Jean-David Bodénan^a,b, Natalie A. Starkey^a, Sara S. Russell^b, Ian P. Wright^a, Ian A. Franchi^a

^aPlanetary and Space Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
^aDepartment of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom

Calcium–aluminium-rich Inclusions (CAIs) and the thin Wark–Lovering (WL) rims of minerals surrounding them offer a record of the nature of changing conditions during the earliest stages of Solar System formation. Considerable heterogeneity in the gas composition in the immediate vicinity of the proto-Sun had previously been inferred from oxygen isotopic variations in the WL rim of a CAI from Allende (Simon et al., 2011). However, high precision and high spatial resolution oxygen isotope measurements presented in this study show that WL rim and pristine core minerals of individual CAIs from meteorites that had experienced only low degrees of alteration or low grade metamorphism (one from Léoville (reduced CV3), two in QUE 99177 (CR3.0) and two in ALHA 77307 (CO3.0)) are uniformly ¹⁶O-rich. This indicates that the previously observed variations are the result of secondary processes, most likely on the asteroid parent body, and that there were no temporal or spatial variations in oxygen isotopic composition during CAI and WL rim formation. Such homogeneity across three groups of carbonaceous chondrites lends further support for a common origin for the CAIs in all chondrites. ¹⁶O-poor oxygen reservoirs such as those associated with chondrule formation, were probably generated by UV photo-dissociation involving self-shielding mechanisms and must have occurred elsewhere in outer regions of the solar accretion disk.

Reference
Bodénan J-D, Starkey NA, Russell SS, Wright IP and Franchi IA (2014) An oxygen isotope study of Wark–Lovering rims on type A CAIs in primitive carbonaceous chondrites. Earth and Planetary Science Letters 401:327.
[doi:10.1016/j.epsl.2014.05.035]
Copyright Elsevier

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P. Mahanti^a, M.S. Robinson^a, D.C. Humm^b, J.D. Stopar^a

^aSchool of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
^bSpace Instrument Calibration Consulting, Annapolis, Maryland, USA

Historically, topographic profiles provided a quantitative means to investigate the morphology and formation processes for impact craters, although no generic mathematical framework was developed to reduce profiles to morphology descriptors. Only need-specific polynomial expressions were utilized in previous studies, thus no standardized automated comparison of craters exists. We employ a Chebyshev polynomial function approximation to describe crater forms in a quantitative and repeatable manner. We show that the Chebyshev polynomials return coefficients that are relatable to crater morphologic characteristics, thus providing a standardized mathematical means for describing crater forms.

Reference
Mahanti P, Robinson MS, Humm DC and Stopar JD (2014) A standardized approach for quantitative characterization of impact crater topography. Icarus
[doi:10.1016/j.icarus.2014.06.023]
Copyright Elsevier

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A. Guilbert-Lepoutre¹, R. Schulz¹, A. Rożek², S. C. Lowry², G. P. Tozzi³ and J. A. Stüwe^{4, 5}

¹European Space Agency – ESTEC, 2200 AG Noordwijk, The Netherlands
²Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent, Canterbury CT2 7NH, UK
³Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁴Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵Wincor Nixdorf Portavis GmbH, Wendenstrasse 21, 20097 Hamburg, Germany

Context. Comets are believed to hold a relatively pristine record of the physical and chemical processes that occurred during the formation and evolution of the solar system. Thorough investigations of these small bodies, such as the one that will be performed by the ESA/Rosetta cornerstone mission, are thus supposed to bring strong and unique constraints on the origins of the solar system.
Aims. Because comet 67P/Churyumov-Gerasimenko was only recently selected as the target for the ESA/Rosetta mission, there has been little opportunity to study its pre-perihelion activity. This phase is, however, very important for the mission, since the global mapping of the nucleus and the choice of landing site for Philae will be performed during this pre-perihelion phase. Here, we report previously unpublished data of the last pre-perihelion passage of this comet, observed between May and September 2008.
Methods. The gas and dust activity of comet 67P/Churyumov-Gerasimenko are studied through visible spectroscopy and broadband imaging, respectively, covering a range of pre-perihelion heliocentric distances between 2.99 and 2.22 AU.
Results. The data we have gathered on the dust activity are consistent with trends observed by other authors and show a strong asymmetry between the pre- and post-perihelion phases of the orbit. The spectra do not show any lines due to the emission of volatiles, and upper limits on their production rates are typically one order of magnitude lower than at the equivalent post-perihelion heliocentric distances. The asymmetry in the pre- and post-perihelion phases of the activity may be due to a dusty crust quenching the activity at the surface of 67P. We estimate that this crust could be about 12 cm thick, although not uniform across the surface. Even if no gas is individually detected, the coma surface brightness profiles might indicate a possible contamination from gaseous species emitted before the comet actually reaches perihelion.

Reference
Guilbert-Lepoutre A, Schulz R, Rożek A, Lowry SC, Tozzi GP and Stüwe JA (2014) Pre-perihelion activity of comet 67P/Churyumov-Gerasimenko. Astronomy & Astrophysics 567:L2.
[doi:10.1051/0004-6361/201424186]
Reproduced with permission © ESO

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Jorge Meléndez¹, Lucas Schirbel¹, TalaWanda R. Monroe¹, David Yong², Iván Ramírez³ and Martin Asplund²

¹Departamento de Astronomia do IAG/USPUniversidade de São Paulo, rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, SP Brazil
²Research School of Astronomy and Astrophysics, The Australian National University, Cotter Road, Weston ACT 2611, Australia
³McDonald Observatory and Department of Astronomy, University of Texas at Austin, USA

Context. The standard solar model fails to predict the very low lithium abundance in the Sun, which is much lower than the proto-solar nebula (as measured in meteorites). This Li problem has been debated for decades, and it has been ascribed either to planet formation or to secular stellar depletion due to additional mixing below the convection zone, either during the pre-main sequence and thus possibly linked to planet formation, or additionally on secular time-scales during the main sequence. In order to test the evolution of Li, it is important to find solar twins in a range of ages, i.e., stars with about one solar mass and metallicity but in different evolutionary stages. Furthermore, the study of stars similar to the Sun is relevant in relation to the signature of terrestrial planet formation around the Sun, and for anchoring photometric and spectroscopic stellar parameter scales.
Aims. We aim to identify and analyse solar twins using high quality spectra, in order to study Li depletion in the Sun and the possible relation between chemical abundance anomalies and planet formation.
Methods. We acquired high-resolution (R ~ 110 000), high S/N (~300) ESO/VLT UVES spectra of several solar twin candidates and the Sun (as reflected from the asteroid Juno). Among the solar twin candidates we identify HIP 114328 as a solar twin and perform a differential line-by-line abundance analysis of this star relative to the Sun.
Results. HIP 114328 has stellar parameters T_eff = 5785 ± 10 K, log g = 4.38 ± 0.03, [ Fe/H] = −0.022 ± 0.009, and a microturbulent velocity 0.05 ± 0.03 km s^-1 higher than solar. The differential analysis shows that this star is chemically very similar to the Sun. The refractory elements seem slightly more depleted than in the Sun, meaning that HIP 114328 may be as likely to form terrestrial planets as the Sun. HIP 114328 is about 2 Gyr older than the Sun, and is thus the second oldest solar twin analysed at high precision. It has a Li abundance of A(Li)_NLTE ≲ 0.46, which is about 4 times lower than in the Sun (A(Li)_NLTE = 1.07 dex), but close to the oldest solar twin known, HIP 102152.
Conclusions. Based on the lower abundances of refractory elements when compared to other solar twins, HIP 114328 seems an excellent candidate to host rocky planets. The low Li abundance of this star is consistent with its old age and fits very well the emerging Li-age relation among solar twins of different ages.

Reference
Melendez J, Schirbel L, Monroe TR, Yong D, Ramírez I and Asplund M(2014) HIP 114328: a new refractory-poor and Li-poor solar twin. Astronomy & Astrophysics 567:L3.
[doi:10.1051/0004-6361/201424172]
Reproduced with permission © ESO

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