Takayuki Tanigawa¹, Akito Maruta², and Masahiro N. Machida²

¹Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan
²Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 812-8581, Japan

We investigate the accretion of solid materials onto circumplanetary disks from heliocentric orbits rotating in protoplanetary disks, which is a key process for the formation of regular satellite systems. In the late stage of the gas-capturing phase of giant planet formation, the accreting gas from protoplanetary disks forms circumplanetary disks. Since the accretion flow toward the circumplanetary disks affects the particle motion through gas drag force, we use hydrodynamic simulation data for the gas drag term to calculate the motion of solid materials. We consider a wide range of size for the solid particles (10^-2-10⁶ m), and find that the accretion efficiency of the solid particles peaks around 10 m sized particles because energy dissipation of drag with circum-planetary disk gas in this size regime is most effective. The efficiency for particles larger than 10 m becomes lower because gas drag becomes less effective. For particles smaller than 10 m, the efficiency is lower because the particles are strongly coupled with the background gas flow, which prevents particles from accretion. We also find that the distance from the planet where the particles are captured by the circumplanetary disks is in a narrow range and well described as a function of the particle size.

Reference
Tanigawa T, Maruta A and Machida MN (2014) Accretion of Solid Materials onto Circumplanetary Disks from Protoplanetary Disks. The Astrophysical Journal 784:109.
[doi:10.1088/0004-637X/784/2/109]

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Matthias Ebert^a,b, Lutz Hecht^a,b, Alexander Deutsch^c, Thomas Kenkmann^d, Richard Wirth^e and Jasper Berndt^f

^aMuseum für Naturkunde (MfN), Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstraße 43, D-10115 Berlin, Germany
^bFreie Universität Berlin (FU Berlin), Institut für Geologische Wissenschaften, Malteserstr. 74-100, D-12249 Berlin, Germany
^cInstitut für Planetologie, Westfälische Wilhelms-Universität Münster (WWU), Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany
^dInstitut für Geo- und Umweltwissenschaften, Albert-Ludwigs-Universität Freiburg (ALU), Albertstr. 23-B, D-79104 Freiburg, Germany
^eHelmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum (GFZ), 3.3, Telegrafenberg, D-14473 Potsdam, Germany
^fInstitut für Mineralogie, Westfälische Wilhelms-Universität Münster (WWU), Correns-Str. 24, D-48149 Münster, Germany

The possibility of fractionation processes between projectile and target matter is critical with regard to the classification of the impactor type from geochemical analysis of impactites from natural craters. Here we present results of five hypervelocity MEMIN impact experiments (Poelchau et al., 2013) using the Cr-V-Co-Mo-W-rich steel D290-1 as projectile and two different silica-rich lithologies (Seeberger sandstone and Taunus quartzite) as target materials. Our study is focused on geochemical target-projectile interaction occurring in highly shocked and projectile-rich ejecta fragments. In all of the investigated impact experiments, whether sandstone or quartzite targets, the ejecta fragments show (i) shock-metamorphic features e.g., planar-deformation features (PDF) and the formation of silica glasses, (ii) partially melting of projectile and target, and (iii) significant mechanical and chemical mixing of the target rock with projectile material. The silica-rich target melts are strongly enriched in the “projectile tracer elements” Cr, V, and Fe, but have just minor enrichments of Co, W, and Mo. Inter-element ratios of these tracer elements within the contaminated target melts differ strongly from the original ratios in the steel. The fractionation results from differences in the reactivity of the respective elements with oxygen during interaction of the metal melt with silicate melt. Our results indicate that the principles of projectile-target interaction and associated fractionation do not depend on impact energies (at least for the selected experimental conditions) and water-saturation of the target. Partitioning of projectile tracer elements into the silicate target melt is much more enhanced in experiments with a non-porous quartzite target compared with the porous sandstone target. This is mainly the result of higher impact pressures, consequently higher temperatures and longer reaction times at high temperatures in the experiments with quartzite as target material.

Reference
Ebert M, Hecht L, Deutsch A, Kenkmann T, Wirth R and Berndt J (in press) Geochemical processes between steel projectiles and silica-rich targets in hypervelocity impact experiments. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.02.034]
Copyright Elsevier

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Steven R. Chesley^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aJet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

The target asteroid of the OSIRIS-REx asteroid sample return mission, (101955) Bennu (formerly 1999RQ₃₆), is a half-kilometer near-Earth asteroid with an extraordinarily well constrained orbit. An extensive data set of optical astrometry from 1999 to 2013 and high-quality radar delay measurements to Bennu in 1999, 2005, and 2011 reveal the action of the Yarkovsky effect, with a mean semimajor axis drift rate  or . The accuracy of this result depends critically on the fidelity of the observational and dynamical model. As an example, neglecting the relativistic perturbations of the Earth during close approaches affects the orbit with 3σ significance in da/dt.

The orbital deviations from purely gravitational dynamics allow us to deduce the acceleration of the Yarkovsky effect, while the known physical characterization of Bennu allows us to independently model the force due to thermal emissions. The combination of these two analyses yields a bulk density of , which indicates a macroporosity in the range 40±10% for the bulk densities of likely analog meteorites, suggesting a rubble-pile internal structure. The associated mass estimate is  and .

Bennu’s Earth close approaches are deterministic over the interval 1654–2135, beyond which the predictions are statistical in nature. In particular, the 2135 close approach is likely within the lunar distance and leads to strong scattering and numerous potential impacts in subsequent years, from 2175 to 2196. The highest individual impact probability is 9.5×10^-5 in 2196, and the cumulative impact probability is 3.7×10^-4, leading to a cumulative Palermo Scale of −1.70.

Reference

Chesley et al. (in press) Orbit and Bulk Density of the OSIRIS-REx Target Asteroid (101955) Bennu. Icarus
[doi:10.1016/j.icarus.2014.02.020]
Copyright Elsevier

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Anna Szynkiewicz^a,b, David M. Borrok^c,b and David T. Vaniman^d

^aEarth and Planetary Sciences, University of Tennessee, 1412 Circle Drive, Knoxville, TN 37996, USA
^bGeological Sciences, University of Texas at El Paso, 500 W University Ave., El Paso, TX 79968, USA
^cUniversity of Louisiana at Lafayette, 611 McKinley Street, Lafayette, LA 70504, USA
^dPlanetary Science Institute, 1700 E Fort Lowell, Tucson, AZ 85719, USA

A distinctive sulfur cycle dominates many geological processes on Mars and hydrated sulfate minerals are found in numerous topographic settings with widespread occurrences on the Martian surface. However, many of the key processes controlling the hydrological transport of sulfur, including sulfur sources, climate and the depositional history that led to precipitation of these minerals, remain unclear. In this paper, we use a model for the formation of sulfate efflorescent salts (Mg–Ca–Na sulfates) in the Rio Puerco watershed of New Mexico, a terrestrial analog site from the semiarid Southwest U.S., to assess the origin and environmental conditions that may have controlled deposition of hydrated sulfates in Valles Marineris on Mars. Our terrestrial geochemical results ( of −36.0 to +11.1‰) show that an ephemeral arid hydrological cycle that mobilizes sulfur present in the bedrock as sulfides, sulfate minerals, and dry/wet atmospheric deposition can lead to widespread surface accumulations of hydrated sulfate efflorescences. Repeating cycles of salt dissolution and reprecipitation appear to be major processes that migrate sulfate efflorescences to sites of surface deposition and ultimately increase the aqueous  flux along the watershed (average 41,273 metric tons/yr). We suggest that similar shallow processes may explain the occurrence of hydrated sulfates detected on the scarps and valley floors of Valles Marineris on Mars. Our estimates of salt mass and distribution are in accord with studies that suggest a rather short-lived process of sulfate formation (minimum rough estimate ∼100 to 1000 years) and restriction by prevailing arid conditions on Mars.ed.

Reference
Szynkiewicz A, Borrok DM and Vaniman DT (2014) Efflorescence as a source of hydrated sulfate minerals in valley settings on Mars. Earth and Planetary Science Letters 393:14–25.
[doi:10.1016/j.epsl.2014.02.035]
Copyright Elsevier

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