J. E. CHAPPELOW
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12853]
Meteorifics Inc., 1148 Sundance Loop, Fairbanks, Alaska 99709, USA
Published by Arrangement with John Wiley & Sons

Recent work on the shapes of small, simple impact craters on the Moon has shown that the parabolic ideal does not well represent the vast majority of these craters. They are hyperbolic in shape and usually resemble a cone more than a parabola. A parabolic shape also does not fit the most commonly held archetype for simple craters in general (Linné), which is also hyperbolic. In addition, Linné itself may not be the best model for fresh simple craters, in terms of cross-sectional shape, although shape data to compare it to have heretofore been lacking. Here, the “free shadowfront method” for determining the shapes of simple craters is used to measure 64 fresh simple craters on five lunar maria to test both assumptions. Laser altimetry cross sections, available for many of the craters measured herein, are used to complement and spot-check the shadow measurement results, and thereby demonstrate the efficacy of the free shadowfront method. A new shape model is established, and two craters that better fit this model than Linné are identified. These are located at 24.45° N/328.12° E and 31.35° N/296.46° E and have diameters of 1.40 and 2.73 km, respectively. An apparent dichotomy between fresh simple craters smaller than 2.5 km and those larger than this is observed. Flat floors are found to be ubiquitous among the larger craters, but rare and small in extent in smaller ones. A slide in one crater which appears to be an incipient flat floor suggests a major mode of formation for these flat floors.

¹Clemens M. RUMPF, ¹Hugh G. LEWIS, and ^2,3,4Peter M. ATKINSON
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12861]
¹Atronautics Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, UK
²Faculty of Science and Technology, Lancaster University, Lancaster, UK
³Geography and Environment, University of Southampton, Southampton, UK
⁴School of Geography, Archaeology and Palaeoecology, Queen’s University Belfast, Belfast, UK
Published by Arrangement with John Wiley & Sons

An asteroid impact is a low probability event with potentially devastating consequences. The Asteroid Risk Mitigation Optimization and Research (ARMOR) software tool calculates whether a colliding asteroid experiences an airburst or surface impact and calculates effect severity as well as reach on the global map. To calculate the consequences of an impact in terms of loss of human life, new vulnerability models are derived that connect the severity of seven impact effects (strong winds, overpressure shockwave, thermal radiation, seismic shaking, ejecta deposition, cratering, and tsunamis) with lethality to human populations. With the new vulnerability models, ARMOR estimates casualties of an impact under consideration of the local population and geography. The presented algorithms and models are employed in two case studies to estimate total casualties as well as the damage contribution of each impact effect. The case studies highlight that aerothermal effects are most harmful except for deep water impacts, where tsunamis are the dominant hazard. Continental shelves serve a protective function against the tsunami hazard caused by impactors on the shelf. Furthermore, the calculation of impact consequences facilitates asteroid risk estimation to better characterize a given threat, and the concept of risk as well as its applicability to the asteroid impact scenario are presented.

^1,2Antoine S. G. Roth,³Reto Trappitsch,⁴Knut Metzler,⁵Beda A. Hofmann,¹Ingo Leya
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12868]
¹Institute of Physics, University of Bern, Bern, Switzerland
²Present Address: Department of Earth Sciences, ETH Zurich, Zurich, Switzerland
³Department of the Geophysical Sciences, The University of Chicago and Chicago Center for Cosmochemistry, Chicago, USA
⁴Institute for Planetology, University of M€unster, Muenster, Germany
⁵Natural History Museum Bern, Bern, Switzerland
Published by Arrangement with John Wiley & Sons

Solar-cosmic-ray-produced Ne (SCR-Ne), in the form of low cosmogenic ²¹Ne/²²Ne ratios (²¹Ne/²²Ne_cos <0.8), is more likely to be found in rare meteorite classes, like Martian meteorites, than in ordinary chondrites. This may be the result of a sampling bias: SCR-Ne is better preserved in meteorites with small preatmospheric radii and these specimens are often only studied if they belong to unusual or rare classes. We measured He and Ne isotopic concentrations and nuclear tracks in 25 small unpaired ordinary chondrites from Oman. Most chondrites have been intensively heated during atmospheric entry as evidenced by the disturbed track records, the low ³He/²¹Ne ratios, the low ⁴He concentrations, and the high peak release temperatures. Concentration depth profiles indicate significant degassing; however, the Ne isotopes are mainly undisturbed. Remarkably, six chondrites have low ²¹Ne/²²Ne_cos in the range 0.711–0.805. Using a new physical model for the calculation of SCR production rates, we show that four of the chondrites contain up to ~20% of SCR-Ne; they are analyzed in terms of preatmospheric sizes, cosmic ray exposure ages, mass ablation losses, and orbits. We conclude that SCR-Ne is preserved, regardless of the meteorite class, in specimens with small preatmospheric radii. Sampling bias explains the predominance of SCR-Ne in rare meteorites, although we cannot exclude that SCR-Ne is more common in Martian meteorites than it is in small ordinary chondrites.

^1,2Eva-Regine Carl,³Ulrich Mansfeld,⁴Hanns-Peter Liermann,²Andreas Danilewsky,³Falko Langenhorst,^5,6Lars Ehm,^4,7Ghislain Trullenque,¹Thomas Kenkmann
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12840]
¹Institut für Geo- und Umweltnaturwissenschaften, Geologie, Albert-Ludwigs-Universität, Albertstr. 23b, 79104 Freiburg,Germany2
²Institut für Geo- und Umweltnaturwissenschaften, Kristallographie, Albert-Ludwigs-Universität, Hermann-Herder-Str. 5, 79104 Freiburg, Germany
³Institut für Geowissenschaften, Mineralogie, Friedrich-Schiller-Universität Jena, Carl-Zeiss-Promenade 10, 07745 Jena, Germany
⁴DESY, Notkestraße 85, 22607 Hamburg, Germany
⁵Stony Brook University, Mineral Physics Institute, Stony Brook, NY 11794-2100, USA
⁶National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY 11973-500, USA
⁷Institut Polytechnique LaSalle Beauvais, Departement GEOS,equipe B2R 19 rue Pierre Waguet – BP 30313, 60026 BeauvaisCedex, France
Published by Arrangement with John Wiley & Sons

Hypervelocity collisions of solid bodies occur frequently in the solar system and affect rocks by shock waves and dynamic loading. A range of shock metamorphic effects and high-pressure polymorphs in rock-forming minerals are known from meteorites and terrestrial impact craters. Here, we investigate the formation of high-pressure polymorphs of α-quartz under dynamic and nonhydrostatic conditions and compare these disequilibrium states with those predicted by phase diagrams derived from static experiments under equilibrium conditions. We create highly dynamic conditions utilizing a mDAC and study the phase transformations in α-quartz in situ by synchrotron powder X-ray diffraction. Phase transitions of α-quartz are studied at pressures up to 66.1 and different loading rates. At compression rates between 0.14 and 1.96 GPa s⁻¹, experiments reveal that α-quartz is amorphized and partially converted to stishovite between 20.7 GPa and 28.0 GPa. Therefore, coesite is not formed as would be expected from equilibrium conditions. With the increasing compression rate, a slight increase in the transition pressure occurs. The experiments show that dynamic compression causes an instantaneous formation of structures consisting only of SiO₆ octahedra rather than the rearrangement of the SiO₄tetrahedra to form a coesite. Although shock compression rates are orders of magnitude faster, a similar mechanism could operate in impact events.

John A. Grant, Sharon A. Wilson
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12843]
Center for Earth and Planetary Studies, Smithsonian National Air and Space Museum, Washington,District of Columbia 20560, USA
Published by Arrangement with John Wiley & Sons

Hale crater formed in the Early to Middle Amazonian and is one of the best preserved large craters on Mars. We focus on the emplacement of previously mapped distal continuous ejecta and newly recognized discontinuous distal ejecta deposits reaching up to 450 km northeast of Hale. The distal continuous ejecta deposits are typically tens of meters thick, likely water-rich, and subsequent dewatering of some resulted in flow along gradients of 10 m km^-1 for distances of tens of kilometers. The discontinuous distal ejecta are typically <10 m thick with volumes generally <0.5 km³ and embay Hale secondaries, which occur up to ~600 km from Hale. Both continuous and discontinuous distal ejecta deposits are typically smooth at scales of tens to hundreds of meters, relatively dark-toned, devoid of eolian bedforms, inferred to be mostly fine-grained, and were likely emplaced within hours to 1–2 days after impact. The occurrence of well-preserved discontinuous distal ejecta at Hale is unusual compared to other large Martian craters and could be due to impact into an ice-rich substrate that enabled their formation and (or) their survival after minimal postimpact degradation relative to older craters. The pristine nature of distal continuous and discontinuous distal deposits at Hale and the preservation of associated secondaries imply (1) low erosion rates after the Hale impact, comparable to those estimated elsewhere during the Amazonian; (2) the impact did not significantly influence long-term global or regional scale geomorphic activity or climate; and (3) the Hale impact occurred after late alluvial fan activity in Margaritifer Terra.