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]
Copyright Elsevier

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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]
Copyright Elsevier

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