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]

K. Righter¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website
¹Department of Earth and Environmental Studies, Montclair State

Three masses of the Chelyabinsk meteorite have been studied with a wide range of analytical techniques to understand the mineralogical variation and thermal history of the Chelyabinsk parent body. The samples exhibit little to no postentry oxidation via Mössbauer and Raman spectroscopy indicating their fresh character, but despite the rapid collection and care of handling some low levels of terrestrial contamination did nonetheless result. Detailed studies show three distinct lithologies, indicative of a genomict breccia. A light-colored lithology is LL5 material that has experienced thermal metamorphism and subsequent shock at levels near S4. The second lithology is a shock-darkened LL5 material in which the darkening is caused by melt and metal-troilite veins along grain boundaries. The third lithology is an impact melt breccia that formed at high temperatures (~1600 °C), and it experienced rapid cooling and degassing of S2 gas. Portions of light and dark lithologies from Chel-101, and the impact melt breccias (Chel-102 and Chel-103) were prepared and analyzed for Rb-Sr, Sm-Nd, and Ar-Ar dating. When combined with results from other studies and chronometers, at least eight impact events (e.g., ~4.53 Ga, ~4.45 Ga, ~3.73 Ga, ~2.81 Ga, ~1.46 Ga, ~852 Ma, ~312 Ma, and ~27 Ma) are clearly identified for Chelyabinsk, indicating a complex history of impacts and heating events. Finally, noble gases yield young cosmic ray exposure ages, near 1 Ma. These young ages, together with the absence of measurable cosmogenic derived Sm and Cr, indicate that Chelyabinsk may have been derived from a recent breakup event on an NEO of LL chondrite composition.

Reference
Righter et al. (2015) Mineralogy, petrology, chronology, and exposure history of the Chelyabinsk meteorite and parent body. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12511]
Published by arrangement with John Wiley & Sons

Rebecca G. Martin¹ and Mario Livio^2
1Department of Physics and Astronomy, University of Nevada, Las Vegas, 4505 South Maryland Parkway, Las Vegas, NV 89154, USA
²Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

With the availability of considerably more data, we revisit the question of how special our solar system is compared to observed exoplanetary systems. To this goal, we employ a mathematical transformation that allows for a meaningful, statistical comparison. We find that the masses and densities of the giant planets in our solar system are very typical, as is the age of the solar system. While the orbital location of Jupiter is something of an outlier, this is most likely due to strong selection effects toward short-period planets. The eccentricities of the planets in our solar system are relatively small compared to those in observed exosolar systems, but are still consistent with the expectations for an 8-planet system (and could, in addition, reflect a selection bias toward high-eccentricity planets). The two characteristics of the solar system that we find to be most special are the lack of super-Earths with orbital periods of days to months and the general lack of planets inside of the orbital radius of Mercury. Overall, we conclude that, in terms of its broad characteristics, our solar system is not expected to be extremely rare, allowing for a level of optimism in the search for extrasolar life.

Reference
Martin RG and Livio M (2015) The solar system as an explanetary system. Astrophysical Journal 811:105.
Link to Article [doi:10.1088/0004-637X/810/2/105]

Shu Wang^1,2, Aigen Li², and B. W. Jiang^1
1Department of Astronomy, Beijing Normal University, Beijing 100875, China
²Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA

The sizes of interstellar grains are widely distributed, ranging from a few angstroms to a few micrometers. The ultraviolet (UV) and optical extinction constrains the dust in the size range of a couple hundredths of micrometers to several submicrometers. The near and mid infrared (IR) emission constrains the nanometer-sized grains and angstrom-sized very large molecules. However, the quantity and size distribution of micrometer-sized grains remain unknown because they are gray in the UV/optical extinction and they are too cold and emit too little in the IR to be detected by IRAS, Spitzer, or Herschel. In this work, we employ the ~3–8 μm mid-IR extinction, which is flat in both diffuse and dense regions to constrain the quantity, size, and composition of the μm-sized grain component. We find that, together with nano- and submicron-sized silicate and graphite (as well as polycyclic aromatic hydrocarbons), μm-sized graphite grains with C/H ≈ 137 ppm and a mean size of ~1.2 μm closely fit the observed interstellar extinction of the Galactic diffuse interstellar medium from the far-UV to the mid-IR, as well as the near-IR to millimeter thermal emission obtained by COBE/DIRBE, COBE/FIRAS, and Planck up to λ lesssim 1000 μm. The μm-sized graphite component accounts for ~14.6% of the total dust mass and ~2.5% of the total IR emission.

Reference
Wang S, Li A and Jiang BW (2015) Very large interstellar grains as evidenced by the mid-infrared extinction. Astrophysical Journal 811:38.
Link to Article [doi:10.1088/0004-637X/811/1/38]

Bernhard Mayer¹, Nadine Wittig^1,2, Munir Humayun1, and Ingo Leya3^1,4
1National High Magnetic Field Laboratory and Department of Earth, Ocean & Atmospheric Science, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310, USA
²Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, K1S 5B6 Ontario, Canada
³Space Science and Planetology, Institute of Physics, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

The origin of ubiquitous nucleosynthetic isotope anomalies in meteorites may represent spatial and/or temporal heterogeneity in the sources that supplied material to the nascent solar nebula, or enhancement by chemical processing. For elements beyond the Fe peak, deficits in s-process isotopes have been reported in some (e.g., Mo, Ru, W) but not all refractory elements studied (e.g., Os) that, among the iron meteorites, are most pronounced in IVB iron meteorites. Palladium is a non-refractory element in the same mass region as Mo and Ru. In this study, we report the first precise Pd isotopic abundances from IVB irons to test the mechanisms proposed for the origin of isotope anomalies. First, this study determined the existence of a cosmogenic neutron dosimeter from the reaction ¹⁰³Rh(n, β−)¹⁰⁴Pd in the form of excess ¹⁰⁴Pd, correlated with excess ¹⁹²Pt, in IVB irons. Second, all IVB irons show a deficit of the s-process only isotope 104Pd (ε¹⁰⁴Pd = −0.48 ± 0.24), an excess of the r-only isotope ¹¹⁰Pd (ε¹¹⁰Pd = +0.46 ± 0.12), and no resolvable anomaly in the p-process ¹⁰²Pd (ε¹⁰²Pd = +1 ± 1). The magnitude of the Pd isotope anomaly is about half that predicted from a uniform depletion of the s-process yields from the correlated isotope anomalies of refractory Mo and Ru. The discrepancy is best understood as the result of nebular processing of the less refractory Pd, implying that all the observed nucleosynthetic anomalies in meteorites are likely to be isotopic relicts. The Mo–Ru–Pd isotope systematics do not support enhanced rates of the ²²Ne(α,n)²⁵Mg neutron source for the solar system s-process.

Reference
Mayer B, Witting N, Human M, and Leya I (2015) Palladium isotopic evidence for nucleosynthetic and cosmogenic isotope anomalies in IVB iron meteorites. Astrophysical Journal 809:180.
Link to Article [doi:10.1088/0004-637X/809/2/180]

Amanda I. Karakas^1,2, Ashley J. Ruiter^1,3, and Melanie Hampel^1,4
1Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT 2611, Australia
²Kavli IPMU (WPI), The University of Tokyo, Japan
³ARC Centre of Excellence for All-Sky Astrophysics (CAASTRO), Australia
⁴Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany

We present a new theoretical estimate for the birthrate of R Coronae Borealis (RCB) stars that is in agreement with recent observational data. We find the current Galactic birthrate of RCB stars to be ≈25% of the Galactic rate of Type Ia supernovae, assuming that RCB stars are formed through the merger of carbon–oxygen and helium-rich white dwarfs. Our new RCB birthrate (1.8 × 10^-3 yr^-1) is a factor of 10 lower than previous theoretical estimates. This results in roughly 180–540 RCB stars in the Galaxy, depending on the RCB lifetime. From the theoretical and observational estimates, we calculate the total dust production from RCB stars and compare this rate to dust production from novae and born-again asymptotic giant branch (AGB) stars. We find that the amount of dust produced by RCB stars is comparable to the amounts produced by novae or born-again post-AGB stars, indicating that these merger objects are a viable source of carbonaceous pre-solar grains in the Galaxy. There are graphite grains with carbon and oxygen isotopic ratios consistent with the observed composition of RCB stars, adding weight to the suggestion that these rare objects are a source of stardust grains.

Reference
Karakas AI, Ruiter AJ and Hampel M (2015) R Coronae Borealis stars are viable factories of pre-solar grains. Astrophysical Journal 809:184.
Link to Article [doi:doi:10.1088/0004-637X/809/2/184]

Evan Groopman¹ et al. (>10)*
¹Laboratory for Space Sciences, Physics Department, Washington University, One Brookings Drive, Campus Box 1105, Saint Louis, MO 63130, USA

We performed an in-depth exploration of the Al–Mg system for presolar graphite, SiC, and Si₃N₄ grains found to contain large excesses of ²⁶Mg, indicative of the initial presence of live ²⁶Al. Ninety of the more than 450 presolar grains processed in this study contain well-correlated δ²⁶Mg/²⁴Mg and ²⁷Al/²⁴Mg ratios, derived from Nano-scale Secondary Ion Mass Spectrometer depth profiles, whose isochron-like regression lines yield inferred initial ²⁶Al/²⁷Al ratios that, on average, are ~1.5–2 times larger than the ratios previously reported for the grains. The majority of presolar graphite and SiC grains are heavily affected by Al contamination, resulting in large negative δ²⁶Mg/²⁴Mg intercepts of the isochron lines. Al contamination is potentially due to etching of the grains’ surfaces and subsequent capture of dissolved Al during the acid dissolution of their meteorite host rocks. From the isochron fits, the magnitude of Al contamination was quantified for each grain. The amount of Al contamination on each grain was found to be random and independent of grain size, following a uniform distribution with an upper bound at 59% contamination. The Al contamination causes conventional whole-grain estimates to underpredict the initial ²⁶Al/²⁷Al ratios. The presolar grains with the highest ²⁶Al/²⁷Al ratios are from Type II supernovae whose isochron-derived initial ²⁶Al/²⁷Alratios greatly exceed those predicted in the He/C and He/N zones of SN models.

Reference
Groopman et al. (2015) Inferred initial 26Al/27Al ratios in presolar stardust grains from supernovae are higher than previously estimated. Astrophysical Journal 809:31.
Link to Article [doi:10.1088/0004-637X/809/1/31]

R. E. Johnson^1,2, A. Oza^3,4, L. A. Young⁵, A. N. Volkov⁶, and C. Schmidt^1,3
1Engineering Physics, University of Virginia, Charlottesville, VA 22904, USA
²Physics, New York University, New York, NY 10003, USA
³Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA
⁴CNRS, LATMOS/IPSL, Université Pierre et Marie Curie, Paris, France
⁵SwRI, 1050 Walnut Street, Boulder, CO 80302-5150, USA
⁶Department of Mechanical Engineering, University of Alabama, Tuscaloosa, AL 35487, USA

Observations indicate that some of the largest Kuiper Belt Objects (KBOs) have retained volatiles in the gas phase (e.g., Pluto), while others have surface volatiles that might support a seasonal atmosphere (e.g., Eris). Since the presence of an atmosphere can affect their reflectance spectra and thermal balance, Schaller & Brown examined the role of volatile escape driven by solar heating of the surface. Guided by recent simulations, we estimate the loss of primordial N2 for several large KBOs, accounting for escape driven by UV/EUV heating of the upper atmosphere as well as by solar heating of the surface. For the latter we present new simulations and for the former we scale recent detailed simulations of escape from Pluto using the energy limited escape model validated recently by molecular kinetic simulations. Unlike what has been assumed to date, we show that unless the N2 atmosphere is thin (<~10¹⁸ N2 cm^-2) and/or the radius small (<~200–300 km), escape is primarily driven by the UV/EUV radiation absorbed in the upper atmosphere. This affects the discussion of the relationship between atmospheric loss and the observed surface properties for a number of the KBOs examined. Our long-term goal is to connect detailed atmospheric loss simulations with a model for volatile transport for individual KBOs.

Reference
Johnson RE, Oza A, Young LA, Volkov AN and Schmidt C (2015) Volatile loss and classification of Kuiper belt objects. Astrophysical Journal 809:43.
Link to Article [doi:10.1088/0004-637X/809/1/43]