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]

Ji Yeon Seok and Aigen Li
Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA

The debris disk around the Vega-type star HD 34700 is detected in dust thermal emission from the near-infrared (IR) to millimeter (mm) and submm wavelength range. Also detected is a distinct set of emission features at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7µm, which are commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules. We model the observed dust IR spectral energy distribution (SED) and PAH emission features of the HD 34700 disk in terms of porous dust and astronomical-PAHs. Porous dust together with a mixture of neutral and ionized PAHs closely explains the dust IR SED and PAH emission features observed in the HD 34700 disk. Due to the stellar radiation pressure and Poynting–Robertson drag together with the photodissociation of PAHs, substantial removal of dust and PAHs has occurred in the disk, and continuous replenishment of these materials is required to maintain their current abundances. This implies that these materials are not primitive but secondary products probably originating from mutual collisions among planetesimals, asteroids, and comets.

Reference
Seok JY and Li A (2015)Dust and polycyclic aromatic hydrocarbon in the HD 34700 debris disk. Astrophysical Journal 809:22.
Link to Article [doi:10.1088/0004-637X/809/1/22]

Alyssa K. Cobb^1,2, Ralph E. Pudritz^1,2, and Ben K. D. Pearce^1,2
1Origins Institute, McMaster University, ABB 241, 1280 Main Street, Hamilton, ONL8S 4M1, Canada; alyssacobb107@gmail.com, pudritz@physics.mcmaster.ca, ben.pearce@alumni.ubc.ca
²Department of Physics and Astronomy, McMaster University, ABB 241, 1280 Main Street, Hamilton, ONL8S 4M1, Canada

Carbonaceous chondrite meteorites are known for having high water and organic material contents, including amino acids. Here we address the origin of amino acids in the warm interiors of their parent bodies (planetesimals) within a few million years of their formation, and we connect this with the astrochemistry of their natal protostellar disks. We compute both the total amino acid abundance pattern and the relative frequencies of amino acids within the CM2 (e.g., Murchison) and CR2 chondrite subclasses based on Strecker reactions within these bodies. We match the relative frequencies to well within an order of magnitude among both CM2 and CR2 meteorites for parent body temperatures <200°C. These temperatures agree with 3D models of young planetesimal interiors. We find theoretical abundances of approximately 7 × 105 parts per billion, which is in agreement with the average observed abundance in CR2 meteorites of (4 ± 7) × 105, but an order of magnitude higher than the average observed abundance in CM2 meteorites of (2 ± 2) × 104. We find that the production of hydroxy acids could be favored over the production of amino acids within certain meteorite parent bodies (e.g., CI1, CM2) but not others (e.g., CR2). This could be due to the relatively lower NH3 abundances within CI1 and CM2 meteorite parent bodies, which leads to less amino acid synthesis. We also find that the water content in planetesimals is likely to be the main cause of variance between carbonaceous chondrites of the same subclass. We propose that amino acid abundances are primarily dependent on the ammonia and water content of planetesimals that are formed in chemically distinct regions within their natal protostellar disks.

Reference
Cobb AK, Pudritz RE and Pearce BKD (2015) Nature’s starships. II. Simulating the synthesis of amino acids in meteorite parted bodies. Astrophysical Journal 809:6.
Link to Article [doi:10.1088/0004-637X/809/1/6]

^1,2Asmaa Boujibar, ²Denis Andrault, ²Nathalie Bolfan-Casanova, ²Mohamed Ali Bouhifd, ²Julien Monteux
¹Astromaterials Research and Exploration Science, NASA Johnson Space Center, 2101 Nasa Parkway, Houston, Texas 77058, USA.
²Laboratoire Magmas et Volcans, Université Blaise Pascal, CNRS UMR-6524, 5 rue Kessler, 63000 Clermont-Ferrand, France

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Reference
Boujibar A, Andrault D, Bolfan-Casanova N, Bouhifd MA, Monteux J (2015) Cosmochemical fractionation by collisional erosion during the Earth’s accretion. Nature Communications 6, 8295
Link to Article [doi:10.1038/ncomms9295]