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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