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

^aPlanetary Science Institute, Tucson, AZ 85719, USA

We investigated the spectral and compositional properties of Chelyabinsk meteorite to identify its possible parent body in the main asteroid belt. Our analysis shows that the meteorite contains two spectrally distinct but compositionally indistinguishable components of LL5 chondrite and shock blackened/impact melt material. Our X-ray diffraction analysis confirms that the two lithologies of the Chelyabinsk meteorite are extremely similar in modal mineralogy. The meteorite is compositionally similar to LL chondrite and its most probable parent asteroid in the main belt is a member of the Flora family. Our work confirms previous studies (e.g., ,  and ), linking LL chondrites to the Flora family. Intimate mixture of LL5 chondrite and shock blackened/impact melt material from Chelyabinsk provides a spectral match with (8) Flora, the largest asteroid in the Flora family. The Baptistina family and Flora family overlap each other in dynamical space. Mineralogical analysis of (298) Baptistina and 11 small family members shows that their surface compositions are similar to LL chondrites, although their absorption bands are subdued and albedos lower when compared to typical S-type asteroids. A range of intimate mixtures of LL5 chondrite and shock blackened/impact melt material from Chelyabinsk provides spectral matches for all these BAF members. We suggest that the presence of a significant shock/impact melt component in the surface regolith of BAF members could be the cause of lower albedo and subdued absorption bands. The conceptual problem with part of this scenario is that impact melts are very rare within ordinary chondrites. Of the ∼42,000 ordinary chondrites, less than 0.5% (203) of them contain impact melts. A major reason that impact melts are rare in meteorites is that high impact velocities (V > 10 km/s) are needed to generate the necessary shock pressures and temperatures (e.g.,Pierazzo and Melosh 1998) unless the target material is highly porous. Nearly all asteroid impacts within the main belt are at ∼5 km/s (Bottke et al., 1994), which prevents them from producing much impact melt unless they are highly porous. However, shock darkening is an equally efficient process that takes place at much lower impact velocities (∼2 km/s) and can cause the observed spectral effects. Spectral effects of shock darkening and impact melt are identical. The parent asteroid of BAF was either a member of the Flora family or had the same basic composition as the Floras (LL Chondrite). The shock pressures produced during the impact event generated enough impact melt or shock blackening to alter the spectral properties of BAF, but keep the BAF composition largely unchanged. Collisional mixing of shock blackened/impact melt and LL5 chondritic material could have created the Baptistina Asteroid Family with composition identical to those of the Floras, but with subdued absorption bands. Shock darkening and impact melt play an important role in altering the spectral and albedo properties of ordinary chondrites and our work confirms earlier work by Britt and Pieters (1994).

Reference
Vishnu Reddy et al. (in press) Chelyabinsk meteorite explains unusual spectral properties of Baptistina Asteroid Family. Icarus
[doi:10.1016/j.icarus.2014.04.027]
Copyright Elsevier

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Francesco L. Villante^1,2, Aldo M. Serenelli³, Franck Delahaye⁴, and Marc H. Pinsonneault⁵

¹Dipartimento di Scienze Fisiche e Chimiche, Università dell’Aquila, I-67100 L’Aquila, Italy
²Istituto Nazionale di Fisica Nucleare (INFN), Laboratori Nazionali del Gran Sasso (LNGS), I-67100 Assergi (AQ), Italy
³Instituto de Ciencias del Espacio (CSIC-IEEC), Facultad de Ciencias, E-08193 Bellaterra, Spain
⁴LERMA, Observatoire de Paris, ENS, UPMC, UCP, CNRS, F-92190 Meudon, France
⁵Astronomy Department, Ohio State University, Columbus, OH 43210, USA

We perform a quantitative analysis of the solar composition problem by using a statistical approach that allows us to combine the information provided by helioseismic and solar neutrino data in an effective way. We include in our analysis the helioseismic determinations of the surface helium abundance and of the depth of the convective envelope, the measurements of the ⁷Be and ⁸B neutrino fluxes, and the sound speed profile inferred from helioseismic frequencies. We provide all the ingredients to describe how these quantities depend on the solar surface composition, different from the initial and internal composition due to the effects of diffusion and nuclear reactions, and to evaluate the (correlated) uncertainties in solar model predictions. We include error sources that are not traditionally considered such as those from inversion of helioseismic data. We, then, apply the proposed approach to infer the chemical composition of the Sun. Our result is that the opacity profile of the Sun is well constrained by the solar observational properties. In the context of a two-parameter analysis in which elements are grouped as volatiles (i.e., C, N, O, and Ne) and refractories (i.e., Mg, Si, S, and Fe), the optimal surface composition is found by increasing the abundance of volatiles by (45 ± 4)% and that of refractories by (19 ± 3)% with respect to the values provided by Asplund et al. (2009, ARA&A, 47, 481). This corresponds to the abundances ε_O = 8.85 ± 0.01 and ε_Fe = 7.52 ± 0.01, which are consistent at the ~1σ level with those provided by Grevesse & Sauval (1998, SSRv, 85, 161). As an additional result of our analysis, we show that the best fit to the observational data is obtained with values of input parameters of the standard solar models (radiative opacities, gravitational settling rate, and the astrophysical factors S₃₄ and S₁₇) that differ at the ~1σ level from those presently adopted.

Reference
Villante FL, Serenelli AM, Delahaye F and Pinsonneault MH (2014) The Chemical Composition of the Sun from Helioseismic and Solar Neutrino Data. The Astrophysical Journal 787:13.
[doi:10.1088/0004-637X/787/1/13]

Link to Article