Cyrena Anne Goodrich^a,b, George E. Harlow^c, James A. Van Orman^d, Stephen R. Sutton^e, Michael J. Jercinovic^b, Takashi Mikouchi^f

^aPlanetary Science Institute, 1700 E. Ft. Lowell, Suite 106, Tucson, AZ 85719 USA
^bDepartment of Geosciences, University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003 USA
^cAmerican Museum of Natural History, Department of Earth and Planetary Sciences, Central Park West at 79[th] Street, New York, NY 10024 USA
^dDept. of Earth, Environmental and Planetary Sciences, Case Western Reserve University, Cleveland, OH 44120 USA
^eDept. of Geophysical Sciences and Center for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637 USA
^fDepartment of Earth and Planetary Sciences, University of Tokyo, Tokyo 113-0033 Japan

Ureilites are ultramafic achondrites thought to be residues of partial melting on a carbon-rich asteroid. They show a trend of FeO-variation (olivine Fo from ~74 to 95) that suggests variation in oxidation state. Whether this variation was established during high-temperature igneous processing on the ureilite parent body (UPB), or preserved from nebular precursors, is a subject of debate. The behavior of chromium in ureilites offers a way to assess redox conditions during their formation and address this issue, independent of Fo. We conducted a petrographic and mineral compositional study of occurrences of chromite (Cr-rich spinel) in ureilites, aimed at determining the origin of the chromite in each occurrence and using primary occurrences to constrain models of ureilite petrogenesis. Chromite was studied in LEW 88774 (Fo 74.2), NWA 766 (Fo 76.7), NWA 3109 (Fo 76.3), HaH 064 (Fo 77.5), LAP 03587 (Fo 74.9), CMS 04048 (Fo 76.4), LAP 02382 (Fo 78.6) and EET 96328 (Fo 85.2).
Chromite occurs in LEW 88774 (~5 vol.%), NWA 766 (<1 vol.%), NWA 3109 (<1 vol.%) and HaH 064 (<1 vol.%) as subhedral to anhedral grains comparable in size (~30 μm to 1 mm) and/or textural setting to the major silicates (olivine and pyroxenes[s]) in each rock, indicating that it is a primary phase. The most FeO-rich chromites in these sample (rare grain cores or chadocrysts in silicates) are the most primitive compositions preserved (fe# = 0.55-0.6; Cr# varying from 0.65 to 0.72 among samples). They record olivine-chromite equilibration temperatures of ~1040-1050°C, reflecting subsolidus Fe/Mg reequilibration during slow cooling from ~1200-1300°C. All other chromite in these samples is reduced. Three types of zones are observed. 1) Inclusion-free interior zones showing reduction of FeO (fe# ~0.4→0.28); 2) Outer zones showing further reduction of FeO (fe# ~0.28→0.15) and containing abundant laths of eskolaite-corundum (Cr₂O₃-Al₂O₃); 3) Outermost zones showing extreme reduction of both FeO (fe# <0.15) and Cr₂O₃ (Cr# as low as 0.2). The grains are surrounded by rims of Si-Al-rich glass, graphite, Fe,Cr-carbides ([Fe,Cr]₃C and [Fe,Cr]₇C₃), Cr-rich sulfides (daubréelite and brezinaite) and Cr-rich symplectic bands on adjacent silicates. Chromite is inferred to have been reduced by graphite, forming eskolaite-corundum and carbides as byproducts, during impact excavation. This event involved initial elevation of T (to 1300-1400°C), followed by rapid decompression and drop in T (to <700°C) at 1-20°C/hr. The kinetics of reduction of chromite is consistent with this scenario. The reduction was facilitated by silicate melt surrounding the chromites, which was partly generated by shock-melting of pyroxenes. Symplectic bands, consisting of fine-scale intergrowths of Ca-pyroxene, chromite and glass, formed by reaction between the Cr-enriched melt and adjacent silicates.
Early chromite also occurs in a melt inclusion in olivine in HaH 064 and in a metallic spherule in olivine in LAP 02382. LAP 03587 and CMS 04048 contain ⩽μm-sized chromite+pyroxene symplectic exsolutions in olivine, indicating high Cr valence in the primary olivine. EET 96328 contains a round grain of chromite that could be a late-crystallizing phase. Tiny chromite grains in melt inclusions in EET 96328 formed in late, closed-system reactions.
For 7 of the 8 ureilites we conclude that the relatively oxidizing conditions evidenced by the presence of primary or early chromite pertain to the period of high-T igneous processing. The observation that such conditions are recorded almost exclusively in low-Fo samples supports the interpretation that the ureilite FeO-variation was established during igneous processing on the UPB.

Reference
Goodrich CA, Harlow GE, Van Orman JA, Sutton SR, Jercinovic MJ and Mikouchi T (in press) Petrology of Chromite in Ureilites: Deconvolution of Primary Oxidation States and Secondary Reduction Processes. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.028]
Copyright Elsevier

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J.-P. Williams^a, A.V. Pathare^b and O. Aharonson^c

^aDept. Earth and Space Sciences, University of California, Los Angeles, CA 90095, USA
^bPlanetary Science Institute, Tucson, AZ 85719, USA
^cHelen Kimmel Center for Planetary Science, Weizmann Institute Of Science, Rehovot, 76100 Israel

We model the primary crater production of small (D < 100 m) primary craters on Mars and the Moon using the observed annual flux of terrestrial fireballs. From the size-frequency distribution (SFD) of meteor diameters, with appropriate velocity distributions for Mars and the Moon, we are able to reproduce martian and lunar crater-count chronometry systems (isochrons) in both slope and magnitude. We include an atmospheric model for Mars that accounts for the deceleration, ablation, and fragmentation of meteors. We find that the details of the atmosphere or the fragmentation of the meteors do not strongly influence our results. The downturn in the crater SFD from atmospheric filtering is predicted to occur at D ~ 10-20 cm, well below the downturn observed in the distribution of fresh craters detected by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) or the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX). Crater counts are conducted on the ejecta blanket of Zunil crater and the interior of Pangboche crater on Mars and North Ray and Cone craters on the Moon. Our model isochrons produce a similar slope and age estimate for the formation of Zunil crater as the Hartmann production function (~1 Ma). We derive an age of 35.1 Ma for Pangboche when accounting for the higher elevation (>20 km higher than Zunil), a factor ~2 younger than estimated using the Hartmann production function which assumes 6 mbar surface pressure. We estimate ages of 52.3 Ma and 23.9 Ma for North Ray and Cone crater respectively, consistent with cosmic ray exposure ages from Apollo samples. Our results indicate that the average cratering rate has been constant on these bodies over these time periods. Since our Monte Carlo simulations demonstrate that the existing crater chronology systems can be applied to date young surfaces using small craters on the Moon and Mars, we conclude that the signal from secondary craters in the isochrons must be relatively small at these locations, as our Monte Carlo model only generates primary craters.

Reference
Williams J-P, Pathare AV and O. Aharonson O (in press) The Production of Small Primary Craters on Mars and the Moon. Icarus
[doi:10.1016/j.icarus.2014.03.011]
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J. Leliwa-Kopystynski^a and M. Arakawa^b

^aUniversity of Warsaw, Institute of Geophysics, 02-093 Warsaw, Pasteura 7, Poland
^bGraduate School of Science, Kobe University, 1-1, Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan

A series of impact experiments onto solid decimeter-sized cylinders made of porous gypsum admixed with approximately one centimeter-sized pebbles have been performed. The target densities and their heterogeneous structures could be representative of those of the asteroids Ida, Eros and many others, because asteroid sub-surface could be the consolidated boulders made by self-compaction and/or by impact compaction. Impact velocities in the experiments ranged from 2.0 km/s to 6.7 km/s (collision velocity in the asteroid main belt is approximately 5 km/s). It was found that the slope of the cumulative number distribution of post-impact fragments strongly depends on the specific energy of the impact. The presence of pebbles strongly influences the impact strength of the target as well as the size distribution of the post-impact fragments. Results of the experiments presented here are aimed at identifying the analogy between the laboratory results and the damage of small asteroids or their catastrophic disruption after impacts.

Reference
Leliwa-Kopystynski J and Arakawa M (in press) Impacts experiments onto heterogeneous targets simulating impact breccia: Implications for impact strength of asteroids and formation of the asteroid families. Icarus
[doi:10.1016/j.icarus.2014.03.012]
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Takeru K. Suzuki and Shu-ichiro Inutsuka

Department of Physics, Nagoya University, Nagoya, Aichi 464-8602, Japan

We report results of three-dimensional magnetohydrodynamical (MHD) simulations of global accretion disks threaded with weak vertical magnetic fields. We perform the simulations in the spherical coordinates with different temperature profiles and accordingly different rotation profiles. In the cases with a spatially constant temperature, because the rotation frequency is vertically constant in the equilibrium condition, general properties of the turbulence excited by magnetorotational instability are quantitatively similar to those obtained in local shearing box simulations. On the other hand, in the cases with a radially variable temperature profile, the vertical differential rotation, which is inevitable in the equilibrium condition, winds up the magnetic field lines in addition to the usual radial differential rotation. As a result, the coherent wound magnetic fields contribute to the Maxwell stress in the surface regions. We obtain nondimensional density and velocity fluctuations ~0.1-0.2 at the midplane. The azimuthal power spectra of the magnetic fields show shallower slopes, ~m⁰ – m^–1, than those of velocity and density. The Poynting flux associated with the MHD turbulence drives intermittent and structured disk winds as well as sound-like waves toward the midplane. The mass accretion mainly occurs near the surfaces, and the gas near the midplane slowly moves outward in the time domain of the present simulations. The vertical magnetic fields are also dragged inward in the surface regions, while they stochastically move outward and inward around the midplane. We also discuss an observational implication of induced spiral structure in the simulated turbulent disks.

Reference
Suzuki TK and Inutsuka S-I (2014) Magnetohydrodynamic Simulations of Global Accretion Disks with Vertical Magnetic Fields. The Astrophysical Journal 784:121.
[doi:10.1088/0004-637X/784/2/121]

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