H. Leroux^a, P. Cuvillier^a, B. Zanda^b, R.H. Hewins^b
aUnité Matériaux et Transformations, Université Lille 1 & CNRS, 59655 Villeneuve d’Ascq, France
^bInstitut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Universités, Muséum National d’Histoire Naturelle, UPMC Université Paris 06, UMR CNRS 7590, IRD UMR 206, 57 rue Cuvier, 75231 Paris, France

The Paris meteorite is a weakly altered CM chondrite that has been discovered recently (Hewins et al. 2014). Its matrix offers the opportunity to search for well-preserved pristine pre-accretional material, as well as to study the earliest stages of aqueous alteration in the CM parent body. The study of the matrix of Paris has been conducted by analytical transmission electron microscopy on focused ion beam sections extracted from matrix areas showing different degrees of aqueous alteration.

The least altered matrix sample consists of amorphous silicate grains, a few hundreds of nm in size, separated from one another by an abundant porosity. The amorphous silicates enclose numerous Fe-sulfide nanograins and their average composition is close to the chondritic composition. They share many similarities with GEMS (glass with embedded metal and sulfides) grains present in chondritic-porous interplanetary dust particles and with primitive type 3.0 carbonaceous chondrites. This first discovery of GEMS-like texture in a CM chondrite suggests that GEMS grains could have been the building blocks of the CM matrices.

In more aqueously altered samples, pronounced microstructural heterogeneities were detected at the micrometer scale. The matrix consists mostly of a mixture of amorphous material and Fe-rich, spongy to fine-fibrous, poorly crystalline phyllosilicates. The porosity fraction is significantly reduced and the mixed amorphous-fibrous material frequently forms a continuous groundmass. The close association between these two material types suggests a replacement mechanism due to aqueous alteration. Chemical compositions correlate strongly with the microstructure. The amorphous material has a composition close to the chondritic value while the fine-fibrous phyllosilicate material is Fe-enriched. This Fe enrichment is found to be continuous from weakly to more heavily altered areas, in which the fibrous morphology is coarser and better crystalline. Cronstedtite with intercalated tochilinite is also found, but in pore spaces. This chemical evolution, concomitant with the maturation of the phyllosilicates, demonstrates that the early aqueous fluids that interacted with silicates in the matrix were enriched in Fe. This composition is probably the consequence of the preferential dissolution of metal and iron sulfides during the first stages of alteration. The enrichment of phyllosilicates in Mg seen in more altered CM chondrites is not observed in Paris.

Reference
Leroux H, Cuvillier P, Zanda B and Hewins RH (2015) GEMS-like material in the matrix of the Paris meteorite and the early stages of alteration of CM chondrites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.09.019]
Copyright Elsevier

V. Adibekyan¹, N. C. Santos^1,2, P. Figueira¹, C. Dorn³, S. G. Sousa¹, E. Delgado-Mena¹, G. Israelian^4,5, A. A. Hakobyan⁶ and C. Mordasini^3
1Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
²Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
³Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
⁴Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁵Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁶Byurakan Astrophysical Observatory, 0213 Byurakan, Aragatsotn province, Armenia

Aims. The main goal of this work is to study element ratios that are important for the formation of planets of different masses.
Methods. We study potential correlations between the existence of planetary companions and the relative elemental abundances of their host stars. We use a large sample of FGK-type dwarf stars for which precise Mg, Si, and Fe abundances have been derived using HARPS high-resolution and high-quality data.
Results. A first analysis of the data suggests that low-mass planet host stars show higher [Mg/Si] ratios, while giant planet hosts present [Mg/Si] that is lower than field stars. However, we found that the [Mg/Si] ratio significantly depends on metallicity through Galactic chemical evolution. After removing the Galactic evolution trend only the difference in the [Mg/Si] elemental ratio between low-mass planet hosts and non-hosts was present in a significant way. These results suggest that low-mass planets are more prevalent around stars with high [Mg/Si].
Conclusions. Our results demonstrate the importance of Galactic chemical evolution and indicate that it may play an important role in the planetary internal structure and composition. The results also show that abundance ratios may be a very relevant issue for our understanding of planet formation and evolution.

Reference
Adibekyan V, Santos NC, Figueira P, Dorn C, Sousa SG, Delgado-Mena E, Israelian G, Hakobyan AA and Mordasini C (2015) From stellar to planetary composition: Galactic chemical evolution of Mg/Si mineralogical ratio. Astronomy & Astrophysics 581:L2.
Link to Article [doi:10.1051/0004-6361/201527059]

V. Alí-Lagoa, M. Delbo’, and G. Libourel
Laboratoire Lagrange, UMR7293, Université de la Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, F-06304 Nice Cedex 4, France

The so-called “early activity” of comet 67P/Churyumov–Gerasimenko has been observed to originate mostly in parts of the concave region or “neck” between its two lobes. Since activity is driven by the sublimation of volatiles, this is a puzzling result because this area is less exposed to the Sun and is therefore expected to be cooler on average. We used a thermophysical model that takes into account thermal inertia, global self-heating, and shadowing, to compute surface temperatures of the comet. We found that, for every rotation in the 2014 August–December period, some parts of the neck region undergo the fastest temperature variations of the comet’s surface precisely because they are shadowed by their surrounding terrains. Our work suggests that these fast temperature changes are correlated to the early activity of the comet, and we put forward the hypothesis that erosion related to thermal cracking is operating at a high rate on the neck region due to these rapid temperature variations. This may explain why the neck contains some ice—as opposed to most other parts of the surface—and why it is the main source of the comet’s early activity. In a broader context, these results indicate that thermal cracking can operate faster on atmosphereless bodies with significant concavities than implied by currently available estimates.

Reference
Alí-Lagoa V, Delbo’ M and Libourel G (2015) Rapid temperature changes and the early activity on comet 67P/Churumov-Gerasimenko. Astrophysical Journal – Letters 811:L22.
Link to Article [doi:10.1088/2041-8205/810/2/L22]