Cyrena Anne Goodrich^1,2,*, Allan H. Treiman³, Justin Filiberto⁴, Juliane Gross⁵, Michael Jercinovic²

¹Planetary Science Institute, Tucson, Arizona, USA
²Department of Geosciences, University of Massachusetts, Amherst, Massachusetts, USA
³Lunar and Planetary Institute, Houston, Texas, USA
⁴Department of Geology, Southern Illinois University, Carbondale, Illinois, USA
⁵Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA

We used new analytical and theoretical methods to determine the major and minor element compositions of the primary trapped liquid (PTLs) represented by melt inclusions in olivine and augite in the Martian clinopyroxenite, Nakhla, for comparison with previously proposed compositions for the Nakhla (or nakhlite) parent magma. We particularly focused on obtaining accurate K₂O contents, and on testing whether high K₂O contents and K₂O/Na₂O ratios obtained in previous studies of melt inclusions in olivine in Nakhla could have been due to unrepresentative sampling, systematic errors arising from electron microprobe techniques, late alteration of the inclusions, and/or boundary layer effects. Based on analyses of 35 melt inclusions in olivine cores, the PTL in olivine, PTL_oliv, contained (by wt) approximately 47% SiO₂, 6.3% Al₂O₃, 9.6% CaO, 1.8% K₂O, and 0.9% Na₂O, with K₂O/Na₂O = 2.0. We infer that the high K₂O content of PTL_oliv is not due to boundary layer effects and represents a real property of the melt from which the host olivine crystallized. This melt was cosaturated with olivine and augite. Its mg# is model-dependent and is constrained only to be ≥19 (equilibrium Fo = 40). Based on analyses of 91 melt inclusions in augite cores, the PTL in augite, PTL_aug, contained (by wt) 53–54% SiO₂, 7–8% Al₂O₃, 0.8–1.1% K₂O, and 1.1–1.4% Na₂O, with K₂O/Na₂O = 0.7–0.8. This K₂O content and K₂O/Na₂O ratio are significantly higher than inferred in studies of melt inclusions in augite in Nakhla by experimental rehomogenization. PTL_aug was saturated only with augite, and in equilibrium with augite cores of mg# 62. PTL_augrepresents the Nakhla parent magma, and does not evolve to PTL_oliv by fractional crystallization. We therefore conclude that olivine cores in Nakhla (and, by extension, other nakhlites) are xenocrystic. We propose that PTL_oliv and PTL_aug were generated from the same source region. PTL_oliv was generated first and emplaced to form olivine-rich cumulate rocks. Shortly thereafter, PTL_aug was generated and ascended through these olivine-rich cumulates, incorporating fragments of wallrock that became the xenocrystic olivine cores in Nakhla. The Nakhla (nakhlite) mantle source region was pyroxenitic with some olivine, and could have become enriched in K relative to Na via metasomatism. A high degree of melting of this source produced the silica-poor, alkali-rich magma PTL_oliv. Further ascension and decompression of the source led to generation of the silica-rich, relatively alkali-poor magma PTL_aug. Potassium-rich magmas like those involved in the formation of the nakhlites represent an important part of the diversity of Martian igneous rocks.

Reference
Goodrich CA, Treiman AH, Filiberto J, Gross J and Jercinovic M (2013) K2O-rich trapped melt in olivine in the Nakhla meteorite: Implications for petrogenesis of nakhlites and evolution of the Martian mantle. Meteoritics & Planetary Science 48:2371–2405.
[doi:10.1111/maps.12226]
Published by arrangement with John Wiley & Sons

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M. Ali-Dib¹, O. Mousis¹, G. S. Pekmezci², J. I. Lunine³, N. Madhusudhan⁴ and J.-M. Petit<sup>1

1</sup>Université de Franche-Comté, Institut UTINAM, CNRS/INSU, UMR 6213, Besançon Cedex, France
²Dipartimento di Astronomia, Universitá di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy
³Center for Radiophysics and Space Research, Space Sciences Building, Cornell University, Ithaca, NY 14853, USA
⁴Department of Physics and Department of Astronomy, Yale University, New Haven, CT 06511, USA

Context. The observation of carbon-rich disks have motivated several studies questioning the influence of the C/O ratio on their gas phase composition in order to establish the connection between the metallicity of hot-Jupiters and that of their parent stars.
Aims. We propose a method that allows the characterization of the adopted C/O ratio in protoplanetary disks independently from the determination of the host star composition. Titanium and vanadium chemistries are investigated because they are strong optical absorbers and also because their oxides are known to be sensitive to the C/O ratio in some exoplanet atmospheres.
Methods. We use a commercial package based on the Gibbs energy minimization technique to compute the titanium and vanadium equilibrium chemistries in protoplanetary disks for C/O ratios ranging from 0.05 to 10. Our calculations are performed for pressures in the 10^-6–10^-2 bar domain, and for temperatures ranging from 50 K to 2000 K.
Results. We find that the vanadium nitride/vanadium oxide and titanium hydride/titanium oxide gas phase ratios strongly depend on the C/O ratio in the hot parts of disks (T ≥ 1000 K). Our calculations suggest that, in these regions, these ratios can be used as tracers of the C/O value in protoplanetary disks.

Reference
Ali-Dib M, Mousis O, Pekmezci GS, Lunine JI, Madhusudhan N and Petit J-M (2014) Influence of the C/O ratio on titanium and vanadium oxides in protoplanetary disks. Astronomy & Astrophysics 561:A60.
[doi:10.1051/0004-6361/201321780]
Reproduced with permission © ESO

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V. Alí-Lagoa^1,2, L. Lionni³, M. Delbo⁴, B. Gundlach⁵, J. Blum⁵ and J. Licandro^1,2

¹Instituto de Astrofísica de Canarias (IAC), c/ Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
²Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
³University Paris VII – Diderot, 5 rue Thomas Mann, 75013 Paris, France
⁴UNS-CNRS-Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France
⁵Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany

Aims. We derive the thermal inertia of 2008 EV₅, the baseline target for the Marco Polo-R mission proposal, and infer information about the size of the particles on its surface.
Methods. Values of thermal inertia were obtained by fitting an asteroid thermophysical model to NASA’s Wide-field Infrared Survey Explorer (WISE) infrared data. Grain size was derived from the constrained thermal inertia and a model of heat conductivity that accounts for different values of the packing fraction (a measure of the degree of compaction of the regolith particles).
Results. We obtain an effective diameter D = 370 ± 6   m, geometric visible albedo p_V = 0.13 ± 0.05 (assuming H = 20.0 ± 0.4), and thermal inertia Γ = 450 ± 60 J m^-2 s^−1/2 K^-1 at the 1σ level of significance for its retrograde spin-pole solution. The regolith particles radius is r = 6.6^+1.3_-1.3 mm for low degrees of compaction and r = 12.5^+2.7_-2.6 mm for the highest packing densities.

Reference
Alí-Lagoa V, Lionni L, Delbo M, Gundlach B, Blum J and Licandro J (2014) Thermophysical properties of near-Earth asteroid (341843) 2008 EV₅ from WISE data. Astronomy & Astrophysics 561:A45.
[doi:10.1051/0004-6361/201322215]
Reproduced with permission © ESO

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Álvaro Ribas^1,2,3, Bruno Merín⁴, Hervé Bouy² and Luke T. Maud⁵

¹European Space Astronomy Centre (ESA), PO Box 78, 28691 Villanueva de la Cañada Madrid Spain
²Centro de Astrobiología, INTA-CSIC, PO Box-Apdo. de correos 78, 28691 Villanueva de la Cañada Madrid, Spain
³Ingeniería y Servicios Aeroespaciales-ESAC, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁴Herschel Science Centre, ESAC-ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁵School of Physics & Astronomy, EC Stoner Building, University of Leeds, Leeds LS2 9JT, UK

Aims. We study the evolution of circumstellar disks in 22 young (1 to 100 Myr) nearby (within 500 pc) associations over the entire mass spectrum using photometry covering from the optical to the mid-infrared.
Methods. We compiled a catalog of 2340 spectroscopically-confirmed members of these nearby associations. We analyzed their spectral energy distributions and searched for excess related to the presence of protoplanetary disks. The dataset has been analyzed in a homogeneous and consistent way, allowing for meaningful inter-comparison of results obtained for individual regions. Special attention was given to the sensitivity limits and spatial completeness of the observations.
Results. We derive disk fractions as probed by mid-infrared excess in the 22 regions. The unprecedented size of our sample allows us to confirm the timescale of disk decay reported in the literature and to find new trends. The fraction of excess sources increases systematically if measured at longer wavelengths. Disk percentages derived using different wavelength ranges should therefore be compared with caution. The dust probed at 22–24 μm evolves slower than that probed at shorter wavelengths (3.4–12 μm). Assuming an exponential decay, we derive a timescale τ = 4.2 − 5.8 Myr at 22–24 μm for primordial disks, compared to 2 ~ 3 Myr at shorter wavelengths (3.4–12 μm). Primordial disks disappear around 10 ~ 20 Myr. Their decline matches in time a brief increase of the number of “evolved” disks (defined here as including transitional and debris disks). There is more dispersion in the fraction of excess sources with age when measured at 22–24 μm in comparison to shorter wavelengths.
Conclusions. The increase in timescale of excess decay at longer wavelength is compatible with inside-out disk clearing scenarios. The increased timescale of decay and larger dispersion in the distribution of disk fractions at 22–24 μm suggest that the inner (terrestrial-planet forming) and outer (giant-planet forming) zones evolve differently, the latter potentially following a variety of evolutionary paths. The drop of primordial disks and the coincident rise of evolved disks at 10 Myr are compatible with planet formation theories suggesting that the disappearance of the gas is immediately followed by the dynamical stirring of the disk.

Reference
Ribas A, Merín B, Bouy H and Maud LT (2014) Disk evolution in the solar neighbourhood – I. Disk frequencies from 1 to 100 Myr. Astronomy & Astrophysics 561:A54.
[doi:10.1051/0004-6361/201322597]
Reproduced with permission © ESO

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