Miriam Sharp^a, Iva Gerasimenko^a, Lorne C. Loudin^b, Jingao Liu^a,c, Odette B. James^d, Igor S. Puchtel^a and Richard J. Walker^a

^aDepartment of Geology, University of Maryland, College Park, MD 20742, USA
^bDepartment of Earth Sciences, University of New Hampshire, Durham, NH 03824, USA
^cDepartment of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G2E3, Canada
^dEmeritus U.S.Geological Survey, Reston, VA 20192, USA

Concentrations of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, and Pd and ¹⁸⁷Os/¹⁸⁸Os isotopic compositions are reported for seven Apollo 17 impact melt rocks. These data are used to examine the dominant chemical signature of the impactor that formed the melts. Six of the samples (72355, 72435, 72535, 76035, 76055, and 76135) have poikilitic textures; one sample (73235) has an aphanitic texture. Data for the samples define linear correlations when Ir is plotted versus other HSE concentrations, with y-intercepts indistinguishable from zero for most HSE in most rocks. Scatter about some of the trends, and occasional trends with positive y-intercepts, indicate either mixing of additional components that are heterogeneously distributed within several rocks, or modest fractionation of some HSE by volatilization, crystal fractionation, or other processes, during formation and evolution of the melt sheet. There is no statistical difference between the aphanitic and poikilitic samples in terms of HSE ratios after visible granulitic clasts were removed from aphanite 73235. Hence, earlier speculations that the two types of impact melt rocks at this site may have been generated by different impactors are not supported by our data.
Most Apollo 17 samples examined here and in prior studies are characterized by very similar HSE signatures, consistent with a common impactor. These samples are characterized by elevated Ru/Ir, Pd/Ir, and Re/Os, relative to most chondrites. Collectively, the data indicate that the impactor was characterized by the following HSE ratios (2σ): Re/Ir 0.093±0.020, Os/Ir 1.03±0.28, Ru/Ir 1.87±0.30, Pt/Ir 2.36±0.31, Pd/Ir 1.85±0.41, and present-day ¹⁸⁷Os/¹⁸⁸Os of 0.1322±0.0013. The results most likely mean that the impactor was a body with a bulk composition that was just outside the range of meteoritic compositions currently sampled on Earth.

Reference
Sharp M, Gerasimenko I, Loudin LC, Liu J, James OB, Puchtel IS and Walker RJ (in press) Characterization of the Dominant Impactor Signature for Apollo 17 Impact Melt Rocks. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.01.014]
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David Polishook^a, Nicholas Moskovitz^a, Richard P. Binzel^a, Francesca E. DeMeo^a,b, David Vokrouhlický^c, Jindřich Žižka^c, Dagmara Oszkiewicz^d

^aDepartment of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
^bHarvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
^cInstitute of Astronomy, Charles University, Prague, V Holešovičkách 8, CZ – 18000 Prague 8, Czech Republic
^dAstronomical Observatory Institute, Faculty of Physics, A. Mickiewicz University, Słoneczna 36, 60-286, Poznań, Poland

The rotational-fission of a “rubble-pile” structured asteroid can result in an “asteroid pair” – two un-bound asteroids sharing nearly identical heliocentric orbits. Models suggest that this mechanism exposes material from below the progenitor surface that previously had never have been exposed to the weathering conditions of space. Therefore, the surfaces of asteroid pairs offer the opportunity to observe non-weathered “fresh” spectra.
Here we report near-infrared spectroscopic observations of 31 asteroids in pairs. In order to search for spectral indications of fresh surfaces we analyze their spectral slopes, parameters of their 1μm absorption band and taxonomic classification. Additionally, through backward dynamical integration we estimate the time elapsed since the disintegration of the pairs’ progenitors.
Analyzing the 19 ordinary chondrite-like (S-complex) objects in our sample, we find two Q-type asteroids (19289 and 54827) that are the first of their kind to be observed in the main-belt of asteroids over the full visible and near-infrared range. This solidly demonstrates that the Q-type taxonomy is not limited to the NEA population.
The pairs in our sample present a range of fresh and weathered surfaces with no clear evidence for a correlation with the ages of the pairs. However, our sample includes “old” pairs (2×10⁶ ⩾ age ⩾ 1×10⁶ years) that present relatively low, meteoritic-like spectral slopes (<0.2% per μm). This illustrates a timescale of at least ∼2 million years before an object develops high spectral slope that is typical for S-type asteroids.
We discuss three mechanisms that explain the existence of weathered pairs with young dynamical ages and find that the “secondary fission” model (Jacobson and Scheeres 2011) is the most robust with our observations. In this mechanism an additional and subsequent fission of the secondary component contributes the lion share of fresh material that re-settles on the primary’s surface and recoats it with fresh material. If the secondary breaks loose from the vicinity of the primary before its “secondary fission”, this main source of fresh dust is avoided. We prefer this secondary fission model since i) the secondary members in our sample present “fresh” parameters that tend to be “fresher” than their weathered primaries; ii) most of the fresh pairs in our sample have low size ratios between the secondary and the primary; iii) 33% of the primaries in our sample are fresh, similar to the prediction set by the secondary fission model (Jacobson and Scheeres 2011); iv) known satellites orbit two of the pairs in our sample with low size ratio (D₂/D₁) and fresh surface; v) there is no correlation between the weathering state and the primary shape as predicted by other models.

Reference
Polishook D, Moskovitz N, Binzel RP, DeMeo FE, Vokrouhlický D, Žižka J and Oszkiewicz D (in press) Observations of “Fresh” and Weathered Surfaces on Asteroid Pairs and Their Implications on the Rotational-Fission Mechanism. Icarus
[doi:10.1016/j.icarus.2014.01.014]
Copyright Elsevier

Link to Article