Large sulfur isotope fractionations in Martian sediments at Gale crater

Variability in the sulfur isotopic composition in sediments can reflect atmospheric, geologic and biological processes. Evidence for ancient fluvio-lacustrine environments at Gale crater on Mars and a lack of efficient crustal recycling mechanisms on the planet suggests a surface environment that was once warm enough to allow the presence of liquid water, at least for discrete periods of time, and implies a greenhouse effect that may have been influenced by sulfur-bearing volcanic gases. Here we report in situ analyses of the sulfur isotopic compositions of SO2 volatilized from ten sediment samples acquired by NASA’s Curiosity rover along a 13 km traverse of Gale crater. We find large variations in sulfur isotopic composition that exceed those measured for Martian meteorites and show both depletion and enrichment in 34S. Measured values of δ34S range from −47 ± 14‰ to 28 ± 7‰, similar to the range typical of terrestrial environments. Although limited geochronological constraints on the stratigraphy traversed by Curiosity are available, we propose that the observed sulfur isotopic signatures at Gale crater can be explained by equilibrium fractionation between sulfate and sulfide in an impact-driven hydrothermal system and atmospheric processing of sulfur-bearing gases during transient warm periods

A Global Fireball Observatory

The world's meteorite collections contain a very rich picture of what the early Solar System would have been made of, however the lack of spatial context with respect to their parent population for these samples is an issue. The asteroid population is equally as rich in surface mineralogies, and mapping these two populations (meteorites and asteroids) together is a major challenge for planetary science. Directly probing asteroids achieves this at a high cost. Observing meteorite falls and calculating their pre-atmospheric orbit on the other hand, is a cheaper way to approach the problem. The Global Fireball Observatory (GFO) collaboration was established in 2017 and brings together multiple institutions (from Australia, USA, Canada, Morocco, Saudi Arabia, the UK, and Argentina) to maximise the area for fireball observation time and therefore meteorite recoveries. The members have a choice to operate independently, but they can also choose to work in a fully collaborative manner with other GFO partners. This efficient approach leverages the experience gained from the Desert Fireball Network (DFN) pathfinder project in Australia. The state-of-the art technology (DFN camera systems and data reduction) and experience of the support teams is shared between all partners, freeing up time for science investigations and meteorite searching. With all networks combined together, the GFO collaboration already covers 0.6% of the Earth's surface for meteorite recovery as of mid-2019, and aims to reach 2% in the early 2020s. We estimate that after 5 years of operation, the GFO will have observed a fireball from virtually every meteorite type. This combined effort will bring new, fresh, extra-terrestrial material to the labs, yielding new insights about the formation of the Solar System.Comment: Accepted in PSS. 19 pages, 9 figure

Insights into complex layered ejecta emplacement and subsurface stratigraphy in Chryse Planitia, Mars, through an analysis of THEMIS brightness temperature data

© 2016. American Geophysical Union. All Rights Reserved. Layered ejecta craters on Mars have been interpreted to indicate the presence of volatiles in the substrate, making them important targets for the investigation of sites of astrobiological significance. If the ejecta are associated with the presence of water in the substratum, specific surface grain size trends are expected. In this study we explore the distribution of grain sizes in the layered ejecta of impact craters located in Chryse Planitia, using Thermal Emission Imaging System (THEMIS) thermal infrared data. Ejecta grain size trends, in conjunction with ejecta mobility and lobateness values, are applied to assess the degree of surface flow of the ejecta, and in turn to constrain the plausible volatile abundance, cohesion, and fine particle content of the target materials. Craters with a larger fraction of small grain sizes in their ejecta showed greater ejecta mobility and lobateness, consistent with a water-rich and/or a low-cohesion target. Craters displaying decreasing grain size with increasing radius had smaller diameters and lower ejecta mobility and lobateness, indicating only a minimal component of surface ejecta flow. Ejecta grain size trends varied with crater diameter, from which the presence of vertical compositional stratigraphy in Chryse Planitia is inferred and interpreted. Our observations are synthesized into a number of plausible geologic scenarios for Chryse Planitia

Formation of complex craters in layered targets with material anisotropy

Meteorite impacts often occur in layered targets, where the strength of the target varies as a function of depth, but this complexity is often not represented in numerical impact simulations because of the high computational cost of resolving thin layers. To address this limitation, we developed a method to approximate the effect of multiple thin weak layers within a sedimentary sequence using a single material layer to represent the entire sequence. Our approach, implemented in the iSALE (impact‐Simplified Arbitrary Lagrangian Eulerian) shock physics code, combines an anisotropic yield criterion with a cell‐based method to track the orientation of layers. To demonstrate the efficacy of the method and constrain parameters of the anisotropic strength model required to replicate the effects of thin, weak layers, we compare results of simulations of a ~20 – 25‐km diameter complex crater on Earth using the new method to those from simulations that explicitly resolve multiple thin weak layers. We show that our approach allows for a reduction in computational cost, negating the need for an increase in spatial resolution to resolve thin layers in the target, while replicating crater formation and final morphology from the high‐resolution models. In keeping with field observations, we also find that anisotropic layers may be responsible for a lack of central uplift expression observed at many craters formed in targets with thick sedimentary layers (e.g., the Haughton and Ries impact structures)

Extensional tectonics of the Outer Hebrides Fault Zone, south Uist, NW Scotland

The Outer Hebrides Fault Zone is a major ESE-dipping reactivated structure within Lewisian basement gneisses of the Laurentian craton, northwest Scotland. Detailed mapping in South Uist reveals important new evidence that contributes to a better understanding of the kinematic evolution of the fault zone. Large quantities of pseudotachylite which characterize the fault zone on South Uist may in part be lithologically controlled, and therefore of little value in determining areas of greatest deformation and displacement. Only limited evidence is preserved for ductile and brittle thrust-sense movements along this portion of the fault zone. The tectonics of the fault zone oil South Uist are dominated by structures associated with several episodes of pervasive top-down-to-the-SE to -ENE brittle extensional deformation, which are progressively overprinted by protophyllonitic and phyllonitic fabrics associated with top-down-to-the-E to -ENE extension. A series of late-stage high-angle normal faults record top-down-to-the-ESE to -ENE extension and cut the phyllonites. Fluid inclusion studies from syntectonic quartz veins constrain the conditions of phyllonite formation at 370 +/- 20 degreesC. Field evidence suggests that this section of the Outer Hebrides Fault Zone may have been largely unaffected by sinistral strike-slip reactivation as reported along-strike to the north, suggesting both a varied and compartmentalized tectonic and evolutionary history along the length of the Outer Hebrides Fault Zone.</p