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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Y. Alibert^1,2, F. Carron¹, A. Fortier¹, S. Pfyffer¹, W. Benz¹, C. Mordasini³ and D. Swoboda¹

¹Physikalisches Institut & Center for Space and Habitability, Universität Bern, 3012 Bern, Switzerland
²Observatoire de Besançon, 41 avenue de l’Observatoire, 25000 Besançon, France
³Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

Context. Planet formation models have been developed during the past years to try to reproduce what has been observed of both the solar system and the extrasolar planets. Some of these models have partially succeeded, but they focus on massive planets and, for the sake of simplicity, exclude planets belonging to planetary systems. However, more and more planets are now found in planetary systems. This tendency, which is a result of radial velocity, transit, and direct imaging surveys, seems to be even more pronounced for low-mass planets. These new observations require improving planet formation models, including new physics, and considering the formation of systems.
Aims. In a recent series of papers, we have presented some improvements in the physics of our models, focussing in particular on the internal structure of forming planets, and on the computation of the excitation state of planetesimals and their resulting accretion rate. In this paper, we focus on the concurrent effect of the formation of more than one planet in the same protoplanetary disc and show the effect, in terms of architecture and composition of this multiplicity.
Methods. We used an N-body calculation including collision detection to compute the orbital evolution of a planetary system. Moreover, we describe the effect of competition for accretion of gas and solids, as well as the effect of gravitational interactions between planets.
Results. We show that the masses and semi-major axes of planets are modified by both the effect of competition and gravitational interactions. We also present the effect of the assumed number of forming planets in the same system (a free parameter of the model), as well as the effect of the inclination and eccentricity damping. We find that the fraction of ejected planets increases from nearly 0 to 8% as we change the number of embryos we seed the system with from 2 to 20 planetary embryos. Moreover, our calculations show that, when considering planets more massive than ~5 M_⊕, simulations with 10 or 20 planetary embryos statistically give the same results in terms of mass function and period distribution.

Reference
Alibert Y, Carron F, Fortier A, Pfyffer S, Benz W, Mordasini C and Swoboda D (in press) Theoretical models of planetary system formation: mass vs. semi-major axis. Astronomy & Astrophysics
[doi:10.1051/0004-6361/201321690]
Reproduced with permission © ESO

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