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

Link to Article

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]
Copyright Elsevier

Link to Article

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]
Copyright Elsevier

Link to Article