Anna Losiak^1,2, Eva Maria Wild³, Leonard Michlmayr³, Christian Koeberl^1,4

¹Department of Lithospheric Research, University of Vienna, Vienna, Austria
²Institute of Geological Sciences, Polish Academy of Sciences, Wrocław, Poland
³VERA Laboratory, Faculty of Physics, Isotope Research and Nuclear Physics, University of Vienna, Vienna, Austria
⁴Natural History Museum, Vienna, Austria

Rocks from drill cores LB-07A (crater fill) and LB-08A (central uplift) into the Bosumtwi impact crater, Ghana, were analyzed for the presence of the cosmogenic radionuclide ¹⁰Be. The aim of the study was to determine the extent to which target rocks of various depths were mixed during the formation of the crater-filling breccia, and also to detect meteoric water infiltration within the impactite layer. ¹⁰Be abundances above background were found in two (out of 24) samples from the LB-07A core, and in none of five samples from the LB-08A core. After excluding other possible explanations for an elevated ¹⁰Be signal, we conclude that it is most probably due to a preimpact origin of those clasts from target rocks close to the surface. Our results suggest that in-crater breccias were well mixed during the impact cratering process. In addition, the lack of a ¹⁰Be signal within the rocks located very close to the lake sediment–impactite boundary suggests that infiltration of meteoric water below the postimpact crater floor was limited. This may suggest that the infiltration of the meteoric water within the crater takes place not through the aerial pore-space, but rather through a localized system of fractures.

Reference
Losiak A, Wild EM, Michlmayr L and Koeberl C (in press) 10Be content in clasts from fallout suevitic breccia in drill cores from the Bosumtwi impact crater, Ghana: Clues to preimpact target distribution Meteoritics & Planetary Science
[doi:10.1111/maps.12256]
Published by arrangement with John Wiley & Sons

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Roger R. Fu, Bradford H. Hager, Anton I. Ermakov, Maria T. Zuber

Department of Earth, Atmospheric, and Planetary Sciences. Massachusetts Institute of Technology. Cambridge, MA

The asteroid Vesta is a differentiated planetesimal from the accretion phase of solar system formation. Although its present-day shape is dominated by a non-hydrostatic fossil equatorial bulge and two large, mostly unrelaxed impact basins, Vesta may have been able to approach hydrostatic equilibrium during a brief early period of intense interior heating. We use a finite element viscoplastic flow model coupled to a 1D conductive cooling model to calculate the expected rate of relaxation throughout Vesta’s early history. We find that, given sufficient non-hydrostaticity, the early elastic lithosphere of Vesta experienced extensive brittle failure due to self-gravity, thereby allowing relaxation to a more hydrostatic figure. Soon after its accretion, Vesta reached a closely hydrostatic figure with <2 km non-hydrostatic topography at degree-2, which, once scaled, is similar to the maximum disequilibrium of the hydrostatic asteroid Ceres. Vesta was able to support the modern observed amplitude of non-hydrostatic topography only >40-200 My after formation, depending on the assumed depth of megaregolith. The Veneneia and Rheasilvia giant impacts, which generated most non-hydrostatic topography, must have therefore occurred >40-200 My after formation. Based on crater retention ages, topography, and relation to known impact generated features, we identify a large region in the northern hemisphere that likely represents relic hydrostatic terrain from early Vesta. The long-wavelength figure of this terrain suggests that, before the two late giant impacts, Vesta had a rotation period of 5.02 hr (6.3% faster than present) while its spin axis was offset by 3.0 from that of the present. The evolution of Vesta’s figure shows that the hydrostaticity of small bodies depends strongly on its age and specific impact history and that a single body may embody both hydrostatic and non-hydrostatic terrains and epochs.

Reference
Fu RR, Hager BH, Ermakov AI and Zuber MT (2014) Efficient early global relaxation of asteroid Vesta. Icarus
[doi:10.1016/j.icarus.2014.01.023]
Copyright Elsevier

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Yuki Shiromoto¹, Hajime Susa¹, and Takashi Hosokawa²

¹Department of Physics, Konan University, Kobe 658-8501, Japan
²Department of Physics and Research Center for the Early Universe, The University of Tokyo, Tokyo 113-0033, Japan

The generation process of a magnetic field around a proto-first-star is studied. Utilizing the recent numerical results of proto-first-star formation based on radiation hydrodynamics simulations, we assess the magnetic field strength generated by the radiative force and the Biermann battery effect. We find that a magnetic field of ~10^-9 G is generated on the surface of the accretion disk around the proto-first-star. The field strength on the accretion disk is smaller by two orders of magnitude than the critical value, above which the gravitational fragmentation of the disk is suppressed. Thus, the generated seed magnetic field hardly affect the dynamics of on-site first star formation directly, unless an efficient amplification process is taken into consideration. We also find that the generated magnetic field is continuously blown out from the disk on the outflows to the poles, that are driven by the thermal pressure of photoheated gas. The strength of the diffused magnetic field in low-density regions is ~10^-14-10^-13 G at n_H = 10³ cm^-3, which could play an important role in the next generation star formation, as well as the seeds of the magnetic field in the present-day universe.

Reference
Shiromoto Y, Susa H and Takashi Hosokawa T (in press) Generation of Magnetic Field on the Accretion Disk around a Proto-first-star. The Astrophysical Journal 782:108.
[doi:10.1088/0004-637X/782/2/108]

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