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Journal articleCollins GS, Lynch E, McAdam R, et al., 2017,
A numerical assessment of simple airblast models of impact airbursts
, Meteoritics & Planetary Science, Vol: 52, Pages: 1542-1560, ISSN: 1086-9379Asteroids and comets 10–100 m in size that collide with Earth disrupt dramatically in the atmosphere with an explosive transfer of energy, caused by extreme air drag. Such airbursts produce a strong blastwave that radiates from the meteoroid's trajectory and can cause damage on the surface. An established technique for predicting airburst blastwave damage is to treat the airburst as a static source of energy and to extrapolate empirical results of nuclear explosion tests using an energy-based scaling approach. Here we compare this approach to two more complex models using the iSALE shock physics code. We consider a moving-source airburst model where the meteoroid's energy is partitioned as two-thirds internal energy and one-third kinetic energy at the burst altitude, and a model in which energy is deposited into the atmosphere along the meteoroid's trajectory based on the pancake model of meteoroid disruption. To justify use of the pancake model, we show that it provides a good fit to the inferred energy release of the 2013 Chelyabinsk fireball. Predicted overpressures from all three models are broadly consistent at radial distances from ground zero that exceed three times the burst height. At smaller radial distances, the moving-source model predicts overpressures two times greater than the static-source model, whereas the cylindrical line-source model based on the pancake model predicts overpressures two times lower than the static-source model. Given other uncertainties associated with airblast damage predictions, the static-source approach provides an adequate approximation of the azimuthally averaged airblast for probabilistic hazard assessment.
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Journal articleForman LV, Bland PA, Timms NE, et al., 2017,
Defining the mechanism for compaction of the CV chondrite parent body
, Geology, Vol: 45, Pages: 559-562, ISSN: 1943-2682The Allende meteorite, a relatively unaltered member of the CV carbonaceous chondrite group, contains primitive crystallographic textures that can inform our understanding of early Solar System planetary compaction. To test between models of porosity reduction on the CV parent body, complex microstructures within ~0.5-mm-diameter chondrules and ~10-μm-long matrix olivine grains were analyzed by electron backscatter diffraction (EBSD) techniques. The large area map presented is one of the most extensive EBSD maps to have been collected in application to extraterrestrial materials. Chondrule margins preferentially exhibit limited intragrain crystallographic misorientation due to localized crystal-plastic deformation. Crystallographic preferred orientations (CPOs) preserved by matrix olivine grains are strongly coupled to grain shape, most pronounced in shortest dimension <a>, yet are locally variable in orientation and strength. Lithostatic pressure within plausible chondritic model asteroids is not sufficient to drive compaction or create the observed microstructures if the aggregate was cold. Significant local variability in the orientation and intensity of compaction is also inconsistent with a global process. Detailed microstructures indicative of crystal-plastic deformation are consistent with brief heating events that were small in magnitude. When combined with a lack of sintered grains and the spatially heterogeneous CPO, ubiquitous hot isostatic pressing is unlikely to be responsible. Furthermore, Allende is the most metamorphosed CV chondrite, so if sintering occurred at all on the CV parent body it would be evident here. We conclude that the crystallographic textures observed reflect impact compaction and indicate shock-wave directionality. We therefore present some of the first significant evidence for shock compaction of the CV parent body.
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Journal articleCollins GS, 2017,
Moon Formation: Punch Combo or Knock-out Blow
, Nature Geoscience, ISSN: 1752-0908 -
Journal articleMorgan JV, Gulick SPS, Bralower T, et al., 2016,
The formation of peak rings in large impact craters
, Science, Vol: 354, Pages: 878-882, ISSN: 0036-8075Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.
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Journal articleJohnson BC, Blair DM, Collins GS, 2016,
Formation of the Orientale lunar multiring basin
, Science, Vol: 354, Pages: 441-444, ISSN: 0036-8075Multiring basins, large impact craters characterized by multiple concentric topographic rings, dominate the stratigraphy, tectonics, and crustal structure of the Moon. Using a hydrocode, we simulated the formation of the Orientale multiring basin, producing a subsurface structure consistent with high-resolution gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) spacecraft. The simulated impact produced a transient crater, ~390 kilometers in diameter, that was not maintained because of subsequent gravitational collapse. Our simulations indicate that the flow of warm weak material at depth was crucial to the formation of the basin’s outer rings, which are large normal faults that formed at different times during the collapse stage. The key parameters controlling ring location and spacing are impactor diameter and lunar thermal gradients.
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Journal articleKring DA, Kramer GY, Collins GS, et al., 2016,
Peak-Ring Structure and Kinematics from a Multi-disciplinary Study of the Schrödinger Impact Basin
, Nature Communications, Vol: 7, ISSN: 2041-1723The Schrödinger basin on the lunar farside is ~320 km in diameter and the best-preservedpeak-ring basin of its size in the Earth–Moon system. Spectral and photogeologic analyses ofdata from the Moon Mineralogy Mapper instrument on the Chandrayaan-1 spacecraft and theLunar Reconnaissance Orbiter Camera (LROC) on the LRO spacecraft indicate the peak ring iscomposed of anorthositic, noritic, and troctolitic lithologies that were juxtaposed by severalcross-cutting faults during peak ring formation. Hydrocode simulations indicate the lithologieswere uplifted from depths up to 30 km, representing the crust of the lunar farside. Combining2geological and remote-sensing observations with numerical modeling, here we show a DisplacedStructural Uplift model is best for peak rings, including that in the K-T Chicxulub impact crateron Earth. These results may help guide sample selection in lunar sample return missions that arebeing studied for the multi-agency International Space Exploration Coordination Group.Determining which lunar landing site may yield information about the lunar interior is veryimportant with impact basins usually the best sites. Kring et al. provide a geological map of theSchrödinger basin on the moon via a multidisciplinary approach of remote sensing and numericalmodeling.
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Conference paperPenny C, Muxworthy AR, Fabian K, 2016,
The Curie temperature of magnetite nanoparticles (poster)
, EMRS Fall 2016 -
Conference paperShah J, Bates H, Muxworthy AR, et al., 2016,
A micro-CT conglomerate test (poster)
, 15th Castle Meeting -
Conference paperPenny C, Muxworthy AR, Fabian K, 2016,
The Curie temperature of magnetite nanoparticles (poster)
, 15th Castle Meeting -
Conference paperShah J, Muxworthy AR, Almeida TP, et al., 2016,
Determining the magnetic recording fidelity of chondrule dusty olivine
, 15th Castle Meeting -
Journal articleAlmeida TP, Muxworthy AR, Kovacs A, et al., 2016,
Direct observation of the thermal demagnetization of magnetic vortex structures in non-ideal magnetite recorders
, Geophysical Research Letters, Vol: 43, Pages: 8426-8434, ISSN: 1944-8007The thermal demagnetization of pseudo-single-domain (PSD) magnetite (Fe3O4) particles, which govern the magnetic signal in many igneous rocks, is examined using off-axis electron holography. Visualization of a vortex structure held by an individual Fe3O4 particle (~ 250 nm in diameter) during in situ heating is achieved through the construction and examination of magnetic-induction maps. Step-wise demagnetization of the remanence-induced Fe3O4 particle upon heating to above the Curie temperature, performed in a similar fashion to bulk thermal demagnetization measurements, revealed its vortex state remains stable under heating close to its unblocking temperature, and is recovered upon cooling with the same or reversed vorticity. Hence, the PSD Fe3O4 particle exhibits thermomagnetic behavior comparable to a single-domain carrier, and thus vortex-states are considered reliable magnetic recorders for paleomagnetic investigations.
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Conference paperShah J, Muxworthy AR, Almeida T, et al., 2016,
Hot Holography: Magnetic recording fidelity of dusty olivine (poster)
, 13th UK Planetary Forum Early Career Scientists’ Meeting -
Journal articleBerndt T, Muxworthy AR, Fabian K, 2016,
Does size matter? Statistical limits of paleomagnetic field reconstruction from small rock specimens
, Journal of Geophysical Research: Solid Earth, Vol: 121, Pages: 15-26, ISSN: 2169-9356As samples of ever decreasing sizes are being studied paleomagnetically, care has to be taken that the underlying assumptions of statistical thermodynamics (Maxwell-Boltzmann statistics) are being met. Here we determine how many grains and how large a magnetic moment a sample needs to have to be able to accurately record an ambient field. It is found that for samples with a thermoremanent magnetic moment larger than 10−11Am2 the assumption of a sufficiently large number of grains is usually given. Standard 25 mm diameter paleomagnetic samples usually contain enough magnetic grains such that statistical errors are negligible, but “single silicate crystal” works on, for example, zircon, plagioclase, and olivine crystals are approaching the limits of what is physically possible, leading to statistic errors in both the angular deviation and paleointensity that are comparable to other sources of error. The reliability of nanopaleomagnetic imaging techniques capable of resolving individual grains (used, for example, to study the cloudy zone in meteorites), however, is questionable due to the limited area of the material covered.
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Conference paperShah J, Muxworthy AR, Almeida TP, et al., 2016,
In-situ heating holography of chondrule dusty olivine
, Magnetic Interactions -
Conference paperShah J, Muxworthy AR, Almeida T, et al., 2015,
Electron Holography of Chondrule Dusty Olivine (poster)
, Meteorites and Solar System formation workshop -
Journal articleAlmeida TP, Muxworthy AR, Kovács A, et al., 2015,
Visualisation of high temperature magnetisation states in magnetite grains using off-axis electron holography
, Journal of Physics: Conference Series, ISSN: 1742-6588 -
Journal articleAbubakar R, Muxworthy AR, Southern P, et al., 2015,
Formation of magnetic minerals in hydrocarbon-generation conditions
, Marine and Petroleum Geology, ISSN: 1873-4073In this paper, we report the pyrolysis and formation of magnetic minerals in three source rock samples from the Wessex Basin in Dorset, southern England. The experimental conditions in the laboratory recreated the catagenesis environment of oil source rocks. Magnetic analysis of both the heated and the unheated samples at room temperature and at very low temperatures (5 K), coupled with transmission electron-microscopy imaging and X-ray analysis, revealed the formation of nanometre-sized (<10 nm), magnetic particles that varied across the rock samples analysed, but more importantly across the pyrolysis temperature range. Magnetic measurements demonstrated the formation of these magnetic minerals peaked at 250 °C for all rock samples and then decreased at 300 °C before rising again at 320 °C. The newly formed magnetic minerals are suggested to be primarily pyrrhotite, though magnetite and greigite are also thought to be present. The sizes of the magnetic minerals formed suggest a propensity to migrate together with oil potentially explaining the magnetic anomalies observed above and within oil fields.
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Journal articlePotter RWK, Kring DA, Collins GS, 2015,
Scaling of basin-sized impacts and the influence of target temperature
, Geological Society of America Special Papers, Vol: 518, Pages: 99-113, ISSN: 0072-1077We produce a set of scaling laws for basin-sized impacts using data from a suiteof lunar basin numerical models. The results demonstrate the importance of preimpacttarget temperature and thermal gradient, which are shown to greatly infl uencethe modifi cation phase of the impact cratering process. Impacts into targets withcontrasting thermal properties also produce very different crustal and topographicprofi les for impacts of the same energy. Thermal conditions do not, however, signifi -cantly infl uence the excavation stage of the cratering process; results demonstrate,as a consequence of gravity-dominated growth, that transient crater radii are generallywithin 5% of each other over a wide range of thermal gradients. Excavationdepth-to-diameter ratios for the basin models (~0.12) agree well with experimental,geological, and geophysical estimates, suggesting basins follow proportional scaling.This is further demonstrated by an agreement between the basin models andPi- scaling laws based upon fi rst principles and experimental data. The results of thiswork should also be applicable to basin-scale impacts on other silicate bodies, includingthe Hadean Earth.
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Journal articleAlmeida TP, Muxworthy AR, Kasama T, et al., 2015,
Effect of maghemization on the magnetic properties of nonstoichiometric pseudo-single-domain magnetite particles
, Geochemistry Geophysics Geosystems, Vol: 16, Pages: 2969-2979, ISSN: 1525-2027The effect of maghemization on the magnetic properties of magnetite (Fe3O4) grains in the pseudo-single-domain (PSD) size range is investigated as a function of annealing temperature. X-ray diffraction and transmission electron microscopy confirms the precursor grains as Fe3O4 ranging from ~ 150 nm to ~ 250 nm in diameter, whilst Mössbauer spectrometry suggests the grains are initially near-stoichiometric. The Fe3O4 grains are heated to increasing reaction temperatures of 120 – 220 ºC to investigate their oxidation to maghemite (γ-Fe2O3). High-angle annular dark field imaging and localized electron energy loss spectroscopy reveals slightly oxidized Fe3O4 grains, heated to 140 ºC, exhibit higher oxygen content at the surface. Off-axis electron holography allows for construction of magnetic induction maps of individual Fe3O4 and γ-Fe2O3 grains, revealing their PSD (vortex) nature, which is supported by magnetic hysteresis measurements, including first order reversal curve analysis. The coercivity of the grains is shown to increase with reaction temperature up to 180 ºC, but subsequently decreases after heating above 200 ºC; this magnetic behavior is attributed to the growth of a γ-Fe2O3 shell with magnetic properties distinct from the Fe3O4 core. It is suggested there is exchange coupling between these separate components that results in a vortex state with reduced vorticity. Once fully oxidized to γ-Fe2O3, the domain states revert back to vortices with slightly reduced coercivity. It is argued that due to a core/shell coupling mechanism during maghemization, the directional magnetic information will still be correct, however, the intensity information will not be retained.
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Conference paperShah J, Muxworthy AR, Russell SS, et al., 2015,
USING MICRO-CT TO MAP METEORITIC MAGNETISM.
, 78th Annual Meeting of the Meteoritical-Society, Publisher: WILEY-BLACKWELL, ISSN: 1086-9379
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