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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