Gernot Arp*, Claudia Kolepka, Klaus Simon, Volker Karius, Nicole Nolte, Bent T. Hansen

Georg-August-Universität Göttingen, Geowissenschaftliches Zentrum, Göttingen, Germany

The extent of impact-generated hydrothermal activity in the 24 km sized Ries impact structure has been controversially discussed. To date, mineralogical and isotopic investigations point to a restriction of hydrothermal activity to the impact-melt bearing breccias, specifically the crater-fill suevite. Here, we present new petrographic, geochemical, and isotopic data of postimpact carbonate deposits, which indicate a hydrothermal activity more extended than previously assumed. Specifically, carbonates of the Erbisberg, a spring mound located upon the inner crystalline ring of the crater, show travertine facies types not seen in any of the previously investigated sublacustrine soda lake spring mounds of the Ries basin. In particular, the streamer carbonates, which result from the encrustation of microbial filaments in subaerial spring effluents between 60 and 70 °C, are characteristic of a hydrothermal origin. While much of the primary geochemical and isotopic signatures in the mound carbonates have been obliterated by diagenesis, a postimpact calcite vein from brecciated gneiss of the subsurface crater floor revealed a flat rare earth element pattern with a clear positive Eu anomaly, indicating a hydrothermal fluid convection in the crater basement. Finally, the strontium isotope stratigraphic correlation of the travertine mound with the crater basin succession suggests a hydrothermal activity for about 250,000 yr after the impact, which would be much longer than previously assumed.

Reference
Arp G, Kolepka C, Simon K, Karius V, Nolte N and Hansen BT (in press) New evidence for persistent impact-generated hydrothermal activity in the Miocene Ries impact structure, Germany. Meteoritics & Planetary Science 
[doi:10.1111/maps.12235]
Published by arrangement with John Wiley & Sons

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Fred J. Ciesla^1,*, Thomas M. Davison², Gareth S. Collins² and David P. O’Brien³

¹Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, 60637, USA
²Impact and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, London, UK
³Planetary Science Institute, Tucson, Arizona, USA

Collisions between planetesimals were common during the first approximately 100 Myr of solar system formation. Such collisions have been suggested to be responsible for thermal processing seen in some meteorites, although previous work has demonstrated that such events could not be responsible for the global thermal evolution of a meteorite parent body. At this early epoch in solar system history, however, meteorite parent bodies would have been heated or retained heat from the decay of short-lived radionuclides, most notably ²⁶Al. The postimpact structure of an impacted body is shown here to be a strong function of the internal temperature structure of the target body. We calculate the temperature–time history of all mass in these impacted bodies, accounting for their heating in an onion-shell–structured body prior to the collision event and then allowing for the postimpact thermal evolution as heat from both radioactivities and the impact is diffused through the resulting planetesimal and radiated to space. The thermal histories of materials in these bodies are compared with what they would be in an unimpacted, onion-shell body. We find that while collisions in the early solar system led to the heating of a target body around the point of impact, a greater amount of mass had its cooling rates accelerated as a result of the flow of heated materials to the surface during the cratering event.

Reference
Ciesla FJ, Davison TM, Collins GS and O’Brien DP (in press) Thermal consequences of impacts in the early solar system. Meteoritics & Planetary Science 
[doi:10.1111/maps.12236]
Published by arrangement with John Wiley & Sons

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Alex Meshik^a, Charles Hohenberg^a, Olga Pravdivtseva^a and Donald Burnett^b

^aDepartment of Physics, Washington University, 1 Brookings Drive, Saint Louis, MO 63130, USA
^bGeological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

One of the major goals of the Genesis Mission was to refine our knowledge of the isotopic composition of the heavy noble gases in solar wind and, by inference, the Sun, which represents the initial composition of the solar system. This has now been achieved with permil precision: ³⁶Ar/³⁸Ar = 5.5005 ± 0.0040, ⁸⁶Kr/⁸⁴Kr = .3012 ± .0004, ⁸³Kr/⁸⁴Kr = .2034 ± .0002, ⁸²Kr/⁸⁴Kr = .2054 ± .0002, ⁸⁰Kr/⁸⁴Kr = .0412 ± .0002, ⁷⁸Kr/⁸⁴Kr = .00642 ± .00005, ¹³⁶Xe/¹³²Xe = .3001 ± .0006, ¹³⁴Xe/¹³²Xe = .3691 ± .0007, ¹³¹Xe/¹³²Xe = .8256 ± .0012,¹³⁰Xe/¹³²Xe = .1650 ± .0004, ¹²⁹Xe/¹³²Xe = 1.0405 ± .0010, ¹²⁸Xe/¹³²Xe = .0842 ± .0003, ¹²⁶Xe/¹³²Xe = .00416 ± .00009, and ¹²⁴Xe/¹³²Xe = .00491 ± .00007 (error-weighted averages of all published data). The Kr and Xe ratios measured in the Genesis solar wind collectors generally agree with the less precise values obtained from lunar soils and breccias, which have accumulated solar wind over hundreds of millions of years, suggesting little if any temporal variability of the isotopic composition of solar wind krypton and xenon. The higher precision for the initial composition of the heavy noble gases in the solar system allows (1) to confirm that, exept ¹³⁶Xe and ¹³⁴Xe, the mathematically derived U-Xe is equivalent to Solar Wind Xe and (2) to provide an opportunity for better understanding the relationship between the starting composition and Xe-Q (and Q-Kr), the dominant current “planetary” component, and its host, the mysterious phase-Q.

Reference
Meshik A, Hohenberg C, Pravdivtseva O and Burnett D (in press) Heavy Noble Gases in Solar Wind Delivered by Genesis Mission. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.11.030]
Copyright Elsevier

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M. Fouchard^a, H. Rickman^b,c, Ch. Froeschlé^d, G.B. Valsecchi^e,f

^aLAL-IMCCE, Université de Lille 1, 1 Impasse de l’Observatoire, F-59000 Lille, France
^bPAS Space Research Center, Bartycka 18A, PL-00-716, Warszawa, Poland
^cDept. of Physics & Astronomy, Uppsala Univ., Box 516, SE-75120 Uppsala, Sweden
^dObservatoire de la Côte d’Azur, UMR Lagrange 7293, Bv. de l’Observatoire, B.P. 4229, F-06304 Nice cedex 4, France
^eIAPS, INAF, via Fosso del Cavaliere 100, I-00133 Roma, Italy
^fIFAC-CNR, Via Madonna del Piano 10, I-50019 Sesto Fiorentino (FI), Italy

We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard et al., 2013b), the planetary perturbations experienced each time the comets penetrate to within 50 AU of the Sun. Each simulation involves an initial sample of four million comets and extends over a maximum of 5 Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. In the companion paper we studied in detail how observable comets are injected from the Oort cloud, when account is taken of the planetary perturbations. In the present paper we concentrate on how the cloud may evolve in the long term and also on the production of decoupled comets, which evolve into semi-major axes less than 1 000AU. Concerning the long-term evolution, we find that the largest stellar perturbations that may statistically be expected during the age of the Solar System induce a large scale migration of comets within the cloud. Thus, comets leave the inner parts, but the losses from the outer parts are even larger, so at the end of our simulations the Oort cloud is more centrally condensed than at the beginning. The decoupled comets, which form a source of centaurs and Halley type comets (roughly in the proportions of 70% and 30%, respectively), are mainly produced by planetary perturbations, Jupiter and Saturn being the most efficient. This effect is dependent on synergies with the Galactic tide and stellar encounters, bringing the perihelia of Oort cloud comets into the planetary region. The star-planet synergy has a large contribution due to the strong encounters that produce major comet showers. However, outside these showers a large majority of decouplings may be attributed to the tide-planet synergy.

Reference
Fouchard M, Rickman H, Froeschlé Ch and Valsecchi GB (in press) Planetary perturbations for Oort cloud comets: III. Evolution of the cloud and production of centaurs and Halley type comets. Icarus
[doi:10.1016/j.icarus.2013.11.034]
Copyright Elsevier

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