Seasonality of the submesoscale dynamics in the Gulf Stream region

Frontogenesis and frontal instabilities in the mixed layer are known to be important processes in the formation of submesoscale features. We study the seasonality of such processes in the Gulf Stream (GS) region. To approach this problem, a realistic simulation with the Hybrid Coordinate Ocean Model is integrated for 18 months at two horizontal resolutions: a high-resolution (1/48°) simulation able to resolve part of the submesoscale regime and the full range of mesoscale dynamics, and a coarser resolution (1/12°) case, in which submesoscales are not resolved. Results provide an insight into submesoscale dynamics in the complex GS region. A clear seasonal cycle is observed, with submesoscale features mostly present during winter. The submesoscale field is quantitatively characterized in terms of deviation from geostrophy and 2D dynamics. The limiting and controlling factor in the occurrence of submesoscales appears to be the depth of the mixed layer, which controls the reservoir of available potential energy available at the mesoscale fronts that are present most of the year. Atmospheric forcings are the main energy source behind submesoscale formation, but mostly indirectly through mixed layer deepening. The mixed layer instability scaling suggested in the (Fox-Kemper et al., J Phys Oceanogr 38:1145–1165, 2008) parametrization appears to hold, indicating that the parametrization is appropriate even in this complex and mesoscale dominated area.     

A DUAL mission for nuclear astrophysics

DUAL will study the origin and evolution of the elements and explores new frontiers of physics: extreme energies that drive powerful stellar explosions and accelerate particles to macroscopic energies; extreme densities that modify the laws of physics around the most compact objects known; and extreme fields that influence matter in a way that is unknown on Earth. The variability of these extreme objects requires continuous all-sky coverage, while detailed study demands an improvement in sensitivity over previous technologies by at least an order of magnitude. The DUAL payload is composed of an All-Sky Compton Imager (ASCI), and two optical modules, the Laue-Lens Optic (LLO) and the Coded-Mask Optic (CMO). The ASCI serves dual roles simultaneously, both as an optimal focal-plane sensor for deep observations with the optical modules and as a sensitive true all-sky telescope in its own right for all-sky surveys and monitoring. While the optical modules are located on the main satellite, the All-Sky Compton Imager is situated on a deployable structure at a distance of 30?m from the satellite. This configuration not only permits to maintain the less massive payload at the focal distance, it also greatly reduces the spacecraft-induced detector background, and, above all it provides ASCI with a continuous all-sky exposure.     

High resolution TEM of chondritic carbonaceous matter: Metamorphic evolution and heterogeneity

Abstract– The insoluble carbonaceous matter from 12 chondrites (CI, CM, CO, CV, EH, and UOC), was characterized by high resolution transmission electron microscopy (HRTEM). Besides ubiquitous nanoglobules, the insoluble organic matter from petrologic type 1 and 2 chondrites and Semarkona (LL 3.0) is composed of a highly disordered polyaromatic component. No structural differences were observed between these IOMs, in agreement with the limited thermal metamorphism they all experienced. In chondrites of petrologic type >3.0, the evolution of the IOM is controlled by the extent of thermal metamorphism. The polyaromatic layers, shorter than 1 nm in petrologic type ≤3.0 chondrites, grow up to sizes between 5 and 10 nm in petrologic type >3.6 chondrites, contributing to the increase of the degree of structural order. In addition, we find rare, but ubiquitous onion‐like carbons, which may be the product of nanodiamond graphitization. The insoluble carbonaceous matter of the enstatite chondrite Sahara 97096 (EH 3) is different from the other meteorites studied here. It is more heterogeneous and displays a high abundance of graphitized particles. This may be the result of a mixture between (1) the disordered carbon located in the matrix, and (2) catalytic graphitized phases associated with metal, potentially originating from partial melting events. The structural and nanostructural evolution are similar in all IOMs. This suggests that the structure of the accreted precursors and the parent body conditions of their secondary thermal modifications (temperature, duration, and pressure) were similar. The limited degree of organization of the most metamorphosed IOMs compared with terrestrial rocks submitted to similar temperature suggests that the conditions are not favorable to graphitization processes, due to the chemical nature of the precursor or the lack of confinement pressure.     

Metal‐saturated sulfide assemblages in NWA 2737: Evidence for impact‐related sulfur devolatilization in Martian meteorites

NWA 2737, a Martian meteorite from the Chassignite subclass, contains minute amounts (0.010 ± 0.005 vol%) of metal‐saturated Fe‐Ni sulfides. These latter bear evidence of the strong shock effects documented by abundant Fe nanoparticles and planar defects in Northwest Africa (NWA) 2737 olivine. A Ni‐poor troilite (Fe/S = 1.0 ± 0.01), sometimes Cr‐bearing (up to 1 wt%), coexists with micrometer‐sized taenite/tetrataenite‐type native Ni‐Fe alloys (Ni/Fe = 1) and Fe‐Os‐Ir‐(Ru) alloys a few hundreds of nanometers across. The troilite has exsolved flame‐like pentlandite (Fe/Fe + Ni = 0.5–0.6). Chalcopyrite is almost lacking, and no pyrite has been found. As a hot desert find, NWA 2737 shows astonishingly fresh sulfides. The composition of troilite coexisting with Ni‐Fe alloys is completely at odds with Chassigny and Nahkla sulfides (pyrite + metal‐deficient monoclinic‐type pyrrhotite). It indicates strongly reducing crystallization conditions (close to IW), several log units below the fO₂ conditions inferred from chromites compositions and accepted for Chassignites (FMQ‐1 log unit). It is proposed that reduction in sulfides into base and precious metal alloys is operated via sulfur degassing, which is supported by the highly resorbed and denticulated shape of sulfide blebs and their spongy textures. Shock‐related S degassing may be responsible for considerable damages in magmatic sulfide structures and sulfide assemblages, with concomitant loss of magnetic properties as documented in some other Martian meteorites.     

Melt inclusions in augite from the nakhlite meteorites: A reassessment of nakhlite parental melt and implications for petrogenesis

Abstract– The nakhlites, a subgroup of eight clinopyroxenites thought to come from a single geological unit at the Martian surface, show melt inclusions in augite and olivine. In contrast to olivine‐hosted melt inclusions, augite‐hosted melt inclusions are not surrounded by fractures, and are thus considered preferential candidates for reconstructing parent liquid compositions. Furthermore, two types of augite‐hosted melt inclusion have been defined and characterized in four different nakhlites (Northwest Africa [NWA] 817, Nakhla, Governador Valadares, and NWA 998): Type‐I isolated inclusions in augite cores that contain euhedral to subhedral augite, Ti‐magnetite, and pigeonite plus silica‐rich glass and a gas bubble; Type‐II microinclusions that form trails crosscutting host augite crystals. Fast‐heating experiments were performed on selected pristine primary augite‐hosted melt inclusions from these four samples. Of these, only data from Nakhla were considered robust for reconstruction of a nakhlite parental magma composition (NPM). Based upon careful petrographic selection and consideration of iron‐magnesium ratios, our data are used to propose an NPM, which is basaltic (49.1 wt% SiO₂), of high Ca/Al (1.95), and K₂O‐poor (0.32 wt%). Thermodynamic modeling at an oxygen fugacity one log unit below the QFM buffer using the MELTS and PETROLOG programs implies that Mg‐rich olivine was not a liquidus phase for this composition. Our analysis is used to suggest that olivine megacrysts found in the nakhlites are unlikely to have coprecipitated with augite, and thus may have been introduced during or subsequent to accumulation in the magma chamber, possibly from more evolved portions of the same chamber.     

Homogeneous distribution of Fe isotopes in the early solar nebula

To examine the iron (Fe) isotopic heterogeneities of CI and ordinary chondrites, we have analyzed several large chips (approximately 1 g) from three CI chondrites and three ordinary chondrites (LL5, L5, and H5). The Fe isotope compositions of five different samples of Orgueil, one from Ivuna and one from Alais (CI chondrites), are highly homogeneous. This new dataset provides a δ⁵⁶Fe average of 0.02 ± 0.04‰ (2SE, n = 7), which represents the best available value for the Fe isotopic composition of CI chondrites and probably the best estimate of the bulk solar system. We conclude that the homogeneity of CI chondrites reflects the initial Fe isotopic homogeneity of the well‐mixed solar nebula. In contrast, larger (up to 0.26‰ in δ⁵⁶Fe) isotopic variations have been found between separate approximately 1 g pieces of the same ordinary chondrite sample. The Fe isotope heterogeneities in ordinary chondrites appear to be controlled by the abundances of chondritic components, specifically chondrules, whose Fe isotope compositions have been fractionated by evaporation and recondensation during multiple heating events.     

Vesta's mineralogical composition as revealed by the visible and infrared spectrometer on Dawn

The Dawn spacecraft mission has provided extensive new and detailed data on Vesta that confirm and strengthen the Vesta–howardite–eucrite–diogenite (HED) meteorite link and the concept that Vesta is differentiated, as derived from earlier telescopic observations. Here, we present results derived by newly calibrated spectra of Vesta. The comparison between data from the Dawn imaging spectrometer—VIR—and the different class of HED meteorites shows that average spectrum of Vesta resembles howardite spectra. Nevertheless, the Vesta spectra at high spatial resolution reveal variations in the distribution of HED‐like mineralogies on the asteroid. The data have been used to derive HED distribution on Vesta, reported in Ammannito et al. (2013), and to compute the average Vestan spectra of the different HED lithologies, reported here. The spectra indicate that, not only are all the different HED lithologies present on Vesta, but also carbonaceous chondritic material, which constitutes the most abundant inclusion type found in howardites, is widespread. However, the hydration feature used to identify carbonaceous chondrite material varies significantly on Vesta, revealing different band shapes. The characteristic of these hydration features cannot be explained solely by infalling of carbonaceous chondrite meteorites and other possible origins must be considered. The relative proportion of HEDs on Vesta's surface is computed, and results show that most of the vestan surface is compatible with eucrite‐rich howardites and/or cumulate or polymict eucrites. A very small percentage of surface is covered by diogenite, and basaltic eucrite terrains are relatively few compared with the abundance of basaltic eucrites in the HED suite. The largest abundance of diogenitic material is found in the Rheasilvia region, a deep basin, where it clearly occurs below a basaltic upper crust. However, diogenite is also found elsewhere; although the depth to diogenite is consistent with one magma ocean model, its lateral extent is not well constrained.     

The Berchtesgaden magnetic anomaly

A total intensity survey of the large magnetic anomaly in the Berchtesgaden area in the Northern Calcareous Alps has been carried out. The anomaly can be simulated by two-dimensional plate-like bodies having a NS extension of about 65 km, lower boundaries between 20 and 10 km and upper boundaries up to 5 km below the surface, depending on the intensity of magnetization (1–2 · 10^–3 G). A comparison with Bouguer gravity data favours a relatively thin magnetic source layer with a lower boundary at about 10 km and an upper boundary at about 5 km. From the magnetization required for modelling it can be concluded that mafic and possibly also ultramafic rocks are the causes of the anomalies. Two interpretations are suggested: 1) The magnetic source layers could form part of the metamorphic complex of the Bohemian Massif, belonging to a paleo-European plate. There would be probably no relation to the processes involved in the formation of the Alps in this case. 2) The magnetic source layers could be part of an ancient tethyan ocean floor, and should then be considered in relation to fossil subduction zones proposed in different models for the geodynamic evolution of the Eastern Alps.

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Zusammenfassung Im Gebiet der Berchtesgadener Anomalie wurde die Totalintensität des Erdmagnetfeldes gemessen. Die Anomalie hat in NS-Richtung eine Halbwertsbreite von rund 40 km. Sie kann durch zweidimensionale, plattenförmige magnetisierte Körper mit einer NS-Erstreckung von rund 65 km, einer unteren Begrenzung zwischen rund 20 und 10 km und einer oberen Begrenzung bis 5 km unter der Oberfläche simuliert werden. Die Bouguer-Anomalie zeigt, daß eine relativ dünne magnetisierte Schicht zwischen 5 und 10 km als wahrscheinlicher anzusehen ist. Aufgrund der zur Modellrechnung erforderlichen Magnetisierung (1–2 · 10^–3 G) müssen basische, zum Teil eventuell auch ultrabasische Gesteine als Ursache der Anomalie angenommen werden. Diese Gesteine könnten einerseits Teile des sich bis weit unter die Alpen erstreckenden Kristallins der Böhmischen Masse sein. Andererseits könnten sie Reste eines ehemaligen Ozeanbodens am Südrand der europäischen Platte sein. Sie sollten dann bei den Modellen für die geodynamische Entwicklung der Alpen, die fossile Subduktionszonen postulieren, in Betracht gezogen werden.

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Résumé Un levé de l'intensité totale du champ magnétique terrestre a été exécuté dans la région de l'anomalie magnétique de Berchtesgaden, dans les Alpes calcaires du Nord. La largeur de valeur moyenne de l'anomalie est d'environ 40 km dans la direction NS. Les calculs de modèles montrent que l'anomalie est causée par un corps magnétique en forme de plaque, ayant une extension NS d'environ 65 km, la limite inférieure pouvant varier entre 20 et 10 km et la limite supérieure pouvant atteindre 5 km. Une comparaison des effets gravimétriques des modèles magnétiques avec l'anomalie de Bouguer indique que les modèles relativement minces, entre 5 et 10 km, sont plus probables. L'aimantation requise pour les calculs (1 à 2 · 10^–3 G) montre que des roches basiques et peut-être en partie ultrabasiques doivent être considérées comme causes probables de l'anomalie. Deux interprétations alternatives sont proposées: 1) Les couches magnétiques peuvent appartenir au complexe métamorphique du Massif Bohémien. 2) Il s'agit d'un ancien fragment de croûte océanique. Dans ce cas il faudrait en tenir compte dans les modèles géodynamiques expliquant la formation des Alpes Orientales par la tectonique des plaques.

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Ground-based detection of terrestrial extrasolar planets by photometry: the case for CM Draconis

The detection of extrasolar planets by measuring a photometric drop in the stellar brightness due to a planetary transit can be statistically improved by observing eclipsing binary systems and photometrically improved by observing small component systems. In particular the system CM Draconis, with two dM4 components, would allow the detection of extrasolar planets in the size range of Earth-to-Neptune requiring a ground-based photometric precision of about 0.08% to 1.1% (photometric precision of about 0.3% is routinely achievable with 1-meter class telescopes at the magnitude of CM Draconis, 11.07 inR-filter). In addition, the transit of extrasolar planets in a binary star system provides a unique, quasi-periodic signal that can be cross-correlated with the observational data to detect sub-noise signals. We examine the importance of making such observations to an understanding of the formation and evolution of terrestrial-type planets in main-sequence star systems. Terrestrial planets could have formed with substancially shorter periods in this lower luminosity system, for example, and might be expected to have accreted essentially in the binary orbital plane (however, non-coplanar planets may also eventually be detectable due to precession). We also report on a network of medium-sized telescopes at varying longitudes that have been organized to provide such constraints on terrestrial-planet formation processes and discuss the extention of near-term observations to other possible binary systems, as well. Finally, we discuss a more speculative, future observation that could be performed on the CM Draconis system that would be of exobiological as well as astrophysical interest.     

How did the Messinian Salinity Crisis end?

The cause of the desiccation of the Mediterranean Sea during the Messinian Salinity Crisis has been widely debated, but its re-flooding remains poorly investigated. Interpretations generally involve tectonic collapse of the Strait of Gibraltar or global sea-level rise, or even a combination of both. The dramatic sea-level fall in the Mediterranean has induced deep fluvial incision all around the desiccated basin. We investigate erosion dynamics related to this base level drop by using the numerical simulator EROS. We show that intense regressive erosion develops inevitably in the Gibraltar area eventually inducing the piracy of the Atlantic waters by an eastward-flowing stream and the subsequent re-flooding of the Mediterranean.