¹G.J. Consolmagno,^2,3G.J. Golabek,⁴D. Turrini,⁵M. Jutzi,⁶S. Sirono,⁷V. Svetsov, ⁸K. Tsiganis
¹Specola Vaticana, V-00120, Vatican City State
²Institute of Geophysics, ETH Zurich, Sonneggstrasse 5, CH-8092 Zürich, Switzerland
³Bayerisches Geoinstitut, University of Bayreuth, D-95440 Bayreuth, Germany
⁴Istituto di Astrofisica e Planetologia Spaziali INAF-IAPS, Via Fosso del Cavaliere 100, 00133 Rome, Italy
⁵Physics Institute, Space Research and Planetary Sciences, Center for Space and Habitability, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
⁶Earth and Environmental Sciences, Nagoya University, Tikusa-ku, Furo-cho, Nagoya 464-8601 Japan
⁷Institute for Dynamics of Geospheres, Leninskiy Prospekt 38-1, Moscow, Russia
⁸Unit of Mechanics, Section of Astrophysics, Astronomy & Mechanics, Department of Physics, Aristotle University of Thessaloniki, GR 54 124 Thessaloniki, Greece

It is difficult to find a Vesta model of iron core, pyroxene and olivine-rich mantle, and HED crust that can match the joint constraints of (a) Vesta’s density and core size as reported by the Dawn spacecraft team; (b) the chemical trends of the HED meteorites, including the depletion of sodium, the FeO abundance, and the trace element enrichments; and (c) the absence of exposed mantle material on Vesta’s surface, among Vestoid asteroids, or in our collection of basaltic meteorites. These conclusions are based entirely on mass-balance and density arguments, independent of any particular formation scenario for the HED meteorites themselves. We suggest that Vesta either formed from source material with non-chondritic composition or underwent after its formation a radical physical alteration, possibly caused by collisional processes, that affected its global composition and interior structure.

Reference
Consolmagno GJ, Golabek GJ, Turrini D, Jutzi M, Sirono S, Svetsov V, Tsiganis K (2015) Is Vesta an Intact and Pristine Protoplanet? Icarus (in Press)
Link im Article [doi:10.1016/j.icarus.2015.03.029]

Copyright Elsevier

¹Alexander Hubbard
¹Department of Astrophysics, American Museum of Natural History, New York, NY 10024-5192, USA

About 4-5% of chondrules are compound: two separate chondrules stuck together. This is commonly believed to be the result of the two component chondrules having collided shortly after forming, while still molten. This allows high velocity impacts to result in sticking. However, at T ∼1100 K, the temperature below which chondrules collide as solids (and hence usually bounce), coalescence times for droplets of appropriate composition are measured in tens of seconds. Even at 1025 K, at which temperature theory predicts that the chondrules must have collided extremely slowly to have stuck together, the coalescence time scale is still less than an hour. These coalescence time scales are too short for the collision of molten chondrules to explain the observed frequency of compound chondrules. We suggest instead a scenario where chondrules stuck together in slow collisions while fully solid; and the resulting chondrule pair was subsequently briefly heated to a temperature in the range of 900-1025 K. In that temperature window the coalescence time is finite but long, covering a span of hours to a decade. This is particularly interesting because those temperatures are precisely the critical window for thermally ionized MRI activity, so compound chondrules provide a possible probe into that vital regime.

Reference
Hubbard A (2015) Compound Chondrules fused Cold. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.02.030]

Copyright Elsevier

¹Scott L. Murchie et al (>10)*
¹The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
*Find the extensive, full author and affiliation list on the publishers website

A principal data product from MESSENGER’s primary orbital mission at Mercury is a global multispectral map in eight visible to near-infrared colors, at an average pixel scale of 1 km, acquired by the Mercury Dual Imaging System. The constituent images have been calibrated, photometrically corrected to a standard geometry, and map projected. Global analysis reveals no spectral units not seen during MESSENGER’s Mercury flybys and supports previous conclusions that most spectral variation is related to changes in spectral slope and reflectance between spectral end-member high-reflectance red plains (HRP) and low-reflectance material (LRM). Comparison of color properties of plains units mapped on the basis of morphology shows that the two largest unambiguously volcanic smooth plains deposits (the interior plains of Caloris and the northern plains) are close to HRP end members and have average color properties distinct from those of most other smooth plains and intercrater plains. In contrast, smaller deposits of smooth plains are nearly indistinguishable from intercrater plains on the basis of their range of color properties, consistent with the interpretation that intercrater plains are older equivalents of smooth plains. LRM having nearly the same reflectance is exposed in crater and basin ejecta of all ages, suggesting impact excavation from depth of material that is intrinsically dark or darkens very rapidly, rather than the product of gradual darkening of exposed material purely by space weathering. A global search reveals no definitive absorptions attributable to Fe2+-containing silicates or to sulfides over regions 20 km or more in horizontal extent, consistent with results from MESSENGER’s Mercury Atmospheric and Surface Composition Spectrometer. The only absorption-like feature identified is broad upward curvature of the spectrum centered near 600 nm wavelength. The feature is strongest in freshly exposed LRM and weak or absent in older exposures of LRM. We modeled spectra of LRM as intimate mixtures of HRP with candidate low-reflectance phases having a similar 600-nm spectral feature, under the assumption that the grain size is 1 μm or larger. Sulfides measured to date in the laboratory and coarse-grained iron are both too bright to produce LRM from HRP. Ilmenite is sufficiently dark but would require Ti abundances too high to be consistent with MESSENGER X-Ray Spectrometer measurements. Three phases or mixtures of phases that could be responsible for the low reflectance of LRM are consistent with our analyses. Graphite, in amounts consistent with upper limits from the Gamma-Ray Spectrometer, may be consistent with geochemical models of Mercury’s differentiation calling for a graphite-enriched primary flotation crust from an early magma ocean and impact mixing of that early crust before or during the late heavy bombardment (LHB) into material underlying the volcanic plains. The grain size of preexisting iron or iron sulfide could have been altered to a mix of nanophase and microphase grains by shock during those impacts, lowering reflectance. Alternatively, iron-bearing phases and carbon in a late-accreting carbonaceous veneer may have been stirred into the lower crust or upper mantle. Decoupling of variations in color from abundances of major elements probably results from the very low content and variation of Fe2+ in crustal silicates, such that reflectance is controlled instead by minor opaque phases and the extent of space weathering.

Reference
Murchie SL et al. (2015) Orbital Multispectral Mapping of Mercury with the MESSENGER Mercury Dual Imaging System: Evidence for the Origins of Plains Units and Low-Reflectance Material. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.03.027]

Copyright Elsevier