Zhiyong Xiao^a,b,*, Robert G. Strom^b, Clark R. Chapman^c, James W. Head^d, Christian Klimczak^e, Lillian R. Ostrach^f, Jörn Helbert^g, Piero D’Incecco^g

^aPlanetary Science Institute, Faculty of Earth Sciences, China University of Geosciences (Wuhan), Wuhan, Hubei, 430074, China
^bLunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, 85719, USA
^cDepartment of Space Studies, Southwest Research Institute, Boulder, Colorado, 80302, USA
^dDepartment of Geological Sciences, Brown University, Providence, Rhode Island, 02912, USA
^eDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C., 20015, USA
^fSchool of Earth and Space Exploration, Arizona State University, Arizona, USA, 85281
^gInstitute of Planetary Research, Deutsches Zentrum für Luft- und Raumfahrt, 12489 Berlin, Germany

The impact cratering process is usually divided into the coupling, excavation, and modification stages, where each stage is controlled by a combination of different factors. Although recognized as the main factors governing impact processes on airless bodies, the relative importance of gravity, target and projectile properties, and impact velocity in each stage is not well understood. We focus on the excavation stage to place better constraints on its controlling factors by comparing the morphology and scale of crater-exterior structures for similar-sized fresh complex craters on the Moon and Mercury. We find that the ratios of continuous ejecta deposits, continuous secondaries facies, and the largest secondary craters on the continuous secondaries facies between same-sized Mercurian and lunar craters are consistent with predictions from gravity-regime crater scaling laws. Our observations support that gravity is a major controlling factor on the excavation stage of the formation of complex impact craters on the Moon and Mercury. On the other hand, similar-sized craters with identical background terrains on Mercury have different spatial densities of secondaries on the continuous secondaries facies, suggesting that impactor velocity may also be important during the excavation stage as larger impactor velocity may also cause greater ejection velocities. Moreover, some craters on Mercury have more circular and less clustered secondaries on the continuous secondaries facies than other craters on Mercury or the Moon. This morphological difference appears not to have been caused by the larger surface gravity or the larger median impact velocity on Mercury. A possible interpretation is that at some places on Mercury, the target material might have unique properties causing larger ejection angles during the impact excavation stage. We conclude that gravity is the major controlling factor on the impact excavation stage of complex craters, impact velocity and target properties may also affect the excavation stage but their importance is to a less degree compared with gravity.

Reference
Xiao Z, Strom RG, Chapman CR, Head JW, Klimczak C, Ostrach LR, Helbert J, and D’Incecco P (in press) Comparisons of fresh complex impact craters on Mercury and the Moon: Implications of controlling factors in impact excavation processes. Icarus
[doi:10.1016/j.icarus.2013.10.002]
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A. J. Timothy Jull

Department of Geosciences, University of Arizona, Tucson Arizona, USA

No abstract is available for this article.

Reference
Jull AJT (in press) Book Review: Meteoriten—Meteorites: Zeitzeugen der Entstehung des Sonnensystems/Witnesses of the origin of the solar system. Meteoritics & Planetary Science
[doi:10.1111/maps.12210]
Published by arrangement with John Wiley & Sons

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G. Jeffrey Taylor

Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822

An accurate assessment of the bulk chemical composition of Mars is fundamental to understanding planetary accretion, differentiation, mantle evolution, the nature of the igneous parent rocks that were altered to produce sediments on Mars, and the initial concentrations of volatiles such as H, Cl and S, important constituents of the Martian surface. This paper reviews the three main approaches that have been used to estimate the bulk chemical composition of Mars: geochemical/cosmochemical, isotopic, and geophysical. The standard model is one developed by H. Wänke and G. Dreibus in a series of papers, which is based on compositions of Martian meteorites. Since their groundbreaking work, substantial amounts of data have become available to allow a reassessment of the composition of Mars from elemental data, including tests of the basic assumptions in the geochemical models. The results adjust some of the concentrations in the Wänke-Dreibus model, but in general confirm its accuracy. Bulk silicate Mars has roughly uniform depletion of moderately volatile elements such as K (0.6 x CI), and strong depletion of highly volatile elements (e.g., Tl). The highly volatile elements are within uncertainties uniformly depleted at about 0.06 CI abundances. The highly volatile chalcophile elements are likewise roughly uniformly depleted, but with more scatter, with normalized abundances of 0.03 CI. Bulk planetary H₂O is much higher than estimated previously: it appears to be slightly less than in Earth, but D/H is similar in Earth and Mars, indicating a common source of water-bearing material in the inner solar system. K/Th ranges from ~3000 to ~5000 among the terrestrial planets, a small range compared to CI chondrites (19,000). FeO varies throughout the inner solar system: ~3 wt% in Mercury, 8 wt % in Earth and Venus, and 18 wt % in Mars. These differences can be produced by varying oxidation conditions, hence do not suggest the terrestrial planets were formed from fundamentally different materials. The broad chemical similarities among the terrestrial planets indicate substantial mixing throughout the inner solar system during planet formation, as suggested by dynamical models

Reference
Taylor GJ (in press) The Bulk Composition of Mars. Chemie der Erde
[doi:10.1016/j.chemer.2013.09.006]
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Emmanuel Jacquet^1,2,*, Marine Paulhiac-Pison^1,3, Olivier Alard⁴, Anton T. Kearsley⁵, Matthieu Gounelle^1,6

¹Laboratoire de Minéralogie et Cosmochimie du Muséum, CNRS & Muséum National d’Histoire Naturelle, Paris, France
²Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, ON, Canada
³Ecole Normale Supérieure de Paris, Paris, France
⁴Géosciences Montpellier, Université de Montpellier II, Montpellier Cedex 5, France
⁵Impacts and Astromaterials Research Centre, Department of Mineralogy, The Natural History Museum, London, UK
⁶Institut Universitaire de France, Maison des Universités, Paris, France

We report trace element analyses by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of metal grains from nine different CR chondrites, distinguishing grains from chondrule interior (“interior grains”), chondrule surficial shells (“margin grains”), and the matrix (“isolated grains”). Save for a few anomalous grains, Ni-normalized trace element patterns are similar for all three petrographic settings, with largely unfractionated refractory siderophile elements and depleted volatile Au, Cu, Ag, S. All three types of grains are interpreted to derive from a common precursor approximated by the least-melted, fine-grained objects in CR chondrites. This also excludes recondensation of metal vapor as the origin of the bulk of margin grains. The metal precursors were presumably formed by incomplete condensation, with evidence for high-temperature isolation of refractory platinum-group-element (PGE)-rich condensates before mixing with lower temperature PGE-depleted condensates. The rounded shape of the Ni-rich, interior grains shows that they were molten and that they equilibrated with silicates upon slow cooling (1–100 K h⁻¹), largely by oxidation/evaporation of Fe, hence their high Pd content, for example. We propose that Ni-poorer, amoeboid margin grains, often included in the pyroxene-rich periphery common to type I chondrules, result from less intense processing of a rim accreted onto the chondrule subsequent to the melting event recorded by the interior grains. This means either that there were two separate heating events, which formed olivine/interior grains and pyroxene/margin grains, respectively, between which dust was accreted around the chondrule, or that there was a single high-temperature event, of which the chondrule margin records a late “quenching phase,” in which case dust accreted onto chondrules while they were molten. In the latter case, high dust concentrations in the chondrule-forming region (at least three orders of magnitude above minimum mass solar nebula models) are indicated.

Reference
Jacquet E, Paulhiac-Pison M, Alard O, Kearsley AT and Gounelle M (in press) Trace element geochemistry of CR chondrite metal. Meteoritics & Planetary Science
[doi:10.1111/maps.12212]
Published by arrangement with John Wiley & Sons

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A. Pierens^1,2, C. Cossou^1,2 and S. N. Raymond^1,2

¹Université de Bordeaux, Observatoire Aquitain des Sciences de l’Univers, BP 89 33271 Floirac Cedex, France
²Laboratoire d’Astrophysique de Bordeaux, BP 89 33271 Floirac Cedex, France

Context. Rapid gas accretion onto gas giants requires the prior formation of ~10 M_⊕ cores, and this presents a continuing challenge to planet formation models. Recent studies of oligarchic growth indicate that in the region around 5 AU growth stalls at ~2 M_⊕. Earth-mass bodies are expected to undergo Type I migration directed either inward or outward depending on the thermodynamical state of the protoplanetary disc. Zones of convergent migration exist where the Type I torque cancels out. These “convergence zones” may represent ideal sites for the growth of giant planet cores by giant impacts between Earth-mass embryos.
Aims. We study the evolution of multiple protoplanets of a few Earth masses embedded in a non-isothermal protoplanetary disc. The protoplanets are located in the vicinity of a convergence zone located at the transition between two different opacity regimes. Inside the convergence zone, Type I migration is directed outward and outside the zone migration is directed inward.
Methods. We used a grid-based hydrodynamical code that includes radiative effects. We performed simulations varying the initial number of embryos and tested the effect of including stochastic forces to mimic the effects resulting from disc turbulence. We also performed N-body runs calibrated on hydrodynamical calculations to follow the evolution on Myr timescales.
Results. For a small number of initial embryos (N = 5–7) and in the absence of stochastic forcing, the population of protoplanets migrates convergently toward the zero-torque radius and forms a stable resonant chain that protects embryos from close encounters. In systems with a larger initial number of embryos, or in which stochastic forces were included, these resonant configurations are disrupted. This in turn leads to the growth of larger cores via a phase of giant impacts between protoplanets, after which the system settles to a new stable resonant configuration. Giant planets cores with masses ≥ 10 M_⊕ formed in about half of the simulations with initial protoplanet masses of m_p = 3 M_⊕ but in only 15% of simulations with m_p = 1 M_⊕, even with the same total solid mass.
Conclusions. If 2−3 M_⊕ protoplanets can form in less than ~1 Myr, convergent migration and giant collisions can grow giant planet cores at Type I migration convergence zones. This process can happen fast enough to allow for a subsequent phase of rapid gas accretion during the disc’s lifetime.

Reference
Pierens A, Cossou C and Raymond SN (in press) Making giant planet cores: convergent migration and growth of planetary embryos in non-isothermal discs. Astronomy & Astrophysics
[doi:10.1051/0004-6361/201322123]
Reproduced with permission © ESO

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Kickapoo Lunar Research Team^a, Georgiana Kramer^b,c,*

^aAbby Delawder, Austen Beason, Victoria Wilson, and Richard D. Snyder, Kickapoo High School, 3710 S. Jefferson Avenue, Springfield, MO 65807, USA
^bCenter for Lunar Science and Exploration, Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058, USA
^cNASA Lunar Science Institute, USA

High resolution images of stratified ejecta boulders on the lunar nearside reveal layers of alternating low and high albedo material. We measured the thickness and albedo of each alternating light and dark layer from twenty nine stratified boulders located in Aristarchus Crater and Mare Undarum. The results were used to test hypotheses to explain the origins of the observed strata in these impact ejected boulders. Morphologically, these boulders demonstrate cross-bedding, trough-shaped layering, tapered layering and cumulate enclaves. We interpret these characteristics to be evidence that these layers result from periodic disruption by convection or density currents within a cooling layered igneous intrusion. We demonstrate that the layering observed in these boulders cannot be the result of known processes occurring on the surface, but instead suggests a history of complex intrusive igneous processes within the lunar crust.

Reference
Kickapoo Lunar Research Team and Georgiana Kramer G (in press) Stratified Ejecta Boulders as Indicators of Layered Plutons on the Moon. Icarus
[doi:10.1016/j.icarus.2013.10.003]
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Frans J. M. Rietmeijer^1,*, Joseph A. Nuth², Aurora Pun¹

¹Department of Earth and Planetary Sciences, 1-University of New Mexico, Albuquerque, New Mexico, USA
²Astrochemistry Laboratory, Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

This thermal annealing experiment at 1000 K for up to 167 h used a physical mixture of vapor phase-condensed magnesiosilica grains and metallic iron nanograins to test the hypothesis that a mixture of magnesiosilica grains and an Fe-source would lead to the formation of ferromagnesiosilica grains. This exploratory study found that coagulation and thermal annealing of amorphous magnesiosilica and metallic grains yielded ferromagnesiosilica grains with the Fe/(Fe + Mg) ratios in interplanetary dust particles. Furthermore, decomposition of brucite present in the condensed magnesiosilica grains was the source for water and the cause of different iron oxidation states, and the formation of amorphous Fe³⁺-ferrosilica, amorphous Fe³⁺-Mg, Fe-silicates, and magnesioferrite during thermal annealing. Fayalite and ferrosilite that formed from silica/FeO melts reacted with forsterite and enstatite to form Mg, Fe-silicates. The presence of iron in different oxidation states in extraterrestrial materials almost certainly requires active asteroid-like parent bodies. If so, the possible presence of trivalent Fe compounds in comet P/Halley suggests that Halley-type comets are a mixture of preserved presolar and processed solar nebula dust. The results from this thermal annealing experiment further suggest that the Fe-silicates detected in the impact-induced ejecta from comet 9P/Temple 1 might be of secondary origin and related to the impact experiment or to processing in a regolith.

Reference
Rietmeijer FJM, Nuth JA and Pun A (in press) The formation of Mg,Fe-silicates by reactions between amorphous magnesiosilica smoke particles and metallic iron nanograins with implications for comet silicate origins. Meteoritics & Planetary Science
[doi:10.1111/maps.12194]
Published by arrangement with John Wiley & Sons

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C.S. Edwards^a,*, J.L. Bandfield^b, P.R. Christensen^c and A.D. Rogers^d

^aCalifornia Institute of Technology, Division of Geological and Planetary Sciences, 1200 E. California Blvd., MC 150-21, Pasadena, CA, 91125
^bSpace Science Institute, 4750 Walnut St., Boulder, Co, 80301
^cArizona State University, School of Earth and Space Exploration, Mars Space Flight Facility, PO BOX 876305, Tempe, AZ, 85287-6305
^dDepartment of Geosciences, Stony Brook University, 255 Earth and Space Sciences, Stony Brook, NY 11794-2100

Flat-floored craters have long been recognized on Mars with early work hypothesizing a sedimentary origin. More recently, high-resolution thermal inertia measurements show that these craters contain some of the rockiest materials on the planet, inconsistent with poorly consolidated sedimentary materials. In this study, the distribution, physical properties (morphology and thermal inertia), and composition of these craters are thoroughly investigated over the entire planet. The majority of the ~2,800 rocky crater floors identified are concentrated in the low albedo (0.1-0.17), cratered southern highlands. These craters were infilled at ~3.5 Ga and are associated with the highest thermal inertia values and some of the most mafic materials identified on the planet. Although several processes may have led to the formation of the crater floors, the most likely scenario is volcanic infilling through fractures created by the impact event. The primitive magma source directly results from decompression melting of the martian mantle by the removal of the crustal material excavated by the impactor. Volcanic infilling of craters by decompression melting appears to only have occurred in early martian history when the lithosphere was still relatively thin and the thermal gradient was high. This process was widespread and responsible for the eruption of significant volumes of primitive material, inside and likely outside of craters. Impact induced decompression melting of the martian mantle accounts for the unusual infilling of martian craters and is a widespread planetary process that has gone previously undocumented.

Reference
Edwards CS, Bandfield JL, Christensen PR and Rogers AD (in press) The formation of infilled craters on mars: evidence for widespread impact induced decompression of the early martian mantle?. Icarus
[doi:10.1016/j.icarus.2013.10.005]
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Justin Filiberto^1,*, Susanne P. Schwenzer^2,3

¹Department of Geology, Southern Illinois University Carbondale, Carbondale, Illinois, USA
²Department of Physical Science, The Open University, Milton Keynes, UK
³Lunar and Planetary Institute, USRA, Houston, Texas, USA

The Mars Exploration Rover Spirit investigated the igneous and alteration mineralogy and chemistry of Home Plate and its surrounding deposits. Here, we focus on using thermochemical modeling to understand the secondary alteration mineralogy at the Home Plate outcrop and surrounding Columbia Hills region in Gusev crater. At high temperatures (300 °C), magnetite occurs at very high W/R ratios, but the alteration assemblage is dominated by chlorite and serpentine over most of the W/R range. Quartz, epidote, and typical high-T phases such as feldspar, pyroxene, and garnet occur at low W/R. At epithermal temperatures (150 °C), hematite occurs at very high W/R. A range of phyllosilicates, including kaolinite, nontronite, chlorite, and serpentine are precipitated at specific W/R. Amphibole, with garnet, feldspar, and pyroxene occur at low W/R. If the CO₂ content of the system is high, the assemblage is dominated by carbonate with increasing amounts of an SiO₂-phase, kaolinite, carpholite, and chlorite with lower W/R. At temperatures of hydrous weathering (13 °C), the oxide phase is goethite, silicates are chlorite, nontronite, and talc, plus an SiO₂-phase. In the presence of CO₂, the mineral assemblage at high W/R remains the same, and only at low W/R, i.e., with increasing salinity, carbonate precipitates. The geochemical gradients observed at Home Plate are attributed to short-lived, initially high (300 °C) temperature, but fast cooling events, which are in agreement with our models and our interpretation of a multistage alteration scenario of Home Plate and Gusev in general. Alteration at various temperatures and during different geological processes within Gusev crater has two effects, both of which increase the habitability of the local environment: precipitation of hydrous sheet silicates, and formation of a brine, which might contain elements essential for life in diluted, easily accessible form.

Reference
Filiberto J and Schwenzer SP (in press) Alteration mineralogy of Home Plate and Columbia Hills—Formation conditions in context to impact, volcanism, and fluvial activity. Meteoritics & Planetary Science
[doi:10.1111/maps.12207]
Published by arrangement with John Wiley & Sons

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F. Pauzat¹, Y. Ellinger¹, O. Mousis², M. Ali Dib² and O. Ozgurel¹

¹Laboratoire de Chimie Théorique, UMR 7616-CNRS, UPMC Univ. Paris 06, F-75005 Paris, France
²Institut UTINAM, CNRS/INSU, UMR 6213, Université de Franche-Comté, F-25030 Besançon Cedex, France

We address the problem of the sequestration of Ar, Kr, and Xe by H₃⁺ in the gas-phase conditions encountered during the cooling of protoplanetary disks when H₃⁺ is competing with other species present in the same environment. Using high-level ab initio simulations, we try to quantify other sequestration possibilities involving He, H₃⁺, H₂O, and H₃O⁺ present in the protosolar nebula. Apart from the fact that H₃⁺ complexes formed with heavy noble gases are found to be by far much more stable than those formed with He or H₂O, we show that H₂D⁺ and H₃O⁺, both products of the reactions of H₃⁺ with HD and H₂O, can also be efficient trapping agents for Ar, Kr, and Xe. Meanwhile, the abundance profile of H₃⁺ in the outer part of the nebula is revisited with the use of an evolutionary accretion disk model that allows us to investigate the possibility that heavy noble gases can be sequestered by H₃⁺ at earlier epochs than those corresponding to their trapping in planetesimals. We find that H₃⁺ might be abundant enough in the outer protosolar nebula to trap Xe and Kr prior their condensation epochs, implying that their abundances should be solar in Saturn’s current atmosphere and below the observational limit in Titan. The same scenario predicts that comets formed at high heliocentric distances should also be depleted in Kr and Xe. In situ measurements, such as those planed with the Rosetta mission on 67P/Churyumov-Gerasimenko, will be critical to check the validity of our hypotheses.

Reference
Pauzat F, Ellinger Y, Mousis O, Ali Dib M and Ozgurel O (in press) Gas-phase Sequestration of Noble Gases in the Protosolar Nebula: Possible Consequences on the Outer Solar System Composition. The Astrophysical Journal
[doi:10.1088/0004-637X/777/1/29]

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