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]
Copyright Elsevier

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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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