Julia Roszjar^a,c, Martin J. Whitehouse^b and Addi Bischoff^c

^aInstitut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Carl-Zeiss-Promenade 10, DE-07745 Jena, Germany
^bDepartment of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
^cInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, DE-48149 Münster, Germany

In common with the remarkable variation in the bulk rock Zr content of distinct meteorite groups, ranging from <1 ppm to >800 ppm, the occurrence and abundance of accessory zircon is also highly diverse and limited to certain meteorite classes. A detailed literature study on the occurrence of meteoritic zircon, along with other Zr-bearing phases reveals that lunar rocks, eucrites and mesosiderites are the prime sources of meteoritic zircon. Rare zircon grains occur in chondrites, silicate-bearing iron meteorites and Martian meteorites, with grain sizes of >5 μm allowing chemical and chronological studies at high spatial resolution using secondary ion mass spectrometry (SIMS) technique. Grain sizes, crystal habits, structural and chemical characteristics of zircon grains derived from various meteorite types, including their REE abundances, minor element concentrations, and Zr/Hf values is diverse. Superchondritic Zr/Hf values (47 ± 8; s.d. with n = 97), i.e., typical for zircon in eucrites and mesosiderites, indicate crystallization from a fractionated, incompatible-element-rich (residual) melt. Differences in REE abundances, occurrence or absence of Ce- and Eu-anomalies, and overall REE patterns that are often fractionated with a depletion in LREE, might be primarily controlled by variable formation conditions of individual grains and/or differences in the residual melt compositions on a small, local scale within single samples. Subsequent fractionation/modification of the chemical fingerprint of meteoritic zircon can involve high-temperature annealing processes during thermal metamorphic reactions and/or impact events along with mixing of lithic fragments since many samples are breccias.

Reference
Roszjar J, Whitehouse MJ and Bischoff A (in press) Meteoritic zircon – Occurrence and chemical characteristics. Chemie der Erde – Geochemistry
[doi:10.1016/j.chemer.2014.05.002]
Copyright Elsevier

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Stefano Orsini, Valeria Mangano, Alessandro Mura, Diego Turrini, Stefano Massetti, Anna Milillo, Christina Plainaki

INAF-IAPS, Istituto di Astrofisica e Planetologia Spaziali, Roma, Italy

Mercury, due to its close location to the Sun, is surrounded by an environment whose conditions may be considered as ‘extreme’ in the entire Solar System. Both solar wind and radiation are stronger with respect to other Solar System bodies, so that their interactions with the planet cause high emission of material from its surface. Moreover, the meteoritic precipitation plays a significant role in surface emission processes. This emitted material is partially lost in space. Although under the present conditions the surface particles loss rate does not seem to be able to produce significant erosion of the planetary mass and volume, the long-term effects over billions of years should be carefully considered to properly understand the evolution of the planet. In the early stages, under even more extreme conditions, some of these processes were much more effective in removing material from the planet’s surface. This study attempts to provide a rough estimation of the material loss rate as a function of time, in order to evaluate whether and how this environmental effect can be applied to understand the Hermean surface evolution. We show that the most potentially effective Sun-induced erosion process in early times is a combination of ion sputtering, photon stimulated desorption and enhanced diffusion, which could have caused the loss of a surface layer down to a depth of 20 m, as well as a relevant Na depletion.

Reference
Orsini S, Mangano V, Mura A, Turrini D, Massetti S, Milillo A and Plainaki C (in press) The Influence of Space Environment on the Evolution of Mercury. Icarus
[doi:10.1016/j.icarus.2014.05.031]
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Andrea Milani^a, Alberto Cellino^b, Zoran Knežević^c, Bojan Novaković^d, Federica Spoto^a and Paolo Paolicchi^e

^aDipartimento di Matematica, Università di Pisa, Largo Pontecorvo 5, 56127 Pisa, Italy
^bINAF–Osservatorio Astrofisico di Torino, 10025 Pino Torinese, Italy
^cAstronomical Observatory, Volgina 7, 11060 Belgrade 38, Serbia
^dDepartment of Astronomy, Faculty of Mathematics, University of Belgrade, Studenski trg 16, 11000 Belgrade, Serbia
^eDipartimento di Fisica, Università di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy

The number of asteroids with accurately determined orbits increases fast, and this increase is also accelerating. The catalogs of asteroid physical observations have also increased, although the number of objects is still smaller than in the orbital catalogs. Thus it becomes more and more challenging to perform, maintain and update a classification of asteroids into families. To cope with these challenges we developed a new approach to the asteroid family classification by combining the Hierarchical Clustering Method (HCM) with a method to add new members to existing families. This procedure makes use of the much larger amount of information contained in the proper elements catalogs, with respect to classifications using also physical observations for a smaller number of asteroids.
Our work is based on a large catalog of high accuracy synthetic proper elements (available from AstDyS), containing data for  numbered asteroids. By selecting from the catalog a much smaller number of large asteroids, we first identify a number of core families; to these we attribute the next layer of smaller objects. Then, we remove all the family members from the catalog, and reapply the HCM to the rest. This gives both satellite families which extend the core families and new independent families, consisting mainly of small asteroids. These two cases are discriminated by another step of attribution of new members and by merging intersecting families. This leads to a classification with 128 families and currently 87095 members. The number of members can be increased automatically with each update of the proper elements catalog; changes in the list of families are not automated.
By using information from absolute magnitudes, we take advantage of the larger size range in some families to analyze their shape in the proper semimajor axis vs. inverse diameter plane. This leads to a new method to estimate the family age, or ages in cases where we identify internal structures. The analysis of the plot above evidences some open problems but also the possibility of obtaining further information of the geometrical properties of the impact process. The results from the previous steps are then analyzed, using also auxiliary information on physical properties including WISE albedos and SDSS color indexes. This allows to solve some difficult cases of families overlapping in the proper elements space but generated by different collisional events.
The families formed by one or more cratering events are found to be more numerous than previously believed because the fragments are smaller. We analyze some examples of cratering families (Massalia, Vesta, Eunomia) which show internal structures, interpreted as multiple collisions. We also discuss why Ceres has no family.

Reference
Milani A, Cellino A, Knežević Z, Novaković B, Spoto F and Paolicchi P (in press) Asteroid families classification: Exploiting very large data sets. Icarus
[doi:10.1016/j.icarus.2014.05.039]
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Andreas Breslau, Manuel Steinhausen, Kirsten Vincke and Susanne Pfalzner

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Most stars do not form in isolation, but as part of a star cluster or association. These young stars are initially surrounded by protoplanetary discs. In these cluster environments tidal interactions with other cluster members can alter the disc properties. Besides the disc frequency, its mass, angular momentum, and energy, the disc’s size is particularly prone to being changed by a passing star. So far the change in disc size has only been investigated for a small number of very specific encounters. Several studies investigated the effect of the cluster environment on the sizes of planetary systems like our own solar system, based on a generalisation of information from this limited sample. We performed numerical simulations covering the wide parameter space typical of young star clusters, to test the validity of this approach. Here the sizes of discs after encounters are presented, based on a size definition that is comparable to the one used in observational studies. We find that, except for encounters between equal-mass stars, the usually applied estimates are insufficient. They tend to severely overestimate the remaining disc size. We show that the disc size after an encounter can be described by a relatively simple dependence on the periastron distance and the mass ratio of the encounter partners. This knowledge allows us, for example, to pin down the types of encounter possibly responsible for the structure of today’s solar system.

Reference
Breslau A, Steinhausen M, Vincke K and Pfalzner S (2014) Sizes of protoplanetary discs after star-disc encounters. Astronomy & Astrophysics 565:A130.
[doi:10.1051/0004-6361/201323043]
Reproduced with permission © ESO

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