the simulation allows to determine the genesis of a particular landform, i.e. a steep-sided cone with associated pyroclastic deposits, which was revealed by images acquired by MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. On Mars, the simulation of the Firsoff crater in Arabia Terra permits a better understanding the subsequent geologic processes that led to crater post modification, defining which rheological structure is more likely in that region. In both cases, we conclude that the numerical modelling of impact process is a powerful tool to improve the comprehension of the Solar System. The second topic of the thesis has been developed after the Rosetta mission got inserted around the comet Churyumov-Gerasimenko 67P. We investigated the surface of comet Churyumov-Gerasimenko 67P focusing on the possible energetic events that lead to the formation of boulders; i.e. blocks that are ubiquitous on the surface of the comet. Different energetic formation processes were invoked to explain the presence of boulders, such as sublimation, fragmentation, outbursts and gravitational falls. Using images acquired by OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) and CIVA (Comet Infrared and Visible Analyser) cameras on board the spacecraft and the lander respectively, a quantitative analysis of different-size boulders has been performed in order to understand if the same energetic formation processes occur equally on different scales on the comet (m, cm and mm). Specifically, by means of different resolution images, we obtained several size-frequency distribution for: (i) boulders larger than 7 m, (ii) boulders larger than 1 m from higher resolution images used to analyse the Abydos site, the location where Philae is supposed to be, and (iii) pebbles (mm-scale structures) visible on CIVA images. The third topic is the icy satellite Europa in view of the future ESA/JUICE mission and because of our involvement in JANUS (Jovis, Amorum ac Natorum Undique Scrutator) visible camera. The presence of a subsurface ocean is a primary topic on Europa, in addition the recent discovery of a transient plume at the south pole by HST observations has raised many questions regarding the interaction between the subsurface/surface and the outer environment of Europa in terms of active processes affecting the icy surface. In this context, a possible plume deposit originating from cryovolcanic events was simulated to understand its detectability by JANUS camera during the Europa flyby phase of the JUICE mission. In addition, since the study of transient plumes has as a mandatory prerequisite an accurate characterisation of the exospheric background, a detailed study of the loss rates of Europa's tenuous atmosphere was performed. In particular, loss rates for electron impact dissociation and ionization processes, for charge-exchange (considering plasma torus, pick up and ionosphere ions) and for photo processes (for both cases of quite and active Sun) were calculated

The effect of target properties on crater morphology: Comparison of central peak craters on the Moon and Ganymede

We examine the morphology of central peak craters on the Moon and Ganymede in order to investigate differences in the near-surface properties of these bodies. We have extracted topographic profiles across craters on Ganymede using Galileo images, and use these data to compile scaling trends. Comparisons between lunar and Ganymede craters show that crater depth, wall slope and amount of central uplift are all affected by material properties. We observe no major differences between similar-sized craters in the dark and bright terrain of Ganymede, suggesting that dark terrain does not contain enough silicate material to significantly increase the strength of the surface ice. Below crater diameters of ~12 km, central peak craters on Ganymede and simple craters on the Moon have similar rim heights, indicating comparable amounts of rim collapse. This suggests that the formation of central peaks at smaller crater diameters on Ganymede than the Moon is dominated by enhanced central floor uplift rather than rim collapse. Crater wall slope trends are similar on the Moon and Ganymede, indicating that there is a similar trend in material weakening with increasing crater size, and possibly that the mechanism of weakening during impact is analogous in icy and rocky targets. We have run a suite of numerical models to simulate the formation of central peak craters on Ganymede and the Moon. Our modeling shows that the same styles of strength model can be applied to ice and rock, and that the strength model parameters do not differ significantly between materials.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

The effect of the oceans on the terrestrial crater size-frequency distribution: Insight from numerical modeling

From the proceedings of the Workshop on Impact Craters as Indicators for Planetary Environmental Evolution and Astrobiology held in June 2006 in Östersund, Sweden.On Earth, oceanic impacts are twice as likely to occur as continental impacts, yet the effect of the oceans has not been previously considered when estimating the terrestrial crater size-frequency distribution. Despite recent progress in understanding the qualitative and quantitative effect of a water layer on the impact process through novel laboratory experiments, detailed numerical modeling, and interpretation of geological and geophysical data, no definitive relationship between impactor properties, water depth, and final crater diameter exists. In this paper, we determine the relationship between final (and transient) crater diameter and the ratio of water depth to impactor diameter using the results of numerical impact models. This relationship applies for normal incidence impacts of stoney asteroids into water-covered, crystalline oceanic crust at a velocity of 15 km s-1. We use these relationships to construct the first estimates of terrestrial crater size-frequency distributions (over the last 100 million years) that take into account the depth-area distribution of oceans on Earth. We find that the oceans reduce the number of craters smaller than 1 km in diameter by about two-thirds, the number of craters ~30 km in diameter by about one-third, and that for craters larger than ~100 km in diameter, the oceans have little effect. Above a diameter of ~12 km, more craters occur on the ocean floor than on land; below this diameter more craters form on land than in the oceans. We also estimate that there have been in the region of 150 impact events in the last 100 million years that formed an impact-related resurge feature, or disturbance on the seafloor, instead of a crater.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202