R. Jaumann^a,b et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aDLR, Inst. of Planetary Research, Berlin, Germany
^bFreie Universitaet Berlin, Inst. of Geosciences, Berlin, Germany

Deposits of dark material appear on Vesta’s surface as features of relatively low-albedo in the visible wavelength range of Dawn’s camera and spectrometer. Mixed with the regolith and partially excavated by younger impacts, the material is exposed as individual layered outcrops in crater walls or ejecta patches, having been uncovered and broken up by the impact. Dark fans on crater walls and dark deposits on crater floors are the result of gravity-driven mass wasting triggered by steep slopes and impact seismicity. The fact that dark material is mixed with impact ejecta indicates that it has been processed together with the ejected material. Some small craters display continuous dark ejecta similar to lunar dark-halo impact craters, indicating that the impact excavated the material from beneath a higher-albedo surface. The asymmetric distribution of dark material in impact craters and ejecta suggests non-continuous distribution in the local subsurface. Some positive-relief dark edifices appear to be impact-sculpted hills with dark material distributed over the hill slopes. Dark features inside and outside of craters are in some places arranged as linear outcrops along scarps or as dark streaks perpendicular to the local topography. The spectral characteristics of the dark material resemble that of Vesta’s regolith. Dark material is distributed unevenly across Vesta’s surface with clusters of all types of dark material exposures. On a local scale, some craters expose or are associated with dark material, while others in the immediate vicinity do not show evidence for dark material. While the variety of surface exposures of dark material and their different geological correlations with surface features, as well as their uneven distribution, indicate a globally inhomogeneous distribution in the subsurface, the dark material seems to be correlated with the rim and ejecta of the older Veneneia south polar basin structure. The origin of the dark material is still being debated, however, the geological analysis suggests that it is exogenic, from carbon-rich low-velocity impactors, rather than endogenic, from freshly exposed mafic material or melt, exposed or created by impacts.

Reference
Jaumann et al. (in press) The Geological Nature of Dark Material on Vesta and Implicatons for the Subsurface Structure. Icarus
[doi:10.1016/j.icarus.2014.04.035]
Copyright Elsevier

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Tomohiro Ono¹, Hideko Nomura² and Taku Takeuchi²

¹Department of Astronomy, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
²Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8551, Japan

We analytically calculate the marginally stable surface density profile for the rotational instability of protoplanetary disks. The derived profile can be utilized for considering the region in a rotating disk where radial pressure gradient force is comparable to the gravitational force, such as an inner edge, steep gaps or bumps, and an outer region of the disk. In this paper, we particularly focus on the rotational instability in the outer region of disks. We find that a protoplanetary disk with a surface density profile of similarity solution becomes rotationally unstable at a certain radius, depending on its temperature profile and a mass of the central star. If the temperature is relatively low and the mass of the central star is high, disks have rotationally stable similarity profiles. Otherwise, deviation from the similarity profiles of surface density could be observable, using facilities with high sensitivity, such as ALMA.

Reference
Ono T, Nomura H and Takeuchi T (2014) Rotational Instability in the Outer Region of Protoplanetary Disks. The Astrophysical Journal 787:37.
[doi:10.1088/0004-637X/787/1/37]

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Naoji Sugiura and Wataru Fujiya

Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan

We look at the relationship between the value of ε⁵⁴Cr in bulk meteorites and the time (after calcium-aluminum-rich inclusion, CAI) when their parent bodies accreted. To obtain accretion ages of chondrite parent bodies, we estimated the maximum temperature reached in the insulated interior of each parent body, and estimated the initial ²⁶Al/²⁷Al for this temperature to be achieved. This initial ²⁶Al/²⁷Al corresponds to the time (after CAI formation) when cold accretion of the parent body would have occurred, assuming ²⁶Al/²⁷Al throughout the solar system began with the canonical value of 5.2 × 10⁻⁵. In cases of iron meteorite parent bodies, achondrite parent bodies, and carbonaceous chondrite parent bodies, we use published isotopic ages of events (such as core formation, magma crystallization, and growth of secondary minerals) in each body’s history to obtain the probable time of accretion. We find that ε⁵⁴Cr correlates with accretion age: the oldest accretion ages (1 ± 0.5 Ma) are for iron and certain other differentiated meteorites with ε⁵⁴Cr of −0.75 ± 0.5, and the youngest ages (3.5 ± 0.5 Ma) are for hydrated carbonaceous chondrites with ε⁵⁴Cr values of 1.5 ± 0.5. Despite some outliers (notably Northwest Africa [NWA] 011 and Tafassasset), we feel that the correlation is significant and we suggest that it resulted from late, localized injection of dust with extremely high ε⁵⁴Cr.

Reference
Sugiura N and Fujiya W (in press) Correlated accretion ages and ε54Cr of meteorite parent bodies and the evolution of the solar nebula. Meteoritics & Planetary Science
[doi:10.1111/maps.12292]
Published by arrangement with John Wiley & Sons

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Elmar Buhl^a,b, Frank Sommer^b, Michael H. Poelchau^b, Georg Dresen^a and Thomas Kenkmann^b

^aHelmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum (GFZ), Telegrafenberg, D-14473 Potsdam, Germany
^bInstitut für Geo- und Umweltnaturwissenschaften, Albert-Ludwigs-Universität Freiburg (ALU), Albertstr. 23-B, D-79104 Freiburg, Germany

The particle size distribution (PSD) of impact crater ejecta is an important parameter that is useful for understanding the formation of natural craters, the distribution of space debris, the influence of impact events on climate and energy partitioning in impact events. 11 impact experiments into dry and water-saturated sandstone were performed and analyzed. The experiments span a range of impact velocities from 2.5 to 5.3 km s^-1 using projectile sizes from 2.5 to 12 mm. Kinetic impact energies between 874 and 80338 J were achieved. Ejecta of these experiments was collected and the PSD was measured and quantified with power law fits. The resulting power law exponents lie between 2.54 and 2.74. Our results do not show an influence of impact energy or impact velocity on the PSD of impact ejecta. A significant increase in the PSD values was found from dry to water-saturated sandstone targets. We suggest that water saturation of the target has multiple effects on ejecta fragmentation. A comparison of our experimental data with data from the literature shows no correlation between the target material lithology and the ejecta PSD. Interestingly, literature data for disruption experiments revealed a strong influence imparted energy density on the D-values. PSD values were used to calculate the energy spent for target fragmentation and show that the fraction of impact energy used for comminution is in the lower single-digit percentage.

Reference
Buhl E, Sommer F, Poelchau MH, Dresen G and Kenkmann T (in press) Ejecta from Experimental Impact Craters: Particle Size Distribution and Fragmentation Energy. Icarus
[doi:10.1016/j.icarus.2014.04.039]
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Ernesto Palomb^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aINAF Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere, 00133 Rome, Italy

Vesta is the asteroid with the largest albedo variation among the known rocky Solar System objects and shows a widespread occurrence of dark material (DM) and bright material (BM) units. In the first observation phases by the Dawn spacecraft, two main extensions of low albedo areas were identified on Vesta and found to be closely correlated with carbonaceous, OH-rich, material. In this work we use the hyperspectral data provided by the VIR-Dawn imaging spectrometer onboard Dawn to detect and analyze individual, well-defined, dark material units. We define DM units assuming a relative criterion, i.e. reflectance lower than the surroundings. By coupling visible and infrared images of the same area we are able to select real dark material units, discarding false detections created by shadowing effects. A detailed final catalogue of 123 dark units is presented, containing the geographical parameters and the main spectral characteristics for each unit. Independently of the geological context of the dark units, all DMs show similar spectral properties, dominated by the pyroxene absorption features, as is the average spectrum of Vesta. This finding suggests a similar composition, with the presence of darkening agents that also weaken pyroxene band depths. The majority (90%) of the DM units shows a positive correlation between low albedo and an OH band centered at 2.8 μm, confirming the hypothesis that the darkening agents are carbonaceous chondrites, probably delivered by low-velocity impacts of primitive asteroids. A comparison with laboratory spectra allows us to better constrain the size and the composition of the darkening agents. These DM areas seem to be made of eucritic material. The regolith grain size seems to be nearly constant around an average value of 25 μm, and is quite homogenous at least in the first hundreds of meters beneath the Vesta surface, suggesting similar processing mechanisms for both DM and BM.

Reference
Ernesto Palomb et al. (in press) Composition and mineralogy of dark material units on Vesta. Icarus
[doi:10.1016/j.icarus.2014.04.040]
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Ansgar Greshake

Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Berlin, Germany

Hydrous carbonaceous microclasts are by far the most abundant foreign fragments in stony meteorites and mostly resemble CI1-, CM2-, or CR2-like material. Their occurrence is of great importance for understanding the distribution and migration of water-bearing volatile-rich matter in the solar system. This paper reports the first finding of a strongly hydrated microclast in a Rumuruti chondrite. The R3-6 chondrite Northwest Africa 6828 contains a 420 × 325 μm sized angular foreign fragment exhibiting sharp boundaries to the surrounding R-type matrix. The clast is dominantly composed of magnetite, pyrrhotite, rare Ca-carbonate, and very rare Mg-rich olivine set in an abundant fine-grained phyllosilicate-rich matrix. Phyllosilicates are serpentine and saponite. One region of the clast is dominated by forsteritic olivine (Fa_<2) supported by a network of interstitial Ca-carbonate. The clast is crosscut by Ca-carbonate-filled veins and lacks any chondrules, calcium-aluminum-rich inclusions, or their respective pseudomorphs. The hydrous clast contains also a single grain of the very rare phosphide andreyivanovite. Comparison with CI1, CM2, and CR2 chondrites as well as with the ungrouped C2 chondrite Tagish Lake shows no positive match with any of these types of meteorites. The clast may, thus, either represent a fragment of an unsampled lithology of the hydrous carbonaceous chondrite parent asteroids or constitute a sample from an as yet unknown parent body, maybe even a comet. Rumuruti chondrites are a unique group of highly oxidized meteorites that probably accreted at a heliocentric distance >1 AU between the formation regions of ordinary and carbonaceous chondrites. The occurrence of a hydrous microclast in an R chondrite attests to the presence of such material also in this region at least at some point in time and documents the wide distribution of water-bearing (possibly zodiacal cloud) material in the solar system.

Reference
Greshake A (in press) A strongly hydrated microclast in the Rumuruti chondrite NWA 6828: Implications for the distribution of hydrous material in the solar system. Meteoritics & Planetary Science
[doi:10.1111/maps.12295]
Published by arrangement with John Wiley & Sons

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R.M. Suggs^a, D.E. Moser^b, W.J. Cooke^a and R.J. Suggs^a

^aNASA, Marshall Space Flight Center, Meteoroid Environment Office, Natural Environments Branch, EV44 Marshall Space Flight Center, Alabama 35812
^bMITS/Dynetics, Marshall Space Flight Center, Meteoroid Environment Office, Natural Environments Branch, EV44 Marshall Space Flight Center, Alabama 35812

The flashes from meteoroid impacts on the Moon are useful in determining the flux of impactors with masses as low as a few tens of grams. A routine monitoring program at NASA’s Marshall Space Flight Center has recorded over 300 impacts since 2006. A selection of 126 flashes recorded during periods of photometric skies was analyzed, creating the largest and most homogeneous dataset of lunar impact flashes to date. Standard CCD photometric techniques were applied to the video and the luminous energy, kinetic energy, and mass are estimated for each impactor. Shower associations were determined for most of the impactors and a range of luminous efficiencies was considered. The flux to a limiting energy of 2.5×10^-6 kT TNT or 1.05×10⁷ J is 1.03×10^-7 km^-2 hr^-1 and the flux to a limiting mass of 30 g is 6.14×10^-10 m^-2 yr^-1 at the Moon. Comparisons made with measurements and models of the meteoroid population indicate that the flux of objects in this size range is slightly lower (but within the error bars) than flux at this size from the power law distribution determined for the near Earth object and fireball population by Brown et al. 2002. Size estimates for the crater detected by Lunar Reconnaissance Orbiter from a large impact observed on March 17, 2013 are also briefly discussed.

Reference
Suggs RM, Moser DE, Cooke WJ and Suggs RJ (in press) The Flux of Kilogram-sized Meteoroids from Lunar Impact Monitoring. Icarus
[doi:0.1016/j.icarus.2014.04.032]
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Claire L. McLeod, Alan D. Brandon, Rosalind M.G. Armytage

Department of Earth and Atmospheric Sciences, Science and Research 1, University of Houston, 4800 Calhoun Road, Houston, TX, 77204-5007, USA

The Moon likely formed as a result of a giant impact between proto-Earth and another large body. The timing of this event and the subsequent lunar differentiation timescales are actively debated. New high-precision Nd isotope data of Apollo mare basalts are used to evaluate the Low-Ti, High-Ti and KREEP mantle source reservoirs within the context of lunar formation and evolution. The resulting models are assessed using both reported ¹⁴⁶Sm half-lives (68 and 103 Myr). The linear relationship defined by ¹⁴²Nd–¹⁴³Nd systematics does not represent multi-component mixing and is interpreted as an isochron recording a mantle closure age for the Sm–Nd system in the Moon. Using a chondritic source model with present day μ  ¹⁴²Nd of −7.3, the mare basalt mantle source reservoirs closed at  () or  (). In a superchondritic, 2-stage evolution model with present day  of 0, mantle source closure ages are constrained to  () or  ().

The lunar mantle source reservoir closure ages <4.5 Ga may be reconciled by 3 potential scenarios. First, the Moon formed later than currently favored models indicate, such that the lunar mantle closure age is near or at the time of lunar formation. Second, the Moon formed ca. 4.55 to 4.47 Ga and small amounts of residual melts were sustained within a crystallizing lunar magma ocean (LMO) for up to ca. 200 Myr from tidal heating or asymmetric LMO evolution. Third, the LMO crystallized rapidly after early Moon formation. Thus the Sm–Nd mantle closure age represents a later resetting of isotope systematics. This may have resulted from a global wide remelting event. While current Earth-Moon formation constraints cannot exclusively advocate or dismiss any of these models, the fact that U–Pb ages and Hf isotopes for Jack Hills zircons from Australia are best explained by an Earth that re-equilibrated at 4.4 Ga or earlier following the Moon-forming impact, does not favor a later forming Moon. If magma oceans crystallize in a few million years as currently advocated, then a global resetting, possibly by a large impact at 4.40 to 4.34 Ga, such as that which formed the South Pole Aitken Basin, best explains the late mantle closure age for the coupled Sm–Nd isotope systematics presented here.

Reference
McLeod CL, Brandon AD and Armytage RMG (2014) Constraints on the formation age and evolution of the Moon from 142Nd–143Nd systematics of Apollo 12 basalts. Earth and Planetary Science Letters 396:179.
[doi:10.1016/j.epsl.2014.04.007]
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C. Samson¹, S. Butler², C. Fry¹, P. J. A. McCausland³, R. K. Herd^1,4, O. Sharomi⁵, R. J. Spiteri⁵ and M. Ralchenko¹

¹Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada
²Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
³Department of Earth Sciences, Western University, London, Ontario, Canada
⁴Earth Sciences Sector, Natural Resources Canada, Ottawa, Ontario, Canada
⁵Department of Computer Science, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

Ten splash-form tektites from the Australasian strewn field, with masses ranging from 21.20 to 175.00 g and exhibiting a variety of shapes (teardrop, ellipsoid, dumbbell, disk), have been imaged using a high-resolution laser digitizer. Despite challenges due to the samples’ rounded shapes and pitted surfaces, the images were combined to create 3-D tektite models, which captured surface features with a high fidelity (≈30 voxel mm⁻²) and from which volume could be measured noninvasively. The laser-derived density for the tektites averaged 2.41 ± 0.11 g cm⁻³. Corresponding densities obtained via the Archimedean bead method averaged 2.36 ± 0.05 g cm⁻³. In addition to their curational value, the 3-D models can be used to calculate the tektites’ moments of inertia and rotation periods while in flight, as a probe of their formation environment. Typical tektite rotation periods are estimated to be on the order of 1 s. Numerical simulations of air flow around the models at Reynolds numbers ranging from 1 to 10⁶ suggest that the relative velocity of the tektites with respect to the air must have been <10 m s⁻¹ during viscous deformation. This low relative velocity is consistent with tektite material being carried along by expanding gases in the early time following the impact.

Reference
Samson C, Butler S, Fry C, McCausland PJA, Herd RK, Sharomi O, Spiteri RJ and Ralchenko M (in press) 3-D laser images of splash-form tektites and their use in aerodynamic numerical simulations of tektite formation. Meteoritics & Planetary Science
[doi:10.1111/maps.12287]
Published by arrangement with John Wiley & Sons

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Krisztian Fintor¹, Changkun Park², Szabolcs Nagy¹, Elemér Pál-Molnár^1,3 and Alexander N. Krot²

¹Department of Mineralogy, Geochemistry and Petrology, University of Szeged, Szeged, Hungary
²Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i, USA
³MTA-ELTE Volcanology Research Group, Budapest, Hungary

We report an occurrence of hexagonal CaAl₂Si₂O₈ (dmisteinbergite) in a compact type A calcium-aluminum-rich inclusion (CAI) from the CV3 (Vigarano-like) carbonaceous chondrite Northwest Africa 2086. Dmisteinbergite occurs as approximately 10 μm long and few micrometer-thick lath-shaped crystal aggregates in altered parts of the CAI, and is associated with secondary nepheline, sodalite, Ti-poor Al-diopside, grossular, and Fe-rich spinel. Spinel is the only primary CAI mineral that retained its original O-isotope composition (Δ¹⁷O ~ −24‰); Δ¹⁷O values of melilite, perovskite, and Al,Ti-diopside range from −3 to −11‰, suggesting postcrystallization isotope exchange. Dmisteinbergite, anorthite, Ti-poor Al-diopside, and ferroan olivine have ¹⁶O-poor compositions (Δ¹⁷O ~ −3‰). We infer that dmisteinbergite, together with the other secondary minerals, formed by replacement of melilite as a result of fluid-assisted thermal metamorphism experienced by the CV chondrite parent asteroid. Based on the textural appearance of dmisteinbergite in NWA 2086 and petrographic observations of altered CAIs from the Allende meteorite, we suggest that dmisteinbergite is a common secondary mineral in CAIs from the oxidized Allende-like CV3 chondrites that has been previously misidentified as a secondary anorthite.

Reference
Fintor K, Park C, Nagy S, Pál-Molnár E and Krot AN (in press) Hydrothermal origin of hexagonal CaAl2Si2O8 (dmisteinbergite) in a compact type A CAI from the Northwest Africa 2086 CV3 chondrite. Meteoritics & Planetary Science
[doi:10.1111/maps.12294]
Published by arrangement with John Wiley & Sons

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