A. V. Fisenko¹, A. B. Verchovsky², L. F. Semjonova¹

¹Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia
²Department of Physical Sciences, Open University, Milton Keynes, UK

The kinetics of the release of the Xe-P3 component from coarse-grained fractions of Orgueil (CI) meteorite nanodiamonds has been investigated using stepped and isothermal pyrolysis. It has been shown that a first-order chemical reaction diffusion model with a single activation energy cannot provide a satisfactory explanation for the observed retention of Xe-P3 during parent body thermal metamorphism and the kinetics of Xe-P3 release from nanodiamonds during isothermal pyrolysis. Using the activation energy and frequency factor calculated according to this model, it is shown that in the course of thermal metamorphism of the Orgueil meteorite almost the entire Xe-P3 component must have been lost in a very short time (<4 yr at approximately 100 °C). However, the calculated retention of Xe-P3 increases significantly if a diffusion model with a spectrum of activation energies is used. In this case, the model can explain not only a high retention of Xe-P3 in the Orgueil nanodiamonds but also the release pattern of the Xe-P3 from Semarkona and Bishunpur nanodiamonds that have experienced a significant gas loss during parent body metamorphism as well as the release of Xe-P3 during isothermal pyrolysis of the Orgueil nanodiamonds. The energetically complicated Xe-P3 distribution is most likely caused by structural damage to the nanodiamond grains or a complex phase composition of carbon in the surface layer of the diamond grains. It is supposed that the structural damage of the diamond grains can have a radiation origin, while the variations of the carbon phase composition in the grain’s mantle can be caused by the radiation-induced reactions and/or a thermal effect.

Reference
Fisenko AV, Verchovsky AB and Semjonova LF (in press) Kinetics of Xe-P3 release during pyrolysis of the coarse-grained fractions of Orgueil (CI) meteorite nanodiamonds. Meteoritics & Planetary Science
[doi:10.1111/maps.12278]
Published by arrangement with John Wiley & Sons

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Robert W. Hilts¹, Christopher D. K. Herd², Danielle N. Simkus³ and Greg F. Slater³

¹Department of Physical Sciences, MacEwan University, Edmonton, Alberta, Canada
²Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
³School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario, Canada

The C2 ungrouped Tagish Lake meteorite preserves a range of lithologies, reflecting variable degrees of parent-body aqueous alteration. Here, we report on soluble organic compounds, including aliphatic and aromatic hydrocarbons, monocarboxylic acids, and amino acids, found within specimens representative of the range of aqueous alteration. We find that differences in soluble organic compounds among the lithologies may be explained by oxidative, fluid-assisted alteration, primarily involving the derivation of soluble organic compounds from macromolecular material. In contrast, amino acids probably evolved from precursor molecules, albeit in parallel with other soluble organic compounds. Our results demonstrate the role of parent-body alteration in the modification of organic matter and generation of prebiotic compounds in the early solar system, and have implications for interpretation of the complement of soluble organic compounds in carbonaceous chondrites.

Reference
Hilts RW, Herd CDK, Simkus DN and Slater GF (in press) Soluble organic compounds in the Tagish Lake meteorite. Meteoritics & Planetary Science
[doi:10.1111/maps.12272]
Published by arrangement with John Wiley & Sons

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Matthias M. M. Meier^1,2, Birger Schmitz², Carl Alwmark¹, Reto Trappitsch³, Colin Maden⁴, Rainer Wieler⁴

¹Lund University, Department of Geology, Lund, Sweden
²Department of Physics, Lund University, Lund, Sweden
³Department of the Geophysical Sciences, University of Chicago and Chicago Center for Cosmochemistry, Chicago, Illinois, USA
⁴Department of Earth Sciences, ETH Zurich, Zurich, Switzerland

We analyzed He and Ne in chromite grains from the regolith breccia Ghubara (L5), to compare it with He and Ne in sediment-dispersed extraterrestrial chromite (SEC) grains from mid-Ordovician sediments. These SEC grains arrived on Earth as micrometeorites in the aftermath of the L chondrite parent body (LCPB) breakup event, 470 Ma ago. A significant fraction of them show prolonged exposure to galactic cosmic rays for up to several 10 Ma. The majority of the cosmogenic noble gases in these grains were probably acquired in the regolith of the LCPB (Meier et al. ). Ghubara, an L chondritic regolith breccia with an Ar-Ar shock age of 470 Ma, is a sample of that regolith. We find cosmic-ray exposure ages of up to several 10 Ma in some Ghubara chromite grains, confirming for the first time that individual chromite grains with such high exposure ages indeed existed in the LCPB regolith, and that the >10 Ma cosmic-ray exposure ages found in recent micrometeorites are thus not necessarily indicative of an origin in the Kuiper Belt. Some Ghubara chromite grains show much lower concentrations of cosmogenic He and Ne, indicating that the 4π (last-stage) exposure age of the Ghubara meteoroid lasted only 4–6 Ma. This exposure age is considerably shorter than the 15–20 Ma suggested before from bulk analyses, indicating that bulk samples have seen regolith pre-exposure as well. The shorter last-stage exposure age probably links Ghubara to a small peak of ⁴⁰Ar-poor L5 chondrites of the same exposure age. Furthermore, and quite unexpectedly, we find a Ne component similar to presolar Ne-HL in the chromite grains, perhaps indicating that some presolar Ne can be preserved even in meteorites of petrologic type 5.

Reference
Meier MMM, Schmitz B, Alwmark C, Trappitsch R, Maden C and Wieler R (in press) He and Ne in individual chromite grains from the regolith breccia Ghubara (L5): Exploring the history of the L chondrite parent body regolith.. Meteoritics & Planetary Science
[doi:10.1111/maps.12275]
Published by arrangement with John Wiley & Sons

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Gonzalo Tancredi

Departamento de Astronomí a, Facultad de Ciencias, Iguá 4225, 11400. Montevideo, URUGUAY

The classification criterion between asteroids and comets has evolved in recent decades, but the main distinction remains unchanged. Comets present gas and dust ejection from the surface at some point of their orbits, therefore, these objects are considered to be active. On the other hand, asteroids do not show any kind of large scale gas and dust ejection, they are inert. Nevertheless, this classification scheme is impractical when we have more than 500,000 asteroids already discovered. In addition, comets are not active all along their orbits. In order for a comet to display activity at present or in the recent past in the inner region of the Solar System (heliocentric distance <2AU), the cometary orbit must be unstable in the time scale on the order of ten thousands of years; otherwise, the object should have completely consumed its volatile component. Close encounters with the most massive planets is the only mechanism that could produce ”macroscopic” instabilities on a short time scale. The macroscopic changes in the orbital elements can be detected in a numerical integration of the dynamical evolution of the object over a time scale of several thousand years. This procedure to identify asteroids in cometary-like orbits is also impractical because it would require months of computing time. Therefore, a classification scheme based on the orbital elements to identify the border cases between the asteroid and comet populations is urgently required.
We present a criterion to classify asteroids and comets and to find the border case based on the Tisserand’s parameter, the Minimum Orbital Intersection Distance (MOID), and considering some information regarding the aphelion and perihelion distances. Objects in mean-motion are disregarded. After applying a filter to the sample of over half a million asteroids already discovered to select the precise orbits and to the sample of 487 short-period comets, we apply the proposed classification criterion. The resulting sample consists of ∼331 Asteroids in Cometary Orbits (ACOs). The ACOs are further classified in subclasses similar to the cometary classification. There are 436 Jupiter Family Comets and 203 ACOs of the Jupiter Family type. This new criterion is more strict that the criteria used by other authors to identify ACOs; nonetheless, with the new criterion we ensure that the ACOs have a chaotic dynamical evolution similar to the periodic comets. The discovered dormant or extinct comets seems, if they exist at all, to be a small fraction of the active comets.
We also analyse the available photometric data of ACOs to identify possible large brightness variations. Among the sample of ACOs, there is only one object with brightness variations typical of an active comet: 174P/(60558) Echeclus. But this object has already been double classified as asteroid and comet.

Reference
Tancredi G (in press) A criterion to classify asteroids and comets based on the orbital parameters. Icarus
[doi:10.1016/j.icarus.2014.02.013]
Copyright Elsevier

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Lauren A. Edgar^a, John P. Grotzinger^b, James F. Bell III^a and Joel A. Hurowitz^c

^aSchool of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
^bDivision of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
^cStony Brook University, Stony Brook, NY, USA

En route to Endeavour crater, the Mars Exploration Rover Opportunity embarked on a short but significant campaign at Santa Maria crater during sols 2450-2551. Santa Maria crater is a relatively young impact crater, approximately 100 m in diameter and 11-17 m deep. Opportunity performed detailed analyses on several ejecta blocks and completed an extensive imaging campaign around the crater. Many of the ejecta blocks are composed of sandstone with abundant wind ripple laminations suggestive of eolian deposition. However, other ejecta blocks are massive, fine-grained, and exhibit a nodular texture. These rocks are interpreted to be the first rocks of a grain size smaller than the Microscopic Imager can resolve, and may represent the first mudstones observed by the rover. Several depositional environments are considered for the origin of the fine-grained rocks, and the observations are best fit by a transient evaporitic lake. If the inferred mudstones were deposited in a lacustrine setting, then surface water may have been present in a broader range of surface environments than previously documented at Meridiani Planum.

Reference
Edgar LA, Grotzinger JP, Bell III JJ and Hurowitz JA (in press) Hypotheses for the origin of fine-grained sedimentary rocks at Santa Maria Crater, Meridiani Planum. Icarus
[doi:10.1016/j.icarus.2014.02.019]
Copyright Elsevier

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