Tomáš Magna^a,*, Magdalena Šimčíková^a, Frédéric Moynier^b

^aCzech Geological Survey, Klárov 3, CZ-118 21 Prague 1, Czech Republic
^bUniversité Paris Diderot, 1 rue Jussieu, F-75238 Paris cedex 05, France

We present lithium (Li) abundances and isotope compositions in a suite of howardites, eucrites and diogenites (HEDs). These meteorites most likely originated from asteroid Vesta and were delivered to Earth by a series of independent impact events. The Li concentrations show striking differences between Li-poor diogenites plus cumulate eucrites and Li-enriched eucrites whilst howardites have Li abundances intermediate between eucrites and diogenites. Contrary to Li elemental inter-group differences, Li isotope compositions are irresolvable among these individual groups of HED meteorites despite their wildly distinct petrography, attesting to insignificant Li isotope fractionation during formation of a thick basaltic crust by melting of the Vestan mantle. The mean Li isotope composition of Bulk Silicate Vesta is estimated at 3.7 ± 0.6‰ (1σ), intermediate to that of the Earth versus Mars and Moon but identical with these terrestrial bodies within uncertainty. This further validates largely homogeneous inner Solar System solids from the Li isotope perspective and supports the lack of loss of moderately volatile elements from planetary embryos during their magmatic histories because Li does not follow depletion trends inferred from more volatile elements. Pasamonte eucrite has the same Li isotope composition of other eucrites although it may not be directly linked to Vesta. These observations are also important for generating Li elemental and isotope signatures in juvenile basaltic crusts of large terrestrial planets and numerous planetary embryos in the early Solar System. A combination of CV+L chondrites may be less suitable for building Vesta from Li perspective but this may face sampling bias of available data and only further analyses may resolve this issue. Alternatively, significant shift of ~1‰ towards heavier Li isotope compositions must have occurred during thermal processing of CV+L (2.2 to 2.8‰) mixture in order to account for the observed Li isotope systematics in HED meteorites. No correlation is observed between Li versus Zn, Fe or Si isotopes, respectively, implying unrelated processes of forming stable isotope variations observed in HED meteorites.

Reference
Magna T, Šimčíková M and Moynier F (in press) Lithium systematics in howardite–eucrite–diogenite meteorites: Implications for crust–mantle evolution of planetary embryos. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.10.015]
Copyright Elsevier

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M. Pajola^1,5, M. Lazzarin², C. M. Dalle Ore^3,4, D. P. Cruikshank⁴, T. L. Roush⁴, S. Magrin¹, I. Bertini¹, F. La Forgia², and C. Barbieri^1,2

¹Center of Studies and Activities for Space, CISAS, “G. Colombo,” University of Padova, I-35131 Padova, Italy
²Department of Physics and Astronomy, University of Padova, I-35131 Padova, Italy
³Carl Sagan Center, SETI Institute, Mountain View, CA 94043, USA
⁴NASA Ames Research Center, Moffett Field, CA 94035, USA
⁵Currently (2012-2013) Visiting Scientist at NASA Jet Propulsion Laboratory, JPL-CALTECH, 4800 Oak Grove Drive, Pasadena, CA 91109, USA.

This paper describes the spectral modeling of the surface of Phobos in the wavelength range between 0.25 and 4.0 μm. We use complementary data to cover this spectral range: the OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System on board the ESA Rosetta spacecraft) reflectance spectrum that Pajola et al. merged with the VSK-KRFM-ISM (Videospectrometric Camera (VSK)-Combined Radiometer and Photometer for Mars (KRFM)-Imaging Spectrometer for Mars (ISM) on board the USSRPhobos 2 spacecraft) spectra by Murchie & Erard and the IRTF (NASA Infrared Telescope Facility, Hawaii, USA) spectra published by Rivkin et al. The OSIRIS data allow the characterization of an area of Phobos covering from 868 N to 90° S in latitude and from 126° W to 286° W in longitude. This corresponds chiefly to the trailing hemisphere, but with a small sampling of the leading hemisphere as well. We compared the OSIRIS results with the Trojan D-type asteroid 624 Hektor and show that the overall slope and curvature of the two bodies over the common wavelength range are very similar. This favors Phobos being a captured D-type asteroid as previously suggested. We modeled the OSIRIS data using two models, the first one with a composition that includes organic carbonaceous material, serpentine, olivine, and basalt glass, and the second one consisting of Tagish Lake meteorite and magnesium-rich pyroxene glass. The results of these models were extended to longer wavelengths to compare the VSK-KRFM-ISM and IRTF data. The overall shape of the second model spectrum between 0.25 and 4.0 μm shows curvature and an albedo level that match both the OSIRIS and Murchie & Erard data and the Rivkin et al. data much better than the first model. The large interval fit is encouraging and adds weight to this model, making it our most promising fit for Phobos. Since Tagish Lake is commonly used as a spectral analog for D-type asteroids, this provides additional support for compositional similarities between Phobos and D-type asteroids.

Reference
Pajola M, Lazzarin M, Dalle Ore CM, Cruikshank DP, Roush TL, Magrin S, Bertini I, La Forgia F and Barbieri C (in press) Phobos as a D-type Captured Asteroid, Spectral Modeling from 0.25 to 4.0 μm. The Astrophysical Journal
[doi:10.1088/0004-637X/777/2/127]

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A. Moór¹, A. Juhász², Á. Kóspál^3,10, P. Ábrahám¹, D. Apai⁴, T. Csengeri⁵, C. Grady^6,7, Th. Henning⁸, A. M. Hughes⁹, Cs. Kiss¹, I. Pascucci⁴, M. Schmalzl², and K. Gabányi¹

¹Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, P.O. Box 67, H-1525 Budapest, Hungary
²Leiden Observatory, Leiden University, Niels Bohrweg 2, NL-2333-CA Leiden, The Netherlands
³Research and Scientific Support Department, European Space Agency (ESA-ESTEC, SRE-SA), P.O. Box 299, 2200-AG Noordwijk, The Netherlands
⁴Department of Astronomy and Department of Planetary Sciences, The University of Arizona, Tucson, AZ 85721, USA
⁵Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
⁶NASA Goddard Space Flight Center, Code 667, Greenbelt, MD 20771, USA
⁷Eureka Scientific, 2452 Delmer Street, Suite 100, Oakland, CA 94602, USA
⁸Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
⁹Wesleyan University Department of Astronomy, Van Vleck Observatory, 96 Foss Hill Dr., Midletown, CT 06457, USA
¹⁰ESA fellow.

Circumstellar disks around stars older than 10 Myr are expected to be gas-poor. There are, however, two examples of old (30-40 Myr) debris-like disks containing a detectable amount of cold CO gas. Here we present Atacama Large Millimeter/Submillimeter Array (ALMA) and Herschel Space Observatory observations of one of these disks, around HD 21997, and study the distribution and origin of the dust and its connection to the gas. Our ALMA continuum images at 886 μm clearly resolve a broad ring of emission within a diameter of ~4.”5, adding HD 21997 to the dozen debris disks resolved at (sub)millimeter wavelengths. Modeling the morphology of the ALMA image with a radiative transfer code suggests inner and outer radii of ~55 and ~150 AU, and a dust mass of 0.09 M _⊕. Our data and modeling hints at an extended cold outskirt of the ring. Comparison with the morphology of the CO gas in the disk reveals an inner dust-free hole where gas nevertheless can be detected. Based on dust grain lifetimes, we propose that the dust content of this gaseous disk is of secondary origin and is produced by planetesimals. Since the gas component is probably primordial, HD 21997 is one of the first known examples of a hybrid circumstellar disk, a thus-far little studied late phase of circumstellar disk evolution.

Reference
Moór A, Juhász A, Kóspál Á, Ábrahám P, Apai D, Csengeri T, Grady C, Henning Th, Hughes AM, Kiss Cs, Pascucci I, Schmalzl M, and Gabányi K (in press) ALMA Continuum Observations of a 30 Myr Old Gaseous Debris Disk around HD 21997. The Astrophysical Journal – Letters
[doi:10.1088/2041-8205/777/2/L25]

Link to Article