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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L.J. Bridgestock^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aDepartment of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK

Zinc isotope compositions (δ  ⁶⁶Zn) and concentrations were determined for metal samples of 15 iron meteorites across groups IAB, IIAB, and IIIAB. Also analyzed were troilite and other inclusions from the IAB iron Toluca. Furthermore, the first Zn isotope data are presented for metal–silicate partitioning experiments that were conducted at 1.5 GPa and 1650 K. Three partitioning experiments with run durations of between 10 and 60 min provide consistent Zn metal–silicate partition coefficients of ∼0.7 and indicate that Zn isotope fractionation between molten metal and silicate is either small (at less than about ±0.2‰) or absent. Metals from the different iron meteorite groups display distinct ranges in Zn contents, with concentrations of 0.08–0.24 μg/g for IIABs, 0.8–2.5 μg/g for IIIABs, and 12–40 μg/g for IABs. In contrast, all three groups show a similar range of δ  ⁶⁶Zn values (reported relative to ‘JMC Lyon Zn’) from +0.5‰ to +3.0‰, with no clear systematic differences between groups. However, distinct linear trends are defined by samples from each group in plots of δ  ⁶⁶Zn vs. 1/Zn, and these correlations are supported by literature data. Based on the high Zn concentration and δ  ⁶⁶Zn ≈ 0 determined for a chromite-rich inclusion of Toluca, modeling is employed to demonstrate that the Zn trends are best explained by segregation of chromite from the metal phase. This process can account for the observed Zn–δ  ⁶⁶Zn–Cr systematics of iron meteorite metals, if Zn is highly compatible in chromite and Zn partitioning is accompanied by isotope fractionation with Δ⁶⁶Zn_chr-met≈−1.5‰. Based on these findings, it is likely that the parent bodies of the IAB complex, IIAB and IIIAB iron meteorites featured δ  ⁶⁶Zn values of about −1.0 to +0.5‰, similar to the Zn isotope composition inferred for the bulk silicate Earth and results obtained for chondritic meteorites. Together, this implies that most solar system bodies formed with similar bulk Zn isotope compositions despite large differences in Zn contents.

Reference
Bridgestock et al. (2014) Unlocking the zinc isotope systematics of iron meteorites. Earth and Planetary Science Letters 400:153.
[doi:10.1016/j.epsl.2014.05.029]
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J.A. Cartwright^a,b, U. Ott^c,a, S. Herrmann^a, C.B. Agee^d,e

^aMax Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
^bCalifornia Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
^cUniversity of West Hungary, Savaria Campus, 9700 Szombathely, Hungary
^dInstitute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA
^eDepartment of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA

The NWA 7034 Martian basaltic breccia, dated at ~4.4 Ga, represents an entirely new type of Martian meteorite. However, due to the unique make-up of NWA 7034 compared to other Martian meteorite types (including its anomalous oxygen isotope ratios), noble gas analyses – a key tool for Martian meteorite identification – are important to confirm its Martian origin. Here, we report the first noble gas results for NWA 7034, which show the presence of a trapped component that resembles the current Martian atmosphere. This trapped component is also similar in composition to trapped gases found in the much younger shergottites (~150–600 Ma). Our formation ages for the sample suggest events at ~1.6 Ga (K–Ar), and ~170 Ma (U–Th/He), which are considerably younger than those observed by Rb–Sr (2.1 Ga), and Sm–Nd (4.4 Ga; zircons ~4.4 Ga). However, our K–Ar age is similar to a disturbance in the U–Pb zircon data at ~1.7 Ga, which could hint that both chronometers have been subjected to disturbance by a common process or event. The U–Th/He age of ~170 Ma could relate to complete loss of radiogenic ⁴He at this time, and is a similar age to the crystallisation age of most shergottites. While this may be coincidental, it could indicate that a single event is responsible for both shergottite formation and NWA 7034 thermal metamorphism. As for cosmic ray exposure ages, our favoured age is ~5 Ma, which is outside the ranges for other Martian meteorite groups, and may suggest a distinct ejection event. NWA 7034 shows evidence for neutron capture on Br, which has caused elevations in Kr isotopes ⁸⁰Kr and ⁸²Kr. These elevated abundances indicate significant shielding, and could relate to either a large meteoroid size, and/or shielding in relation to a regolithic origin. We have also applied similar neutron capture corrections to Ar and Xe data, which further refine the likelihood of a modern atmospheric component, though such corrections remain speculative. Cosmogenic production rates and noble gas data are consistent with a meteoroid radius of >50 cm. Fission contributions are clear in the Xe data, with evidence to suggest that NWA 7034 contains both ²³⁸U and ²⁴⁴Pu derived fission Xe components. If the gas in NWA 7034 was trapped at its ancient igneous formation, this would suggest little evolution of the Martian atmosphere between ~4.4 Ga and present day. However, as NWA 7034 is a regolith breccia with multiple lithologies and a strong compositional similarity to Gusev soils, the timing and incorporation of trapped atmospheric gases is unclear. With hints of resetting events at ~1.5–2.1 Ga, the atmospheric component may have been incorporated during breccia formation – possibly in the Amazonian, though it could also have been incorporated on ejection from the surface.

Reference
Cartwright JA, Ott U, Herrmann S and Agee CB (2014) Modern atmospheric signatures in 4.4 Ga Martian meteorite NWA 7034. Earth and Planetary Science Letters 400:77.
[doi:10.1016/j.epsl.2014.05.008]
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Stuart J. Robbins^a and Brian M. Hynek^b

^aLaboratory for Atmospheric and Space Physics, University of Colorado at Boulder, 3665 Discovery Dr., Boulder, CO 80309, United States
^bLaboratory for Atmospheric and Space Physics and Department of Geological Sciences, University of Colorado at Boulder, 3665 Discovery Dr., Boulder, CO 80309, United States

Impact craters (“craters”) are ubiquitous across most solid surfaces in the Solar System. The most common use of crater populations (populations as defined by diameter- or “size-” frequency) is to estimate relative and absolute model surface ages based on two assumptions: Craters will form spatially randomly across the planetary body, and craters will form following a random distribution around a known or assumed temporal flux. Secondary craters – craters that form from the ejecta of a crater formed by an extraplanetary-sourced impactor – belie both of these assumptions and so will affect crater-based ages if not removed from crater counts. A question unanswered with observational data to this point has been, what is the population of primary versus secondary craters on a given planet? We have answered this question for Mars for craters larger than 1 km in diameter by using a recently published global crater database, classifying craters as primary or secondary, and creating maps of the population statistics. Our approach was to err on the side of a crater being primary by default and hence our work is a conservative measurement. We show that, globally, secondary craters are at least 24% as numerous as primary craters (comprising 19% of the total population) for diameters D≥1 km. However, there are many “hot spots” across the globe where secondary craters are more numerous than primary craters for diameters as large as 9 km. This is the first time such a study has been conducted globally for any body and it shows that, not only are secondary craters numerous, but they can significantly affect crater populations in a non-uniform way across a planetary surface

Reference
Robbins SJ and Hynek BM (2014) The secondary crater population of Mars. Earth and Planetary Science Letters 400:66.
[doi:10.1016/j.epsl.2014.05.005]
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Arpita Roy^1,2, Jason T. Wright^1,2 and Steinn Sigurðsson^1,2

¹Department of Astronomy and Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
²Center for Exoplanets and Habitable Worlds, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA

The lunar farside highlands problem refers to the curious and unexplained fact that the farside lunar crust is thicker, on average, than the nearside crust. Here we recognize the crucial influence of Earthshine, and propose that it naturally explains this hemispheric dichotomy. Since the accreting Moon rapidly achieved synchronous rotation, a surface and atmospheric thermal gradient was imposed by the proximity of the hot, post-giant impact Earth. This gradient guided condensation of atmospheric and accreting material, preferentially depositing crust-forming refractories on the cooler farside, resulting in a primordial bulk chemical inhomogeneity that seeded the crustal asymmetry. Our model provides a causal solution to the lunar highlands problem: the thermal gradient created by Earthshine produced the chemical gradient responsible for the crust thickness dichotomy that defines the lunar highlands.

Reference
Roy A, Wright JT, and Sigurðsson S (2014) Earthshine on a Young Moon: Explaining the Lunar Farside Highlands. The Astrophysical Journal – Letters 788:L42.
[doi:10.1088/2041-8205/788/2/L42]

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F. Jourdan^a, G. Benedix^b, E. Eroglu^c, P.A. Bland^b and A. Bouvier^d

^aWestern Australian Argon Isotope Facility, Department of Applied Geology and JdL-CMS, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
^bDepartment of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845
^cSchool of Chemistry and Biochemistry, The University of Western Australia, Crawley, WA 6009, Australia
^dDepartment of Earth Sciences, University of Western Ontario, London, ON, N6A 3K7, Canada

The Bunburra Rockhole meteorite is a brecciated anomalous basaltic achondrite containing coarse-, medium- and fine-grained lithologies. Petrographic observations constrain the limited shock pressure to between ca. 10 GPa and 20 GPa. In this study, we carried out nine ⁴⁰Ar/³⁹Ar step-heating experiments on distinct single-grain fragments extracted from the coarse and fine lithologies. We obtained six plateau ages and three mini-plateau ages. These ages fall into two internally concordant populations with mean ages of 3640 ± 21 Ma (n=7; P=0.53) and 3544 ± 26 Ma (n=2; P=0.54), respectively. Based on these results, additional⁴⁰Ar/³⁹Ar data of fusion crust fragments, argon diffusion modeling, and petrographic observations, we conclude that the principal components of the Bunburra Rockhole basaltic achondrite are from a melt rock formed at ~3.64 Ga by a medium to large impact event. The data imply this impact generated high enough energy to completely melt the basaltic target rock and reset the Ar systematics, but only partially reset the Pb-Pb age. We also conclude that a complete ⁴⁰Ar∗ resetting of pyroxene and plagioclase at this time could not have been achieved at solid-state conditions. Comparison with a terrestrial analogue (Lonar crater) shows that the time-temperature conditions required to melt basaltic target rocks upon impact are relatively easy to achieve. Ar data also suggest that a second medium-size impact event occurred on a neighboring part of the same target rock at ~3.54 Ga. Concordant low-temperature step ages of the nine aliquots suggest that, at ~3.42 Ga, a third smaller impact excavated parts of the ~3.64 Ga and ~3.54 Ga melt rocks and brought the fragments together. The lack of significant impact activity after 3.5 Ga, as recorded by the Bunburra Rockhole suggest that (1) either the meteorite was ejected in a small secondary parent body where it resided untouched by large impacts, or (2) it was covered by a porous heat-absorbing regolith blanket which, when combined with the diminishing frequency of large impacts in the solar system, protected Bunburra from subsequent major heating events. Finally we note that the total (K/Ar) resetting impact event history recorded by some of the brecciated eucrites (peak at 3.8-3.5 Ga) is similar to the large impact history recorded by the Bunburra Rockhole parent body (ca. 3.64-3.54 Ga; this study) and could indicate a similar position in the asteroid belt at that time.

Reference
Jourdan F, Benedix G, Eroglu E, Bland PA and Bouvier A (in press) 40Ar/39Ar impact ages and time-temperature argon diffusion history of the Bunburra Rockhole anomalous basaltic achondrite. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.039]
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E.L. Walton^a,b, T.G. Sharp^c, J. Hu^c and J. Filiberto^d

^aMacEwan University, Department of Physical Sciences, 10700-104 Ave, City Centre Campus, Edmonton, AB, T5J 4S2, Canada
^bUniversity of Alberta, Department of Earth & Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, AB, T6G 2E3, Canada
^cArizona State University, School of Earth and Space Exploration, Tempe, AZ, 85287-1404, U.S.A
^dSouthern Illinois University, Department of Geology, Carbondale, IL

The microtexture and mineralogy of shock melts in the Tissint martian meteorite were investigated using scanning electron microscopy, Raman spectroscopy, transmission electron microscopy and synchrotron micro X-ray diffraction to understand shock conditions and duration. Distinct mineral assemblages occur within and adjacent to the shock melts as a function of the thickness and hence cooling history. The matrix of thin veins and pockets of shock melt consists of clinopyroxene + ringwoodite ± stishovite embedded in glass with minor Fe-sulfide. The margins of host rock olivine in contact with the melt, as well as entrained olivine fragments, are now amorphosed silicate perovskite + magnesiowüstite or clinopyroxene + magnesiowüstite. The pressure stabilities of these mineral assemblages are ~15 GPa and >19 GPa, respectively. The ~200-μm-wide margin of thicker, mm-size (up to 1.4 mm) shock melt vein contains clinopyroxene + olivine, with central regions comprising glass + vesicles + Fe-sulfide spheres. Fragments of host rock within the melt are polycrystalline olivine (after olivine) and tissintite + glass (after plagioclase). From these mineral assemblages the crystallization pressure at the vein edge was as high as 14 GPa. The interior crystallized at ambient pressure. The shock melts in Tissint quench-crystallized during and after release from the peak shock pressure; crystallization pressures and those determined from olivine dissociation therefore represent the minimum shock loading. Shock deformation in host rock minerals and complete transformation of plagioclase to maskelynite suggest the peak shock pressure experienced by Tissint ⩾29−30 GPa. These pressure estimates support our assessment that the peak shock pressure in Tissint was significantly higher than the minimum 19 GPa required to transform olivine to silicate perovskite plus magnesiowüstite
Small volumes of shock melt (<100 μm) quench rapidly (0.01 s), whereas thermal equilibration will occur within 1.2 seconds in larger volumes of melt (1 mm²). The apparent variation in shock pressure recorded by variable mineral assemblages within and around shock melts in Tissint is consistent with a shock pulse on the order of 10−20 ms combined with a longer duration of post-shock cooling and complex thermal history. This implies that the impact on Mars that shocked and ejected Tissint at ~1 Ma was not exceptionally large

Reference
Walton EL, Sharp TG, Hu J and Filiberto J (in press) Heterogeneous mineral assemblages in Martian meteorite Tissint as a result of a recent small impact event on Mars. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.023]
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Hurwitz D. M. and D. A. Kring

Center for Lunar Science and Exploration, Lunar and Planetary Institute, Houston, Texas, USA

We modeled the differentiation of the South Pole–Aitken (SPA) impact melt sheet to determine whether noritic lithologies observed within SPA formed as a result of the impact. Results indicate differentiation of SPA impact melt can produce noritic layers that may accommodate observed surface compositions but only in specific scenarios. One of nine modeled impact melt compositions yielded layers of noritic materials that account for observations of noritic lithologies at depths of ~6 km. In this scenario, impact occurred before a hypothesized lunar magma ocean cumulate overturn. The 50 km deep melt sheet would have formed an insulating quenched layer at the surface before differentiating. The uppermost differentiated layers in this scenario have FeO and TiO₂ contents consistent with orbital observations if they were subsequently mixed with the uppermost quenched melt layer and with less FeO- and TiO₂-enriched materials such as ejecta emplaced during younger impacts. These results verify that noritic lithologies observed within SPA could have formed as a direct result of the impact. Therefore, locations within SPA that contain noritic materials represent potential destinations for collecting samples that can be analyzed to determine the age of the SPA impact. Potential destinations include central peaks of Bhabha, Bose, Finsen, and Antoniadi craters, as well as walls of Leibnitz and Schrödinger basins. Additionally, potential remnants of the uppermost quenched melt may be preserved in gabbroic material exposed in “Mafic Mound.” Exploring and sampling these locations can constrain the absolute age of SPA, a task that ranks among the highest priorities in lunar science.

Reference
Hurwitz DM and Kring DA (in press) Differentiation of the South Pole–Aitken basin impact melt sheet: Implications for lunar exploration. Journal of Geophysical Research: Planets 
[doi:10.1002/2013JE004530]
Published by arrangement with John Wiley & Sons

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Vasavada¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website

¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA

The Mars Science Laboratory mission reached Bradbury Landing in August 2012. In its first 500 sols, the rover Curiosity was commissioned and began its investigation of the habitability of past and present environments within Gale Crater. Curiosity traversed eastward toward Glenelg, investigating a boulder with a highly alkaline basaltic composition, encountering numerous exposures of outcropping pebble conglomerate, and sampling aeolian sediment at Rocknest and lacustrine mudstones at Yellowknife Bay. On sol 324, the mission turned its focus southwest, beginning a year-long journey to the lower reaches of Mt. Sharp, with brief stops at the Darwin and Cooperstown waypoints. The unprecedented complexity of the rover and payload systems posed challenges to science operations, as did a number of anomalies. Operational processes were revised to include additional opportunities for advance planning by the science and engineering teams.

Reference
Vasavada et al. (in press) Overview of the Mars Science Laboratory mission: Bradbury Landing to Yellowknife Bay and beyond. Journal of Geophysical Research: Planets
[doi:10.1002/2014JE004622]
Published by arrangement with John Wiley & Sons

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