A. Garenne^a, P. Beck^a, G. Montes-Hernandez^b, R. Chiriac^c, F. Toche^c, E. Quirico^a, L. Bonal^a and B. Schmitt^a

^aCNRS and University Joseph Fourier, IPAG, OSUG/INSU, BP 53, 38041 Grenoble Cedex 9, France
^bCNRS, ISTerre, F-38041 Grenoble, France
^cUniversité de Lyon, Université Lyon 1, Laboratoire des Multimatériaux et Interfaces UMR CNRS 5615, 22 Avenue Gaston Berger, 69622 Villeurbanne Cedex, France.

Carbonaceous chondrites record processes of aqueous alteration in the presence of hydrated and hydroxylated minerals, which could have provided a source of water in the inner solar system (Alexander et al., 2012, 2013; Trigo-Rodriguez and Martin-Torres, 2011). In this study, thermogravimetric analysis (TGA) was performed on 26 CM chondrites, which cover a range of degree of aqueous alteration from 2.0, such as Meteorite Hills (MET) 01070, to 2.6, such as Queen Alexandra Range (QUE) 97990, in order to quantify their water content. In addition, by measuring the release of volatile elements as a function of temperature, we obtained information on the mineralogy of water-bearing phases and provide indicators of aqueous alteration based on water released by phyllosilicates. These analyses are combined with infrared spectroscopy (IR) made on meteorite pellets heated up to 300°C. The infrared features (-OH band at 3-μm and SiO₄ around 10-μm) revealed a correlation with TGA. The two techniques are in agreement with the scheme of aqueous alteration proposed by Rubin et al. (2007) and Alexander et al. (2013) based on phyllosilicate abundance.
The low temperature (200-400°C) mass loss observed in TGA is attributed to Fe-oxy-hydroxydes (ferrihydrite, goethite). However, the proportion of these minerals formed by terrestrial alteration remains unknown. TGA also revealed two anomalous CM chondrites, Pecora Escarpment (PCA) 02012 and PCA 02010. Their TGA curves are significantly different from those of “regular” CMs with little mass loss, which can be related to the dehydration history of these meteorites in response to a heating event (Raman measurements also point toward a thermal event, Quirico et al., 2013). In the case of more mildly heated chondrites, such as with Wisconsin Range (WIS) 91600, the TGA curve presents similar mass loss to the other CMs.
Seven bulk measurements of CR chondrites and 3 measurements of matrix-enriched parts of CR meteorites were also studied by TGA, and confirm the low hydration level of chondrules and a significant alteration of the matrix. The water content of the matrix of the CM 2.6 QUE 97990 was estimated and compared to TGA of the matrix enriched portion of the CR2 EET 92159 and that of Orgueil.
Results suggest a similar aqueous alteration degree between Orgueil and the matrix of CMs (around 25 wt%) and a lower alteration of the CR2 matrix (11 wt% of H₂O).

Reference
Garenne A, Beck P, Montes-Hernandez G, Chiriac R, Toche F, Quirico E, Bonal L and B. Schmitt B (in press) The abundance and stability of “water” in type 1 and 2 carbonaceous chondrites (CI, CM and CR). Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.03.034]
Copyright Elsevier

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K. Stephan¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹DLR, Institute of Planetary Research, Berlin, Germany

Small morphologically fresh impact craters (<10 km in diameter) on Vesta’s surface with a photometrically distinct ejecta blanket are expected to represent fresh surface material and thus provide the opportunity to study the composition of the unweathered surface. Dawn-Framing Camera and Visual and Infrared Spectrometer (VIR) data reveal impact craters with bright, dark, and mixed, i.e., partly bright and dark, ejecta existing on Vesta’s surface, which not only differ in the visible albedo from their surroundings but also in their composition. Differences in the composition are related to the visible albedo and/or the geographic location of the impact craters. Bright ejecta, only seen in the southern Vestan hemisphere, are dominated by howardite/eucrite-like material as expected for Vesta’s upper crust. Dark ejecta associated with dark impact craters are dominated by a strongly absorbing, spectrally neutral compound, supporting an origin from carbon-rich impactors. Few impact craters of intermediate albedo in Vesta’s southern hemisphere contain material resembling diogenites, which are expected to exist in the deeper parts of Vesta’s interior. The geological settings suggest that the diogenite-like material represents a part of a layer of diogenitic material surrounding the Rheasilvia basin or local concentrations of diogenitic material as part of the ejecta excavated during the latter stage of the Rheasilvia impact event. The spectral differences between eucrite- and diogenite-dominated materials also could be verified due to spin-forbidden absorptions in the visible spectral range, which are known from laboratory spectra of pyroxenes, but, which have been identified in the VIR spectra of Vesta for the first time.

Reference
Stephan et al. (in press) Small fresh impact craters on asteroid 4 Vesta: A compositional and geological fingerprint. Journal of Geophysical Research: Planets
[doi:10.1016/j.gca.2014.03.034]
Published by arrangement with John Wiley & Sons

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V. Piétu¹, S. Guilloteau^2,3, E. Di Folco^2,3, A. Dutrey^2,3 and Y. Boehler⁴

¹IRAM, 300 rue de la piscine, 38406 Saint Martin d’Hères France
²Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
³CNRS, LAB, UMR 5804, 33270 Floirac, France
⁴Centro de Radioastronomìa y Astrofìsica, UNAM, Apartado Postal 3-72, 58089 Morelia, Michoacàn, Mexico

Context. Most Class II sources (of nearby star-forming regions) are surrounded by disks with weak millimeter continuum emission. These “faint” disks may hold clues to the disk dissipation mechanism. However, the physical properties of protoplanetary disks have been directly constrained by imaging only the brightest sources.
Aims. We attempt to determine the characteristics of such faint disks around classical T Tauri stars and to explore the link between disk faintness and the proposed disk dispersal mechanisms (accretion, viscous spreading, photo-evaporation, planetary system formation).
Methods. We performed high angular resolution (0.3′′) imaging of a small sample of disks (9 sources) with low 1.3 mm continuum flux (mostly <30 mJy) with the IRAM Plateau de Bure interferometer and simultaneously searched for ¹³CO (or CO) J = 2−1 line emission. Using a simple parametric disk model, we determined characteristic sizes for the disks in dust and gas, and we constrained surface densities in the central 50 AU.
Results. All disks are much smaller than the bright disks imaged so far, both in continuum and ¹³CO lines (5 detections). In continuum, half of the disks are very small, with characteristic radii less than 10 AU, but still have high surface density values. Small sizes appear to be the main cause of the low disk luminosity. Direct evidence for grain growth is found for the three disks that are sufficiently resolved. Low continuum opacity is attested in only two systems, but we cannot firmly distinguish between a low gas surface density and a lower dust emissivity resulting from grain growth. Finally, we report a tentative discovery of a ~20 AU radius cavity in DS Tau, which with the (unresolved) “transition” disk of CX Tau, brings the proportion of “transitional” disks to a similar value to that of brighter sources. The existence of cavities cannot by itself explain their observed low mm flux.
Conclusions. This study highlights a category of very compact dust disks that still exhibit high surface densities, which may represent up to 25% of the whole disk population. While its origin is unclear with the current data alone, it may be related to the compact planetary systems found by the Kepler mission.

Reference
V. Piétu V, Guilloteau S, Di Folco E, Dutrey A and Boehler Y (2014) Faint disks around classical T Tauri stars: Small but dense enough to form planets Astronomy & Astrophysics 564:A95.
[doi:10.1051/0004-6361/201322388]
Reproduced with permission © ESO
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