Lorin S. Matthews¹, Babak Shotorban², and Truell W. Hyde¹

The coagulation of cosmic dust grains is a fundamental process which takes place in astrophysical environments, such as presolar nebulae and circumstellar and protoplanetary disks. Cosmic dust grains can become charged through interaction with their plasma environment or other processes, and the resultant electrostatic force between dust grains can strongly affect their coagulation rate. Since ions and electrons are collected on the surface of the dust grain at random time intervals, the electrical charge of a dust grain experiences stochastic fluctuations. In this study, a set of stochastic differential equations is developed to model these fluctuations over the surface of an irregularly shaped aggregate. Then, employing the data produced, the influence of the charge fluctuations on the coagulation process and the physical characteristics of the aggregates formed is examined. It is shown that dust with small charges (due to the small size of the dust grains or a tenuous plasma environment) is affected most strongly.

Reference
Matthews LS, Shotorban B and Hyde TW (in press) Cosmic Dust Aggregation with Stochastic Charging. The Astrophysical Journal
[doi:10.1088/0004-637X/776/2/103]

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Christian Klimczak^1,*, Carolyn M. Ernst², Paul K. Byrne¹, Sean C. Solomon^1,3, Thomas R. Watters⁴, Scott L. Murchie², Frank Preusker⁵, Jeffrey A. Balcerski⁶

¹Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C., USA
²The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
³Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA
⁴Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, D.C., USA
⁵German Aerospace Center, Institute of Planetary Research, Berlin, Germany
⁶Department of Earth, Environmental, and Planetary Sciences, Case Western Reserve University, Cleveland, Ohio, USA

The volcanic plains that fill the Caloris basin, the largest recognized impact basin on Mercury, are deformed by many graben and wrinkle ridges, among which the multitude of radial graben of Pantheon Fossae allow us to resolve variations in the depth extent of associated faulting. Displacement profiles and displacement-to-length scaling both indicate that faults near the basin center are confined to a ~ 4-km-thick mechanical layer, whereas faults far from the center penetrate more deeply. The fault scaling also indicates that the graben formed in mechanically strong material, which we identify with dry basalt-like plains. These plains were also affected by changes in long-wavelength topography, including undulations with wavelengths of up to 1300 km and amplitudes of 2.5 to 3 km. Geographic correlation of the depth extent of faulting with topographic variations allows a first-order interpretation of the subsurface structure and mechanical stratigraphy in the basin. Further, crosscutting and superposition relationships among plains, faults, craters, and topography indicate that development of long-wavelength topographic variations followed plains emplacement, faulting, and much of the cratering within the Caloris basin. As several examples of these topographic undulations are also found outside the basin, our results on the scale, structural style, and relative timing of the topographic changes have regional applicability and may be the surface expression of global-scale interior processes on Mercury.

Reference
Klimczak C, Ernst CM, Byrne PK, Solomon SC, Watters TR, Murchie SL, Preusker F and Balcerski JA (in press) Insights into the subsurface structure of the Caloris basin, Mercury, from assessments of mechanical layering and changes in long-wavelength topography. Journal of Geophysical Research – Planets, 118
[doi:10.1002/jgre.20157]
Published by arrangement with John Wiley & Sons

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A. C. Boley^1,4, M. A. Morris², and S. J. Desch³

A fundamental, unsolved problem in solar system formation is explaining the melting and crystallization of chondrules found in chondritic meteorites. Theoretical models of chondrule melting in nebular shocks have been shown to be consistent with many aspects of thermal histories inferred for chondrules from laboratory experiments; but, the mechanism driving these shocks is unknown. Planetesimals and planetary embryos on eccentric orbits can produce bow shocks as they move supersonically through the disk gas, and are one possible source of chondrule-melting shocks. We investigate chondrule formation in bow shocks around planetoids through three-dimensional radiation hydrodynamics simulations. A new radiation transport algorithm that combines elements of flux-limited diffusion and Monte Carlo methods is used to capture the complexity of radiative transport around bow shocks. An equation of state that includes the rotational, vibrational, and dissociation modes of H₂ is also used. Solids are followed directly in the simulations and their thermal histories are recorded. Adiabatic expansion creates rapid cooling of the gas, and tail shocks behind the embryo can cause secondary heating events. Radiative transport is efficient, and bow shocks around planetoids can have luminosities ~few× 10⁻⁸ L_☉. While barred and radial chondrule textures could be produced in the radiative shocks explored here, porphyritic chondrules may only be possible in the adiabatic limit. We present a series of predicted cooling curves that merit investigation in laboratory experiments to determine whether the solids produced by bow shocks are represented in the meteoritic record by chondrules or other solids.

Reference
Boley AC, Morris MA and Desch SJ (in press) High-temperature Processing of Solids through Solar Nebular Bow Shocks: 3D Radiation Hydrodynamics Simulations with Particles. The Astrophysical Journal
[doi:10.1088/0004-637X/776/2/101]

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Christoph Duermann, Gerhard Wurm and Markus Kuepper

Faculty of Physics, University of Duisburg-Essen, Lotharstr. 1, 47057 Duisburg, Germany

In optically thin parts of protoplanetary disks photophoresis is a significant force not just for dust grains, but also for macroscopic bodies. The absolute strength on the supposedly highly porous objects is not known in detail as yet. We set up a low pressure torsion balance and studied photophoretic forces down to 100 nN on plates at a light flux of 100 W/m². We investigated the dependence on plate dimensions and on ambient pressure and considered the influence of channels through the plates. As samples for full (no channel) plates we used tissue with 2 mm thickness and circular shape with diameters of 10 mm, 30 mm and 50 mm. The influence of channels was probed on rectangular-shaped circuit boards of 35 mm × 35 mm area and 1.5 mm thickness. The number of channels was 169 and 352. The pressure was varied over three decades between 0.001 and 1 mbar. At low pressure, the absolute photophoretic force is proportional to the cross section of the plates. At high pressure, gas flow through the channels enhances the photophoretic force. The pressure dependence of the radiative force can (formally) be calculated by photophoresis on particles with a characteristic length. We derived two characteristic length scales l depending on the plate radius r₁, the channel radius r₂, and the thickness of the plate, which equals the length of the channel d asl = r^0.35 × d^0.65. The highest force is found at a pressure p_max = 15 × l^-1 Pa mm. In total, the photophoretic force on a plate with channels can be well described by a superposition of the two components: photophoresis due to the overall size and cross section of the plate and photophoresis due to the channels, both with their characteristic pressure dependencies. We applied these results to the transport of large solids in protoplanetary disks and found that the influence of porosity on the photophoretic force can reverse the inward drift of large solids, for instance meter-sized bodies, and push them outward within the optically thin parts of the disk.

Reference
Duermann C, Wurm G and Kuepper M (in press) Radiative forces on macroscopic porous bodies in protoplanetary disks: laboratory experiments. Astronomy & Astrophysics
[doi:10.1051/0004-6361/201321365]
Reproduced with permission © ESO

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B. J. Buratti^1,*, P. A. Dalba¹, M. D. Hicks¹, V. Reddy², M. V. Sykes², T. B. McCord³, D. P. O’Brien², C. M. Pieters⁴, T. H. Prettyman², L. A. McFadden⁵, Andreas Nathues⁶, Lucille Le Corre⁶, S. Marchi⁷, Carol Raymond¹, Chris Russell⁸

¹California Institute of Technology, Jet Propulsion Laboratory, Pasadena, California, USA
²Planetary Science Institute, Tucson,Arizona, USA
³Bear Fight Institute, Winthrop, Washington, USA
⁴Department of Geological Sciences, Brown University, Providence, RI, USA
⁵NASA Goddard Space Flight Center, Greenbelt, MD
⁶Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany
⁷NASA Lunar Science Institute, Boulder, Colorado, USA
⁸Department of Earth and Space Sciences and the Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA, USA

The Framing Camera (FC) on the Dawn spacecraft provided the first view of 4 Vesta at sufficiently high spatial resolution to enable a detailed correlation of the asteroid’s spectral properties with geologic features and with the vestoid (V-type) asteroids and the Howardite-Eucrite-Diogenite (HED) class of meteorites, both of which are believed to originate on Vesta. We combine a spectral analysis of the basin with visible and near-IR spectroscopy of vestoids and with archived data over the same spectral range for HED meteorites. The vestoids are only slightly more akin to the Rheasilvia basin than to Vesta as a whole, suggesting that the crustal material ejected is a well-mixed collection of eucritic and diogenitic materials. The basin itself is more diogenitic, implying Vesta is differentiated and the impact that created Rheasilvia uncovered a mineralogically distinct layer. The Rheasilvia basin exhibits a larger range in pyroxene band strengths than Vesta as a whole, further implying that the basin offers a view into a complex, differentiated protoplanet. The discrepancy between the spectral properties of the HED meteorites and Vesta, in particular the meteorites’ deeper pyroxene absorption band and the redder color of the vestoids, can be explained by the abundance of smaller particles on Vesta and by the addition of low-albedo exogenous particles to its surface, which in turn are due to its larger gravity and longer exposure time to impact processing. Solar phase effects are slight and do not explain the spectral discrepancies between the HEDs, Vesta, and the vestoids.

Reference
Buratti BJ, Dalba PA, Hicks MD, Reddy V, Sykes MV, McCord TB, O’Brien DP, Pieters CM, Prettyman TH, McFadden LA, Nathues A, Le Corre L, Marchi S, Raymond C, Russell C (in press) Vesta, vestoids, and the HED meteorites: Interconnections and differences based on Dawn Framing Camera observations. Journal of Geophysical Research – Planets, 118
[doi:10.1002/jgre.20152]
Published by arrangement with John Wiley & Sons

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A. P. Jones^1,2, L. Fanciullo^1,2, M. Köhler^1,2, L. Verstraete^1,2, V. Guillet^1,2, M. Bocchio^1,2 and N. Ysard^1,2

¹CNRS, Institut d’Astrophysique Spatiale, UMR 8617, 91405 Orsay, France
²Université Paris Sud, Institut d’Astrophysique Spatiale, UMR 8617, 91405 Orsay, France

Context. The evolution of amorphous hydrocarbon materials, a-C(:H), principally resulting from ultraviolet (UV) photon absorption-induced processing, are likely at the heart of the variations in the observed properties of dust in the interstellar medium.
Aims. The consequences of the size-dependent and compositional variations in a-C(:H), from aliphatic-rich a-C:H to aromatic-rich a-C, are studied within the context of the interstellar dust extinction and emission.
Methods. Newly-derived optical property data for a-C(:H) materials, combined with that for an amorphous forsterite-type silicate with iron nano-particle inclusions, a-Sil_Fe, are used to explore dust evolution in the interstellar medium.
Results. We present a new dust model that consists of a power-law distribution of small a-C grains and log-normal distributions of large a-Sil_Fe and a-C(:H) grains. The model, which is firmly anchored by laboratory-data, is shown to quite naturally explain the variations in the infrared (IR) to far-ultraviolet (FUV) extinction, the 217 nm UV bump, the IR absorption and emission bands and the IR-mm dust emission.
Conclusions. The major strengths of the new model are its inherent simplicity and built-in capacity to follow dust evolution in interstellar media. We show that mantle accretion in molecular clouds and UV photo-processing in photo-dominated regions are likely the major drivers of dust evolution.

Reference
Jones AP, Fanciullo L, Köhler M, Verstraete L, Guillet V, Bocchio M and Ysard N (in press) The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point. Astronomy & Astrophysics
[doi:10.1051/0004-6361/201321686]
Reproduced with permission © ESO

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Tomas Kohout^a,b,*, Maria Gritsevich^c,d,e, Victor I. Grokhovsky^f, Grigoriy A. Yakovlev^f, Jakub Haloda^g,h, Patricie Halodova^g, Radoslaw M. Michallikⁱ, Antti Penttilä^a, Karri Muinonen^a,c

^aDepartment of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki University, Finland
^bInstitute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 269, 16500 Prague 6, Czech Republic
^cFinnish Geodetic Institute, Geodeetinrinne 2, P.O. Box 15, FI-02431 Masala, Finland
^dInstitute of Mechanics, Lomonosov Moscow State University, Michurinsky prt., 1, 119192, Moscow, Russia
^eRussian Academy of Sciences, Dorodnicyn Computing Centre, Department of Computational Physics, Vavilova ul. 40, 119333 Moscow, Russia
^fUral Federal University, Ekaterinburg, Russia
^gCzech Geological Survey, Geologická 6, 152 00 Praha 5, Czech Republic
^hOxford Instruments NanoAnalysis, Halifax Road, High Wycombe, Bucks, HP12 3SE, United Kingdom
ⁱDepartment of Geosciences and Geography, University of Helsinki, P.O. Box 64, 00014 Helsinki University, Finland

The mineralogy and physical properties of Chelyabinsk meteorites (fall, February 15, 2013) are presented. Three types of meteorite material are present, described as the light-colored, dark-colored, and impact-melt lithologies. All are of LL5 composition with the impact-melt lithology being close to whole-rock melt and the dark-colored lithology being shock-darkened due to partial melting of iron metal and sulfides. This enables us to study the effect of increasing shock on material with identical composition and origin. Based on the magnetic susceptibility, the Chelyabinsk meteorites are richer in metallic iron as compared to other LL chondrites. The measured bulk and grain densities and the porosity closely resemble other LL chondrites. Shock-darkening does not have a significant effect on the material physical properties, but causes a decrease of reflectance and decrease in silicate absorption bands in the reflectance spectra. This is similar to the space weathering effects observed on asteroids. However, compared to space weathered materials, there is a negligible to minor slope change observed in impact-melt and shock-darkened meteorite spectra. Thus, it is possible that some dark asteroids with invisible silicate absorption bands may be composed of relatively fresh shock-darkened chondritic material.

Reference
Kohout T, Gritsevich M, Grokhovsky VI, Yakovlev GA, Haloda J, Halodova P, Michallik RM, Penttilä A and Muinonen K (in press) Mineralogy, reflectance spectra, and physical properties of the Chelyabinsk LL5 chondrite – insight into shock-induced changes in asteroid regoliths. Icarus
[doi:10.1016/j.icarus.2013.09.027]
Copyright Elsevier

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P. D. Spudis^1,*, D. B. J. Bussey², S. M. Baloga³, J. T. S. Cahill², L. S. Glaze⁴, G. W. Patterson², R. K. Raney², T. W. Thompson⁵, B. J. Thomson⁶, E. A. Ustinov⁵

¹Lunar and Planetary Institute, Houston, Texas, USA
²Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
³Proxemy Research, Laytonsville, Maryland, USA
⁴NASA Goddard Spaceflight Center, Greenbelt, Maryland, USA
⁵Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
⁶Center for Remote Sensing, Boston University, Boston, Massachusetts, USA

The Mini-RF radar instrument on the Lunar Reconnaissance Orbiter spacecraft mapped both lunar poles in two different RF wavelengths (complete mapping at 12.6 cm S-band and partial mapping at 4.2 cm X-band) in two look directions, removing much of the ambiguity of previous Earth- and spacecraft-based radar mapping of the Moon’s polar regions. The poles are typical highland terrain, showing expected values of radar cross section (albedo) and circular polarization ratio (CPR). Most fresh craters display high values of CPR in and outside the crater rim; the pattern of these CPR distributions is consistent with high levels of wavelength-scale surface roughness associated with the presence of block fields, impact melt flows, and fallback breccia. A different class of polar crater exhibits high CPR only in their interiors, interiors that are both permanently dark and very cold (less than 100 K). Application of scattering models developed previously suggests that these anomalously high-CPR deposits exhibit behavior consistent with the presence of water ice. If this interpretation is correct, then both poles may contain several hundred million tons of water in the form of relatively “clean” ice, all within the upper couple of meters of the lunar surface. The existence of significant water ice deposits enables both long-term human habitation of the Moon and the creation of a permanent cislunar space transportation system based upon the harvest and use of lunar propellant.

Reference
Spudis PD, Bussey DBJ, Baloga SM. Cahill JTS, Glaze LS, Patterson GW, Raney RK, Thompson TW, Thomson BJ and Ustinov EA (in press) Evidence for water ice on the moon: Results for anomalous polar craters from the LRO Mini-RF imaging radar. Journal of Geophysical Research – Planets, 118
[doi:10.1002/jgre.20156]
Published by arrangement with John Wiley & Sons

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Jijin Yang^a,e,*, Joseph I. Goldstein^a, Edward R.D. Scott^b, Paul G. Kotula^c, Ansgar Grimberg^d, Ingo Leya^d

^aDept. of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA 01003, USA
^bHawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI 96822, USA
^cMaterials Characterization Dept., Sandia National Laboratories, Albuquerque, NM 87185, USA
^dPhysical Institute , Universität Bern, CH-3012 Bern, Switzerland
^eCarl Zeiss Microscopy, One Zeiss Drive, Thornwood, NY 10594, USA

Tishomingo is a chemically and structurally unique iron with 32.5 wt.% Ni that contains 20% residual taenite and 80% martensite plates, which formed on cooling to between -75 and -200 °C, probably the lowest temperature recorded by any meteorite. Our studies using transmission (TEM) and scanning electron microscopy (SEM), x-ray microanalysis (AEM) and electron backscatter diffraction (EBSD) show that martensite plates in Tishomingo formed in a single crystal of taenite and decomposed during reheating forming 10-100 nm taenite particles with ~50 wt.% Ni, kamacite with ~4 wt.% Ni, along with martensite or taenite with 32 wt.% Ni. EBSD data and experimental constraints show that Tishomingo was reheated to 320-400 °C for about a year transforming some martensite to kamacite and to taenite particles and some martensite directly to taenite without composition change. Fizzy-textured intergrowths of troilite, kamacite with 2.7 wt.% Ni and 2.6 wt.% Co, and taenite with 56 wt.% Ni and 0.15 wt.% Co formed by localized shock melting. A single impact probably melted the sub-mm sulfides, formed stishovite, and reheated and decomposed the martensite plates. Tishomingo and its near-twin Willow Grove, which has 28 wt.% Ni, differ from IAB-related irons like Santa Catharina and San Cristobal that contain 25-36 wt.% Ni, as they are highly depleted in moderately volatile siderophiles and enriched in Ir and other refractory elements. Tishomingo and Willow Grove therefore resemble IVB irons but are chemically distinct. The absence of cloudy taenite in these two irons shows that they cooled through 250 °C abnormally fast at >0.01 °C/yr. Thus this grouplet, like the IVA and IVB irons, suffered an early impact that disrupted their parent body when it was still hot. Our noble gas data show that Tishomingo was excavated from its parent body about 100 to 200 Myr ago and exposed to cosmic rays as a meteoroid with a radius of ~50-85 cm.

Reference
Yang J, Goldstein JI, Scott ERD, Kotula PG, Grimberg A and Leya I (in press) Thermal and Collisional History of Tishomingo: More evidence for early disruption of differentiated planetesimals. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.09.023]
Copyright Elsevier

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N. Dalcher¹, M. W. Caffee², K. Nishiizumi³, K. C. Welten³, N. Vogel^4,†, R. Wieler⁴, I. Leya^1,*

¹Space Sciences and Planetology, University of Bern, Bern, Switzerland
²Department of Physics, PRIME Laboratory, Purdue University, West Lafayette, Indiana, USA
³Space Sciences Laboratory, University of California, Berkeley, California, USA
⁴Institute of Geochemistry and Petrology, ETH Zurich, Switzerland
^†EAWAG, Dübendorf, Switzerland

We measured the concentrations and isotopic compositions of He, Ne, and Ar in bulk samples and metal separates of 14 ordinary chondrite falls with long exposure ages and high metamorphic grades. In addition, we measured concentrations of the cosmogenic radionuclides ¹⁰Be,²⁶Al, and ³⁶Cl in metal separates and in the nonmagnetic fractions of the selected meteorites. Using cosmogenic ³⁶Cl and ³⁶Ar measured in the metal separates, we determined ³⁶Cl-³⁶Ar cosmic-ray exposure (CRE) ages, which are shielding-independent and therefore particularly reliable. Using the cosmogenic noble gases and radionuclides, we are able to decipher the CRE history for the studied objects. Based on the correlation ³He/²¹Ne versus ²²Ne/²¹Ne, we demonstrate that, among the meteorites studied, only one suffered significant diffusive losses (about 35%). The data confirm that the linear correlation ³He/²¹Ne versus ²²Ne/²¹Ne breaks down at high shielding. Using ³⁶Cl-³⁶Ar exposure ages and measured noble gas concentrations, we determine ²¹Ne and ³⁸Ar production rates as a function of ²²Ne/²¹Ne. The new data agree with recent model calculations for the relationship between ²¹Ne and ³⁸Ar production rates and the ²²Ne/²¹Ne ratio, which does not always provide unique shielding information. Based on the model calculations, we determine a new correlation line for ²¹Ne and ³⁸Ar production rates as a function of the shielding indicator ²²Ne/²¹Ne for H, L, and LL chondrites with preatmospheric radii less than about 65 cm. We also calculated the ¹⁰Be/²¹Ne and ²⁶Al/²¹Ne production rate ratios for the investigated samples, which show good agreement with recent model calculations.

Reference
Dalcher N, Caffee MW, Nishiizumi K, Welten KC, Vogel N, Wieler R and Leya I (in press) Calibration of cosmogenic noble gas production in ordinary chondrites based on ³⁶Cl-³⁶Ar ages. Part 1: Refined produced rates for cosmogenic ²¹Ne and ³⁸Ar. Meteoritics & Planetary Science
[doi:10.1111/maps.12203]
Published by arrangement with John Wiley & Sons

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