Replacement of glass in the Nakhla meteorite by berthierine: Implications for understanding the origins of aluminum‐rich phyllosilicates on Mars

A scanning and transmission electron microscope study of aluminosilicate glasses within melt inclusions from the Martian meteorite Nakhla shows that they have been replaced by berthierine, an aluminum‐iron serpentine mineral. This alteration reaction was mediated by liquid water that gained access to the glasses along fractures within enclosing augite and olivine grains. Water/rock ratios were low, and the aqueous solutions were circumneutral and reducing. They introduced magnesium and iron that were sourced from the dissolution of olivine, and exported alkalis. Berthierine was identified using X‐ray microanalysis and electron diffraction. It is restricted in its occurrence to parts of the melt inclusions that were formerly glass, thus showing that under the ambient physico‐chemical conditions, the mobility of aluminum and silicon were low. This discovery of serpentine adds to the suite of postmagmatic hydrous silicates in Nakhla that include saponite and opal‐A. Such a variety of secondary silicates indicates that during aqueous alteration compositionally distinct microenvironments developed on sub‐millimeter length scales. The scarcity of berthierine in Nakhla is consistent with results from orbital remote sensing of the Martian crust showing very low abundances of aluminum‐rich phyllosilicates.     

Assessing water stress in Mediterranean lotic systems: insights from an artificially intermittent river in Greece

Water stress in Mediterranean countries is the result of both variable and changing climatic conditions and widespread anthropogenic pressures. Evrotas, an intermittent river located in Southern Greece, was used as a case study to assess the impacts of water stress on Mediterranean lotic ecosystems. Based on hydrological analyses, it was revealed that during prolonged drought years, such as the summers of 2007 and 2008, the vast majority of the Evrotas riverbed was completely desiccated, primarily as a result of substantial water abstraction for irrigation. The effects of desiccation on the riverine ecosystem were evaluated using fish and macroinvertebrate assemblages according to the demands of the Water Framework Directive 2000/60/EC (WFD). Faunal responses to water stress were assessed through comparisons of assemblages attributes in perennial and intermittent reaches and pre-drought versus post-drought communities. Effects of hydrological disturbance on fish species richness, density, percentage composition and size structure were more pronounced in intermittent than in perennial sites. The most obvious and immediate impact was the elimination of populations in the intermittent reaches. However, upon flow resumption, the recolonisation from upstream perennial reaches began, thereby permitting partial re-establishment of the depleted fish communities. Nevertheless, the structural integrity of fish communities remained severely impacted and recovery was markedly slow. On the contrary, post-drought macroinvertebrate assemblages were not affected by summer droughts, and the recruitment processes were rapid after flow resumption. Our findings point to the necessity of establishing a distinction between naturally and artificially driven intermittent rivers. We, therefore, propose the introduction of an “artificially intermittent Mediterranean river” condition within the context of the WFD assessment applications.     

Long-term variation of the semiannual amplitude in the aa index

The long-term variation of the semiannual amplitude in the geomagnetic activity index aa is analyzed with the purpose of contributing to the understanding of solar variability, directly linked to geomagnetic variability. The time series of the semiannual oscillation amplitude, obtained through a wavelet analysis of the daily aa series, presents a long-term variation similar to that shown by solar and geomagnetic indices, like aa itself or Dst. However, the maximum in the semiannual amplitude series occurs around 1947, almost 10 years before it occurs in solar and geomagnetic indices time series. The phase of the semiannual oscillation fluctuates around the values predicted by the equinoctial and Russell–McPherron models, with a predominance of the equinoctial mechanism during the period of maximum semiannual amplitude. A possible source of changes in the equinoctial mechanism would be the secular variation of the Earth's dipole tilt. But, since it does not follow the semiannual amplitude trend, at first sight, it seems not to be responsible for the equinoctial predominance around 1947. The analysis of quiet and disturbed days separately indicates that only disturbed days present the semiannual annual amplitude maximum around 1947, so the 10 year time shift could be due to the mechanism responsible for the semiannual variation in geomagnetically active periods.     

RC buildings retrofitted by converting frame bays into RC walls

Adding concrete walls by infilling certain frame bays with reinforced concrete is popular for seismic retrofitting, but is covered by codes only if the connection of the old concrete to the new ensures monolithic behavior. To avoid penalizing the foundation of the new wall with a very high moment resistance, the new concrete should not be thicker than, or surround, the old frame members. A cost-effective connection of these members to a thin new web is proposed, alongside a design procedure and detailing that conform to current codes. Owing to practical difficulties, footings of added walls are often small and weakly connected to the other footings, hence they uplift and rock during the earthquake. The model for uplift of 3D footings consists of two pairs of nonlinear-elastic springs in a cross layout and approximates also moderate nonlinearities in the soil continuum. It is used in nonlinear, static or dynamic, analyses of three buildings with added walls. The analyses of a clean, regular 4-story building show the benefit from uplift to the added walls and a certain adverse effect on some columns but not on beams, as well as the lack of a clear positive effect of tie-beams. The application to a 7-story and a 2-story real building with extreme, yet typical, irregularities in plan and elevation exemplifies the real-life restrictions in the use of added walls and shows their limits for the improvement of seismic performance; certain deficiencies in flexure or shear remain in both and are corrected at very low cost with local fiber reinforced polymer (FRP) jackets without new analysis of the building, as FRPs do not change the member effective stiffness or moment resistance.     

X-ray emission from dwarf galaxies: IC 2574 revisited

Until recently, few unequivocal detections had been reported of the hot, X-ray emitting gas thought to be associated with the large, coherent structures variously described as supershells or superbubbles in dwarf irregular (dIrr) galaxies. In this contribution we report follow-up XMM-Newton and Chandra observations of our ROSAT detection of X-ray emission associated with the supergiant shell in the nearby dIrr galaxy IC 2574, a member of the M 81 group of galaxies. The spectral properties of the X-ray source suggest that we are dealing with a young (age < 2000 yr) supernova remnant (SNR). This SNR is most likely one of the many supernovae which have exploded in that region and which have created the impressive supergiant HI shell. This revised version was published online in August 2006 with corrections to the Cover Date.     

On-Going Galaxy Formation

We investigate the process of galaxy formation as can be observed in the only currently forming galaxies - the so-called Tidal Dwarf Galaxies, hereafter TDGs - through observations of the molecular gas detected via its CO (Carbon Monoxide) emission. These objects are formed of material torn off of the outer parts of a spiral disk due to tidal forces in a collision between two massive galaxies. Molecular gas is a key element in the galaxy formation process, providing the link between a cloud of gas and a bona fide galaxy. We have detected CO in 8 TDGs (Braine, Lisenfeld, Duc and Leon, 2000: Nature 403, 867; Braine, Duc, Lisenfeld, Charmandaris, Vallejo, Leon and Brinks: 2001, A&A 378, 51), with an overall detection rate of 80%, showing that molecular gas is abundant in TDGs, up to a few 10⁸ M _⊙. The CO emission coincides both spatially and kinematically with the HI emission, indicating that the molecular gas forms from the atomic hydrogen where the HI column density is high. A possible trend of more evolved TDGs having greater molecular gas masses is observed, in accord with the transformation of HI into H₂. Although TDGs share many of the properties of small irregulars, their CO luminosity is much greater (factor ∼ 100) than that of standard dwarf galaxies of comparable luminosity. This is most likely a consequence of the higher metallicity (≳sim 1/3 solar) of TDGs which makes CO a good tracer of molecular gas. This allows us to study star formation in environments ordinarily inaccessible due to the extreme difficulty of measuring the molecular gas mass. The star formation efficiency, measured by the CO luminosity per Hα flux, is the same in TDGs and full-sized spirals. CO is likely the best tracer of the dynamics of these objects because some fraction of the HI near the TDGs may be part of the tidal tail and not bound to the TDG. Although uncertainties are large for individual objects, as the geometry is unknown, our sample is now of eight detected objects and we find that the ‘dynamical’ masses of TDGs, estimated from the CO line widths, seem not to be greater than the ‘visible’ masses (HI + H₂ + a stellar component). Although higher spatial resolution CO (and HI) observations would help reduce the uncertainties, we find that TDGs require no dark matter, which would make them the only galaxy-sized systems where this is the case. Dark matter in spirals should then be in a halo and not a rotating disk. Most dwarf galaxies are dark matter-rich, implying that they are not of tidal origin. We provide strong evidence that TDGs are self-gravitating entities, implying that we are witnessing the ensemble of processes in galaxy formation: concentration of large amounts of gas in a bound object, condensation of the gas, which is atomic at this point, to form molecular gas and the subsequent star formation from the dense molecular component. This revised version was published online in September 2006 with corrections to the Cover Date.     

Mass loss from galaxies: feeding the IGM, recycling in the IGM

As a result of internal processes or environmental effects like ram-pressure stripping or collisions, galaxies lose a significant part of their stellar and gaseous content. Whereas the impact of such stripping on galaxy evolution has been well studied, much less attention has been given to the fate of the expelled material in the intergalactic or intra cluster medium (IGM/ICM). Observational evidence exists showing that a fraction of the injected matter is actually recycled to form a new generation of galaxies, such as the Tidal Dwarf Galaxies discovered near numerous interacting systems. Using a set of multiwavelength data, we are now able to roughly analyze the processes pertaining to their formation: from an instability in the HI clouds, through the formation of molecular gas, and to the onset of star formation. This revised version was published online in September 2006 with corrections to the Cover Date.     

Spatial–temporal variations of water vapor content over Ethiopia: a study using GPS observations and the ECMWF model

GPS Solutions - We characterize the spatial–temporal variability of integrated water vapor (IWV) in Ethiopia from a network of global positioning system (GPS) stations and the European Center...     

Uncertainty quantification in numerical simulation of particle-laden flows

Numerical models can help to push forward the knowledge about complex dynamic physical systems. Modern approaches employ detailed mathematical models, taking into consideration inherent uncertainties on input parameters (phenomenological parameters or boundary and initial conditions, among others). Particle-laden flows are complex physical systems found in nature, generated due to the (possible small) spatial variation on the fluid density promoted by the carried particles. They are one of the main mechanisms responsible for the deposition of sediments on the seabed. A detailed understanding of particle-laden flows, often referred to as turbidity currents, helps geologists to understand the mechanisms that give rise to reservoirs, strategic in oil exploration. Uncertainty quantification (UQ) provides a rational framework to assist in this task, by combining sophisticated computational models with a probabilistic perspective in order to deepen the knowledge about the physics of the problem and to access the reliability of the results obtained with numerical simulations. This work presents a stochastic analysis of sediment deposition resulting from a turbidity current considering uncertainties on the initial sediment concentrations and particles settling velocities. The statistical moments of the deposition mapping, like other important features of the currents, are approximated by a Sparse Grid Stochastic Collocation method that employ a parallel flow solver for the solution of the deterministic problems associated to the grid points. The whole procedure is supported and steered by a scientific workflow management engine designed for high performance computer applications.