T. Kelling, G. Wurm, and M. Köster

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

For dust aggregates in protoplanetary disks, a transition between sticking and bouncing in individual collisions at mm to cm sizes has been observed in the past. This leads to the notion of a bouncing barrier for which growth gets stalled. Here, we present long-term laboratory experiments on the outcome of repeated aggregate collisions at the bouncing barrier. About 100 SiO₂ dust aggregates 1 mm in size were observed interacting with each other. Collisions occurred within a velocity range from below mm s^-1 up to cm s^-1. Aggregates continuously interacted with each other over a period of 900 s. During this time, more than 10⁵ collisions occurred. Nearly 2000 collisions were analyzed in detail. No temporal stable net growth of larger aggregates was observed even though sticking collision occurred. Larger ensembles of aggregates sticking together were formed but were disassembled again during further collisional evolution. The concept of a bouncing barrier supports the formation of planetesimals by seeded collisional growth, as well as by gravitational instability favoring a significant total mass being limited to certain size ranges. Within our parameter set, the experiments confirm that bouncing barriers are one possible and likely evolutionary limit of self-consistent particle growth.

Reference
Kelling T, Wurm G and Köster M (2014) Experimental Study on Bouncing Barriers in Protoplanetary Disks.  The Astrophysical Journal 783:111.
[doi:10.1088/0004-637X/783/2/111]

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J.P. Emery^a, Y.R. Fernández^b, M.S.P. Kelley^c, K.T. Warden (nèe Crane)^a,1, C. Hergenrother^d, D.S. Lauretta^d, M.J. Drake^d, H. Campins^b, J. Ziffer^e

^aEarth and Planetary Science Dept & Planetary Geosciences Institute, University of Tennessee, Knoxville, TN 37996
^bPhysics Department, University of Central Florida, Orlando, FL 32816
^cDepartment of Astronomy, University of Maryland, College Park, MD 20742-2421
^dDepartment of Planetary Sciences, University of Arizona, Tucson, AZ 85721
^eDepartment of Physics, University of Southern Maine, Portland, ME 04104
¹Department of Earth, Atmospheric, and Planetary Science, Purdue University, West Lafayette, IN 47907.

Near-Earth asteroids (NEAs) have garnered ever increasing attention over the past few years due to the insights they offer into Solar System formation and evolution, the potential hazard they pose, and their accessibility for both robotic and human spaceflight missions. Among the NEAs, carbonaceous asteroids hold particular interest because they may contain clues to how the Earth got its supplies of water and organic materials, and because none has yet been studied in detail by spacecraft. (101955) Bennu is special among NEAs in that it will not only be visited by a spacecraft, but the OSIRIS-REx mission will also return a sample of Bennu’s regolith to Earth for detailed laboratory study. This paper presents analysis of thermal infrared photometry and spectroscopy that test the hypotheses that Bennu is carbonaceous and that its surface is covered in fine-grained (sub-cm) regolith. The Spitzer Space Telescope observed Bennu in 2007, using the Infrared Spectrograph (IRS) to obtain spectra over the wavelength range 5.2 – 38 μm and images at 16 and 22 μm at 10 different longitudes, as well as the Infrared Array Camera (IRAC) to image Bennu at 3.6, 4.5, 5.8, and 8.0 μm, also at 10 different longitudes. Thermophysical analysis, assuming a spherical body with the known rotation period and spin-pole orientation, returns an effective diameter of 484±10 m, in agreement with the effective diameter calculated from the radar shape model at the orientation of the Spitzer observations (492±20 m, Nolan et al. 2013) and a visible geometric albedo of 0.046±0.005 (using H_v=20.51, Hergenrother et al. 2013). Including the radar shape model in the thermal analysis, and taking surface roughness into account, yields a disk-averaged thermal inertia of 310±70 J m^-2K^-1s^-1/2, which is significantly lower than several other NEAs of comparable size. There may be a small variation of thermal inertia with rotational phase (±60 J m^-2K^-1s^-1/2). The spectral analysis is inconclusive in terms of surface mineralogy; the emissivity spectra have a relatively low signal-to-noise ratio and no spectral features are detected. The thermal inertia indicates average regolith grain size on the scale of several millimeters to about a centimeter. This moderate grain size is also consistent with low spectral contrast in the 7.5 – 20 μm spectral range. If real, the rotational variation in thermal inertia would be consistent with a change in average grain size of only about a millimeter. The thermophysical properties of Bennu’s surface appear to be fairly homogeneous longitudinally. A search for a dust coma failed to detect any extended emission, putting an upper limit of about 10⁶ g of dust within 4750 km of Bennu. Three common methodologies for thermal modeling are compared, and some issues to be aware of when interpreting the results of such models are discussed. We predict that the OSIRIS-REx spacecraft will find a low albedo surface with abundant sub-cm sized grains, fairly evenly distributed in longitude.

Reference
Emery JP, Fernández YR, Kelley MSP, Warden (nèe Crane) KT, Hergenrother C, Lauretta DS, Drake MJ, Campins H and Ziffer J (in press) Thermal Infrared Observations and Thermophysical Characterization of OSIRIS-REx Target Asteroid (101955) Bennu. Icarus
[doi:10.1016/j.icarus.2014.02.005]
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Tsukagoshi¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹College of Science, Ibaraki University, Bunkyo 2-1-1, Mito 310-8512, Japan

To reveal the structures of a transition disk around a young stellar object in Lupus, Sz 91 , we have performed aperture synthesis 345 GHz continuum and CO(3-2) observations with the Submillimeter Array (~1”-3” resolution) and high-resolution imaging of polarized intensity at the K_s -band using the HiCIAO instrument on the Subaru Telescope (0.”25 resolution). Our observations successfully resolved the inner and outer radii of the dust disk to be 65 and 170 AU, respectively, which indicates that Sz 91 is a transition disk source with one of the largest known inner holes. The model fitting analysis of the spectral energy distribution reveals an H₂ mass of 2.4 × 10^-3 M_☉ in the cold (T < 30 K) outer part at 65 AU <r < 170 AU by assuming a canonical gas-to-dust mass ratio of 100, although a small amount (>3 × 10^-9 M_☉) of hot (T ~ 180 K) dust possibly remains inside the inner hole of the disk. The structure of the hot component could be interpreted as either an unresolved self-luminous companion body (not directly detected in our observations) or a narrow ring inside the inner hole. Significant CO(3-2) emission with a velocity gradient along the major axis of the dust disk is concentrated on the Sz 91 position, suggesting a rotating gas disk with a radius of 420 AU. The Sz 91 disk is possibly a rare disk in an evolutionary stage immediately after the formation of protoplanets because of the large inner hole and the lower disk mass than other transition disks studied thus far.

Reference
Tsukagoshi et al. (2014) High-resolution Submillimeter and Near-infrared Studies of the Transition Disk around Sz 91.  The Astrophysical Journal 783:90.
[doi:10.1088/0004-637X/783/2/90]

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Renaud Deguen^a,c, Maylis Landeau^b, Peter Olson^a

^aDepartment of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218, USA
^bDynamique des Fluides Géologiques, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, CNRS UMR 7154, 1 rue Jussieu, 75238, Paris cedex 05, France
^cInstitut de Mécanique des Fluides de Toulouse, Université de Toulouse (INPT, UPS) and CNRS, Allée C. Soula, Toulouse, 31400, France

Much of the Earth was built by high-energy impacts of planetesimals and embryos, many of these impactors already differentiated, with metallic cores of their own. Geochemical data provide critical information on the timing of accretion and the prevailing physical conditions, but their interpretation depends critically on the degree of metal–silicate chemical equilibration during core–mantle differentiation, which is poorly constrained. Efficient equilibration requires that the large volumes of iron derived from impactor cores mix with molten silicates down to scales small enough to allow fast metal–silicate mass transfer. Here we use fluid dynamics experiments to show that large metal blobs falling in a magma ocean mix with the molten silicate through turbulent entrainment, with fragmentation into droplets eventually resulting from the entrainment process. In our experiments, fragmentation of the dense fluid occurs after falling a distance equal to 3–4 times its initial diameter, at which point a sizable volume of ambient fluid has already been entrained and mixed with the dense falling fluid. Contrary to previous assumptions, we demonstrate that fragmentation of the metallic phase into droplets may not be required for efficient equilibration: turbulent mixing, by drastically increasing the metal–silicate interfacial area, may result in fast equilibration even before fragmentation. Efficient re-equilibration is predicted for impactors of size small compared to the magma ocean depth. In contrast, much less re-equilibration is predicted for large impacts in situations where the impactor core diameter approaches the magma ocean thickness.

Reference
Deguen R, Landeau M and Olson P (nèe Crane) KT, Hergenrother C, Lauretta DS, Drake MJ, Campins H and Ziffer J (2014) Turbulent metal–silicate mixing, fragmentation, and equilibration in magma oceans. Earth and Planetary Science Letters 391:274–287.
[doi:10.1016/j.epsl.2014.02.007]
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Rebecca A. Fischer and Fred J. Ciesla

Department of the Geophysical Sciences, University of Chicago, 5734 S Ellis Ave, Chicago, IL 60637, USA

The agglomeration of planetary embryos and planetesimals was the final stage of terrestrial planet formation. This process is modeled using N-body accretion simulations, whose outcomes are tested by comparing to observed physical and chemical Solar System properties. The outcomes of these simulations are stochastic, leading to a wide range of results, which makes it difficult at times to identify the full range of possible outcomes for a given dynamic environment. We ran fifty high-resolution simulations each with Jupiter and Saturn on circular or eccentric orbits, whereas most previous studies ran an order of magnitude fewer. This allows us to better quantify the probabilities of matching various observables, including low probability events such as Mars formation, and to search for correlations between properties. We produce many good Earth analogues, which provide information about the mass evolution and provenance of the building blocks of the Earth. Most observables are weakly correlated or uncorrelated, implying that individual evolutionary stages may reflect how the system evolved even if models do not reproduce all of the Solar System’s properties at the end. Thus individual N-body simulations may be used to study the chemistry of planetary accretion as particular accretion pathways may be representative of a given dynamic scenario even if that simulation fails to reproduce many of the other observed traits of the Solar System.

Reference
Fischer RA and Ciesla FJ (2014) Dynamics of the terrestrial planets from a large number of N-body simulations. Earth and Planetary Science Letters 392:28–38.
[doi:10.1016/j.epsl.2014.02.011]
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Minami Yasui^a, Ryo Hayama^b, Masahiko Arakawa^b

^aOrganization of Advanced Science and Technology, Kobe University, 1-1, Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
^bGraduate School of Science, Kobe University, 1-1, Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan

Frequent collisions are common for small bodies in the solar system, and the cumulative damage to these bodies is thought to significantly affect their evolution. It is important to study the effects of multiple impacts such as the number of impacts on the impact strength and the ejection velocity of impact fragments. Here we conducted multiple-impact experiments using a polycrystalline water ice target, varying the number of impacts from 1 to 10 times. An ice cylindrical projectile was impacted at 84 to 502 m s⁻¹ by using a single-stage gas gun in a cold room between −10 and −15°C. The impact strength of the ice target that experienced a single impact and multiple impacts is expressed by the total energy density applied to the same target, ΣQ, and this value was observed to be 77.6 J kg⁻¹. The number of fine impact fragments at a fragment mass normalized by an initial target mass, m/M_t0~10⁻⁶, n_m, had a good correlation with the single energy density at each shot, Q_j, and the relationship was shown to be n_m = 10^1.02±0.22⋅Q_j^1.31±0.12. We also estimated the cumulative damage of icy bodies as a total energy density accumulated by past impacts, according to the crater scaling laws proposed by Housen et al. (1983) of ice and the crater size distributions observed on Phoebe, a saturnian icy satellite. We found that the cumulative damage of Phoebe depended significantly on the impact speed of the impactor that formed the craters on Phoebe; and the cumulative damage was about one-third of the impact strength ΣQ∗ at 500 m s⁻¹ whereas it was almost zero at 3.2 km s⁻¹.

Reference
Yasui M, Hayama R and Masahiko Arakawa M (in press) Impact strength of small icy bodies that experienced multiple collisions. Icarus
[doi:10.1016/j.icarus.2014.02.008]
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S. Xu (许偲艺)¹, M. Jura¹, D. Koester², B. Klein¹ and B. Zuckerman¹

¹Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1562, USA
²Institut fur Theoretische Physik und Astrophysik, University of Kiel, D-24098 Kiel, Germany

We report Keck/HIRES and Hubble Space Telescope/COS spectroscopic studies of extrasolar rocky planetesimals accreted onto two hydrogen atmosphere white dwarfs, G29-38 and GD 133. In G29-38, eight elements are detected, including C, O, Mg, Si, Ca, Ti, Cr, and Fe while in GD 133, O, Si, Ca, and marginally Mg are seen. These two extrasolar planetesimals show a pattern of refractory enhancement and volatile depletion. For G29-38, the observed composition can be best interpreted as a blend of a chondritic object with some refractory-rich material, a result from post-nebular processing. Water is very depleted in the parent body accreted onto G29-38, based on the derived oxygen abundance. The inferred total mass accretion rate in GD 133 is the lowest of all known dusty white dwarfs, possibly due to non-steady state accretion. We continue to find that a variety of extrasolar planetesimals all resemble to zeroth order the elemental composition of bulk Earth.

Reference
Xu S, Jura M, Koester D, Klein B and Zuckerman B (2014) Elemental compositions of two extrasolar rocky planetesimals. The Astrophysical Journal 783:79.
[doi:10.1088/0004-637X/783/2/79]

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Christoph Burkhardt^a, Remco C. Hin^a,1, Thorsten Kleine^b, Bernard Bourdon^c

^aInstitute of Geochemistry and Petrology, Clausiusstrasse 25, ETH Zürich, CH-8092 Zürich, Switzerland
^bInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm Klemm-Strasse 10, 48149 Münster, Germany
^cLaboratoire de Géologie de Lyon, ENS Lyon and Université Claude Bernard Lyon 1, 46 Allée d’Italie, F-69364 Lyon, France
¹School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK.

Mass-dependent Mo isotope fractionation has been investigated for a wide range of meteorites including chondrites (enstatite, ordinary and carbonaceous chondrites), iron meteorites, and achondrites (eucrites, angrites and martian meteorites), as well as for lunar and terrestrial samples. Magmatic iron meteorites together with enstatite, ordinary and most carbonaceous chondrites define a common δ^98/95Mo value of−0.16±0.02‰ (relative to the NIST SRM 3134 Mo standard), which is interpreted to reflect the Mo isotope composition of bulk planetary bodies in the inner solar system. Heavy Mo isotope compositions for IAB iron meteorites most likely reflect impact-induced evaporative losses of Mo from these meteorites. Carbonaceous chondrites define an inverse correlation between δ^98/95Mo and metal content, and a positive correlation between δ^98/95Mo and matrix abundance. These correlations are mainly defined by CM and CK chondrites, and may reflect the heterogeneous distribution of an isotopically light metal and/or an isotopically heavy matrix component in the formation region of carbonaceous chondrites. Alternatively, the elevated δ^98/95Mo of the CM and CK chondrites could result from the loss of volatile, isotopically light Mo oxides, that formed under oxidized conditions typical for the formation of these chondrites.
The Mo isotope compositions of samples derived from the silicate portion of differentiated planetary bodies are heavy compared to the mean composition of chondrites and iron meteorites. This difference is qualitatively consistent with experimental evidence for Mo isotope fractionation between metal and silicate. The common δ^98/95Mo values of −0.05±0.03‰ of lunar samples derived from different geochemical reservoirs indicate the absence of significant Mo isotope fractionation by silicate differentiation or impact metamorphism/volatilization on the Moon. The most straightforward interpretation of the Mo isotope composition of the lunar mantle corresponds to the formation of a lunar core at a metal–silicate equilibration temperature of . The investigated martian meteorites, angrites and eucrites exhibit more variable Mo isotope compositions, which for several samples extend to values above the maximumδ^98/95Mo=+0.14‰ that can be associated with core formation. For these samples post-core formation processes such as partial melting, metamorphism and in the case of meteorite finds terrestrial weathering must have resulted in Mo isotope fractionation. Estimates of the metal–silicate equilibration temperatures for Mars () and the angrite parent body () are thus more uncertain than that derived for the Moon. Although the Mo isotope composition of the bulk silicate Earth has not been determined as part of this study, a value of −0.16‰<δ^98/95Mo<0 can be predicted based on the chondrite and iron meteorite data and by assuming a reasonable temperature range for core formation in the Earth. This estimate is in agreement with four analyzed basalt standards (−0.10±0.10). Improved application of mass-dependent Mo isotope fractionation to investigate core formation most of all requires an improved understanding of potential Mo isotope fractionation during processes not related to metal–silicate differentiation.

Reference
Burkhardt C, Hin RC, Kleine T and Bernard Bourdon B (2014) Evidence for Mo isotope fractionation in the solar nebula and during planetary differentiation. Earth and Planetary Science Letters 391:201–211.
[doi:10.1016/j.epsl.2014.01.037]
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Edgard G. Rivera-Valentin^a,b and Amy C. Barr^a,b

^aBrown University, Department of Geological Sciences, 324 Brook St., Box 1846, Providence, RI 02912, United States
^bCenter for Lunar Origin and Evolution, Southwest Research Institute, Boulder, CO 80302, United States

Remote sensing data suggest Mercury’s surface has compositional variations spatially associated with crater and basin ejecta, the so-called “Low-Reflectance Material” (LRM), which has been suggested to be enriched in a subsurface native darkening agent that is excavated and redeposited onto the surface. This unit may record the evidence of impact-induced mixing of Mercury’s outer layers during its early history. Here, we develop a fully three-dimensional Monte Carlo model of impact cratering, excavation, and ejecta blanket deposition on a global scale for Mercury.
New dynamical simulations of the early evolution of the asteroid belt hint at the presence of additional asteroids in a region interior to the present-day belt, known as the “E-belt”. We use Monte Carlo methods to show that the predicted bombardment from this population matches the observed spatial crater densities on Mercury. Impacts large enough to pierce through the crust create surface ejecta deposits rich in mantle material. Later impacts onto enriched ejecta deposits redistribute mantle material away from the basins. For the suggested average mercurian crustal thickness of 50 km, the surface has, on average, ~0.4% mantle material by volume; the most enriched areas have ~30% mantle by volume.
The regional coverage of impact-induced compositional changes is strongly dependent on the thickness to the subsurface source. Because observations indicate LRM covers ~15% of Mercury’s surface, our model suggests the darkening agent is ~30 km deep. Considering the current estimated average mercurian crustal thickness of 50 km, this implies the darkening agent is likely located within a chemically distinct lower crust.

Reference
Rivera-Valentin EG and Barr AC (2014) Impact-induced compositional variations on Mercury. Earth and Planetary Science Letters 391:201–211.
[doi:10.1016/j.epsl.2014.02.003]
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S. N. Borovikov¹ and N. V. Pogorelov^1,2

¹Center for Space Physics and Aeronomic Research, The University of Alabama in Huntsville, Huntsville, AL 35805, USA
²Department of Space Sciences, The University of Alabama in Huntsville, Huntsville, AL 35805, USA

Recent observations from the Voyager 1 spacecraft show that it is sampling the local interstellar medium (LISM). This is quite surprising because no realistic, steady-state model of the solar wind (SW) interaction with the LISM gives an inner heliosheath width as narrow as ~30 AU. This includes models that assume a strong redistribution of the ion energy to the tails in the pickup ion distribution function. We show that the heliopause (HP), which separates the SW from the LISM, is not a smooth tangential discontinuity, but rather a surface subject to Rayleigh–Taylor-type instabilities which can result in LISM material penetration deep inside the SW. We also show that the HP flanks are always subject to a Kelvin–Helmholtz instability. The instabilities are considerably suppressed near the HP nose by the heliospheric magnetic field in steady-state models, but reveal themselves in the presence of solar cycle effects. We argue that Voyager 1 may be in one such instability region and is therefore observing plasma densities much higher than those in the pristine SW. These results may explain the early penetration of Voyager 1 into the LISM. They also show that there is a possibility that the spacecraft may start sampling the SW again before it finally leaves the heliosphere.

Reference
Borovikov SN and Pogorelov NV (2014) Voyager 1 near the heliopause.  The Astrophysical Journal Letters 783:L16.
[doi:10.1088/2041-8205/783/1/L16]

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