Shun-ichiro Karato

Yale University, Department of Geology and Geophysics, New Haven, CT 06520, USA

Although the Moon was considered to be “dry”, recent measurements of hydrogen content in some of the lunar samples showed a substantial amount of water comparable to the water content in the Earthʼs asthenosphere. However, the interpretation of these observations in terms of the distribution of water in the lunar interior is difficult because the composition of these rocks reflects a complicated history involving melting and crystallization. In this study, I analyze geophysically inferred properties to obtain constraints on the distribution of water (and temperature) in the lunar interior. The electrical conductivity inferred from electromagnetic induction observations and the geodetically or geophysically inferred Q are interpreted in terms of laboratory data and the theoretical models on the influence of water (hydrogen) on these properties. Both electrical conductivity and Q are controlled by defect-related processes that are sensitive to the water (hydrogen) content and temperature but less sensitive to the major element chemistry. After a correction for the influence of the major element chemistry constrained by geophysical observations and geochemical considerations, I estimate the temperature–water content combinations that are consistent with the geophysically inferred electrical conductivity and Q. I conclude that the lunar interior is cooler than Earth (at the same depth) but the water content of the lunar mantle is similar to that of Earthʼs asthenosphere. A possible model is presented to explain the not-so-dry Moon where a small degree of water loss during the Moon formation is attributed to the role of liquid phases that play an important role in the Moon-forming environment.

Reference
Karato S-I (2013) Geophysical constraints on the water content of the lunar mantle and its implications for the origin of the Moon. Earth and Planetary Science Letters 384:144–153.
[doi:10.1016/j.epsl.2013.10.001]
Copyright Elsevier

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Sachiko Amari^1,*, Jun-ichi Matsuda², Rhonda M. Stroud³, and Matthew F. Chisholm⁴

¹McDonnell Center for the Space Sciences and the Physics Department, Washington University, St. Louis, MO 63130, USA
²Department of Earth and Space Science, Osaka University, Osaka 560-0043, Japan
³Code 6360, Naval Research Laboratory, Washington, DC 20375, USA
⁴Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

The majority of heavy noble gases (Ar, Kr, and Xe) in primitive meteorites are stored in a poorly understood phase called Q. Although Q is thought to be carbonaceous, the full identity of the phase has remained elusive for almost four decades. In order to better characterize phase Q and, in turn, the early solar nebula, we separated carbon-rich fractions from the Saratov (L4) meteorite. We chose this meteorite because Q is most resistant in thermal alteration among carbonaceous noble gas carriers in meteorites and we hoped that, in this highly metamorphosed meteorite, Q would be present but not diamond: these two phases are very difficult to separate from each other. One of the fractions, AJ, has the highest ¹³²Xe concentration of 2.1 × 10^–6 cm³ STP g^–1, exceeding any Q-rich fractions that have yet been analyzed. Transmission electron microscopy studies of the fraction AJ and a less Q-rich fraction AI indicate that they both are primarily porous carbon that consists of domains with short-range graphene orders, with variable packing in three dimensions, but no long-range graphitic order. The relative abundance of Xe and C atoms (6:10⁹) in the separates indicates that individual noble gas atoms are associated with only a minor component of the porous carbon, possibly one or more specific arrangements of the nanoparticulate graphene.

Reference
Amari S, Matsuda J-I, Stroud RM and Chisholm MF (2013) Highly Concentrated Nebular Noble Gases in Porous Nanocarbon Separates from the Saratov (L4) Meteorite. The Astrophysical Journal 778:37.
[doi:10.1088/0004-637X/778/1/37]

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S. Richard¹, P. Barge¹ and S. Le Dizès²

¹Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France
²Aix-Marseille Université, CNRS, Centrale Marseille, IRPHE (Institut de Recherche sur les Phénomènes Hors Equilibre), UMR 7342, 49 rue F. Joliot Curie, 13013 Marseille, France

Context. Large-scale persistent vortices could play a key role in the evolution of protoplanetary disks, particularly in the dead zone where no turbulence associated with a magnetic field is expected. These vortices are known to form easily in 2D disks via the Rossby wave or the baroclinic instability. In three dimensions, however, their formation and stability is a complex problem and still a matter of debate.
Aims. We study the formation of vortices by the Rossby wave instability in a stratified inviscid disk and describe their 3D structure, stability, and long-term evolution.
Methods. Numerical simulations were performed using a fully compressible hydrodynamical code based on a second-order finite volume method. We assumed a perfect-gas law and a non-homentropic adiabatic flow.
Results. The Rossby wave instability is found to proceed in 3D in a similar way as in 2D. Vortices produced by the instability look like columns of vorticity in the whole disk thickness; the weak vertical motions are related to the weak inclination of the vortex axis that appears during the development of the RWI. Vortices with aspect ratios higher than 6 are unaffected by the elliptical instability. They relax into a quasi-steady columnar structure that survives hundreds of rotations while slowly migrating inward toward the star at a rate that reduces with the vortex aspect ratio. Vortices with a lower aspect ratio are by contrast affected by the elliptic instability. Short aspect ratio vortices (χ < 4) are completely destroyed in a few orbital periods. Vortices with an intermediate aspect ratio (4 < χ < 6) are partially destroyed by the elliptical instability in a region away from the midplane where the disk stratification is sufficiently strong.
Conclusions. Elongated Rossby vortices can survive many orbital periods in protoplanetary disks in the form of vorticity columns. They could play a significant role in the evolution of the gas and the gathering of solid particles to form planetesimals or planetary cores, a possibility that receives a renewed interest with the recent discovery of a particle trap in the disk of Oph IRS 48.

Reference
Richard S, Barge P and Le Dizès S (2013) Structure, stability, and evolution of 3D Rossby vortices in protoplanetary disks. Astronomy & Astrophysics 559:A30.
[doi:10.1051/0004-6361/201322175]
Reproduced with permission © ESO

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Corentin Le Guillou^a,b,* and Adrian Brearley^a

^aDepartment of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA
^bPresent address: Department of geology, mineralogy and geophysics, Ruhr-Universität Bochum, Universität Strasse 150, 44780 Bochum, Germany

In order to investigate the nature of organics at the time of their accretion into chondrite parent bodies, as well as their subsequent evolution with aqueous alteration, we have conducted a study of the morphologies, spatial distribution and relationships between organic particles and the surrounding matrix phases in one of the most pristine carbonaceous chondrites currently known (MET 00426, CR), which is therefore likely to contain the best preserved record of pre-accretion features. Focused ion beam sections were extracted for transmission electron microscope observations from its matrix.
Organic matter (OM) shows a heterogeneous population of grains, most of them smaller than 1 micron. Diverse morphologies are observed, such as compact, rounded aggregates, individual and aggregated nanoglobules, and micron- to nanometer-sized veins. A common feature is the systematic presence of cracks connected to the grains and filled with OM. The surrounding matrix groundmass consists of amorphous iron-rich silicate particles intimately mixed with phyllosilicates, sulphides and occasional tochilinite, with sizes ranging from several hundreds of nanometers to below 10 nm. A close spatial relationship is commonly observed between some of the organic matter particles and alteration phases, such as tochilinite and phyllosilicates. Phyllosilicates sometimes occur intimately intercalated with organic matter at a scale below 10 nm.
We used TEM/EDS techniques to quantify the water concentration in the matrix amorphous silicate material and the phyllosilicates.The water contents of both materials are identical at 10 (± 6) wt.% H₂O and demonstrate, that the amorphous silicate material in this meteorite is hydrated. Therefore, even though these CR3.0 chondrites are the least altered objects from a mineralogical point of view, their matrices contain significant amounts of water in the amorphous silicate. This coupled in situ study of organics and aqueous alteration suggests that a significant population of the OM accreted as a mixture of soluble and insoluble molecules together with water ice grains and that the OM was mobile at the micrometer scale. The spatial distribution of the OM grains can therefore, in part, be attributed to parent body processes. We suggest that as accreted ice melted, hydration of the amorphous silicates and formation of tochilinite and phyllosilicates occurred in the immediate vicinity of the composite water ice / organic matter grains. The water-soluble component of the organics was likely transported and redistributed in the surrounding porosity (cracks, grain boundaries) as water circulated. The textural settings suggest that some of the OM material could have been polymerized during aqueous alteration and transformed into insoluble molecules, perhaps during the last stages of alteration as water was consumed by silicate hydration reactions.

Reference
Le Guillou C and Brearley A (in press) Relationships between organics, water and early stages of aqueous alteration in the pristine CR3.0 chondrite MET 00426. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.10.024]
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A. Seizinger¹, R. Speith² and W. Kley¹

¹Institut für Astronomie and Astrophysik, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
²Physikalisches Institut, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 14, 72076 Tübingen, Germany

Context. Within the sequential accretion scenario of planet formation, planets are built up through a sequence of sticking collisions. The outcome of collisions between porous dust aggregates is very important for the growth from very small dust particles to planetesimals. In this work we determine the necessary material properties of dust aggregates as a function of porosity.
Aims. Continuum models such as SPH that are capable of simulating collisions of macroscopic dust aggregates require a set of material parameters. Some of them, such as the tensile and shear strength, are difficult to obtain from laboratory experiments. The aim of this work is to determine these parameters from ab initio molecular dynamics simulations.
Methods. We simulated the behavior of porous dust aggregates using a detailed micro-physical model of the interaction of spherical grains that includes adhesion forces, rolling, twisting, and sliding. Using different methods of preparing the samples, we studied the strength behavior of our samples with varying porosity and coordination number of the material.
Results. For the tensile strength, we can reproduce data from laboratory experiments very well. For the shear strength, there are no experimental data available. The results from our simulations differ significantly from previous theoretical models, which indicates that the latter might not be sufficient to describe porous dust aggregates.
Conclusions. We have provided the functional behavior of tensile and shear strength of porous dust aggregates as a function of the porosity, which can be directly applied to continuum simulations of these objects in planet formation scenarios.

Reference
Seizinger A, Speith R and Kley W (2013) Tensile and shear strength of porous dust agglomerates. Astronomy & Astrophysics
[doi:10.1051/0004-6361/201322046]
Reproduced with permission © ESO

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