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]