Jemma Davidson^a, Henner Busemann^a,b, Larry R. Nittler^c, Conel M.O’D. Alexander^c, François-Régis Orthous-Daunay^d, Ian A. Franchi^a, Peter Hoppe^e

^aPlanetary and Space Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
^bSchool of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
^cDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington DC, 20015-1305, USA
^dInstitut de Planétologie et d’Astrophysique de Grenoble, UJF CNRS/INSU 38000 Grenoble, France
^eMax-Planck-Institut für Chemie, P.O. Box 3060, 55020 Mainz, Germany

It has been suggested that the matrices of all chondrites are dominated by a common material with Ivuna-like (CI) abundances of volatiles, presolar grains and insoluble organic matter (IOM) (e.g., Alexander, 2005). However, matrix-normalized abundances of presolar silicon carbide (SiC) grains estimated from their noble gas components show significant variations in even the most primitive chondrites (Huss and Lewis, 1995 and Huss et al., 2003), in contradiction to there being a common chondrite matrix material. Here we report presolar SiC abundances determined by NanoSIMS raster ion imaging of IOM extracted from primitive members of different meteorite groups. We show that presolar SiC abundance determinations are comparable between NanoSIMS instruments located at three different institutes, between residues prepared by different demineralization techniques, and between microtomed and non-microtomed samples. Our derived SiC abundances in CR chondrites are comparable to those found in the CI chondrites (~30 ppm) and are much higher than previously determined by noble gas analyses. The revised higher CR SiC abundances are consistent with the CRs being amongst the most primitive chondrites in terms of the isotopic compositions and disordered nature of their organic matter. Similar abundances between CR1, CR2, and CR3 chondrites indicate aqueous alteration on the CR chondrite parent body has not progressively destroyed SiC grains in them. A low SiC abundance for the reduced CV3 RBT 04133 can be explained by parent body thermal metamorphism at an estimated temperature of ~440°C. Minor differences between primitive members of other meteorite classes, which did not experience such high temperatures, may be explained by prolonged oxidation at lower temperatures under which SiC grains formed outer layers of SiO₂ that were not thermodynamically stable, leading to progressive degassing/destruction of SiC.

Reference
Davidson J, Busemann H, Nittler LR, Alexander CMO’D, Orthous-Daunay F-R, Franchi IA and Hoppe P (in press) Abundances of presolar silicon carbide grains in primitive meteorites determined by NanoSIMS. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.026]
Copyright Elsevier

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Graziella Caprarelli

Division of IT, Engineering and the Environment (DITEE), University of South Australia, GPO Box 2471, Adelaide, SA 5001, Australia

Impact craters on the surface of Mars are degraded by erosion and infilling due to combinations of geological processes. These result in modifications of relative crater dimensions, including diameter increase and reduction of rim-floor depths. In principle, the longer a crater is exposed to geological processes, the more pronounced the modifications. Visualization and analysis of these effects are achieved by plotting the measured depths (M) of impact craters versus the corresponding theoretical depths (predicted: P) calculated from the crater diameters using depth/Diameter power laws. This type of diagram is referred to as MPD (measured depth versus predicted depth diagram). The advantage of using the MPD representation consists in the fact that the data plot along linear regressions, more easily interpreted than standard depth vs. diameter diagrams.
As an example of application of the method, the MPD was used to discriminate different generations of impact craters in Terra Sabaea into four groups: T₀ (fresh craters), T₁, T₂ and T₃ (from younger to older), all located on the most ancient geological unit in the area (Npld). Other units in the area are Hpl₃ and Hr, impacted only by craters belonging to group T₀, suggesting that these units are stratigraphically correlated. The data of 5 craters in superposition relationships with the eastern reaches of Evros Vallis, one of the major valley networks in the area, were plotted in the diagram and assigned each to a regression depending on the location of their data points in relation to the prediction bands of the regressions. The craters superposed to the valley all belonged to T₀, indicating that Evros Vallis has the same relative age of units Hpl₃ and Hr.
A conceptual discussion of the results demonstrates that MPD statistics (a) are unaffected by the procedures used to acquire depths and diameters of impact craters and by the power laws used, and (b) can be interpreted irrespective of the sequence or combination of processes leading to modification of the crater morphometric data. These properties make the diagram a powerful statistical tool.

Reference
Caprarelli G (in press) Introducing and discussing a novel diagrammatic representation of impact crater dimensions. Icarus
[doi:10.1016/j.icarus.2014.04.051]
Copyright Elsevier

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R. Stevenson¹, J. M. Bauer^1,2, E. A. Kramer³, T. Grav⁴, A. K. Mainzer¹ and J. R. Masiero¹

¹Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS 183-427, Pasadena, CA 91109, USA
²Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
³Department of Physics, University of Central Florida, 4000 Central Florida Blvd, Orlando, FL 32816, USA
⁴Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA

Comet 17P/Holmes underwent a massive outburst in 2007 October, brightening by a factor of almost a million in under 48 hr. We used infrared images taken by the Wide-Field Infrared Survey Explorer mission to characterize the comet as it appeared at a heliocentric distance of 5.1 AU almost 3 yr after the outburst. The comet appeared to be active with a coma and dust trail along the orbital plane. We constrained the diameter, albedo, and beaming parameter of the nucleus to 4.135 ± 0.610 km, 0.03 ± 0.01, and 1.03 ± 0.21, respectively. The properties of the nucleus are consistent with those of other Jupiter family comets. The best-fit temperature of the coma was 134 ± 11 K, slightly higher than the blackbody temperature at that heliocentric distance. Using Finson–Probstein modeling, we found that the morphology of the trail was consistent with ejection during the 2007 outburst and was made up of dust grains between 250 μm and a few cm in radius. The trail mass was ~1.2–5.3 × 10¹⁰ kg.

Reference
Stevenson R, Bauer JM, Kramer EA, Grav T, Mainzer AK and Masiero JR (2014) Lingering Grains of Truth around Comet 17P/Holmes. The Astrophysical Journal 787:116.
[doi:10.1088/0004-637X/787/2/116]

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