Albedo control of seasonal South Polar cap recession on Mars

Over the last few decades, General Circulation Models (GCM) have been used to simulate the current martian climate. The calibration of these GCMs with the current seasonal cycle is a crucial step in understanding the climate history of Mars. One of the main climatic signals currently used to validate GCMs is the annual atmospheric pressure cycle. It is difficult to use changes in seasonal deposits on the surface of Mars to calibrate the GCMs given the spectral ambiguities between CO₂ and H₂O ice in the visible range. With the OMEGA imaging spectrometer covering the near infra-red range, it is now possible to monitor both types of ice at a spatial resolution of about 1 km. At global scale, we determine the change with time of the Seasonal South Polar Cap (SSPC) crocus line, defining the edge of CO₂ deposits. This crocus line is not symmetric around the geographic South Pole. At local scale, we introduce the snowdrop distance, describing the local structure of the SSPC edge. Crocus line and snowdrop distance changes can now be used to calibrate GCMs. The albedo of the seasonal deposits is usually assumed to be a uniform and constant parameter of the GCMs. In this study, albedo is found to be the main parameter controlling the SSPC recession at both global and local scale. Using a defrost mass balance model (referred to as D-frost) that incorporates the effect of shadowing induced by topography, we show that the global SSPC asymmetry in the crocus line is controlled by albedo variations. At local scale, we show that the snowdrop distance is correlated with the albedo variability. Further GCM improvements should take into account these two results. We propose several possibilities for the origin of the asymmetric albedo control. The next step will be to identify and model the physical processes that create the albedo differences.     

OMEGA long wavelength channel: Data reduction during non-nominal stages

The Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) instrument is a visible and near-infrared imaging spectrometer on board the European Mars Express (MEx) mission. The on-board calibration (OBC) performed at the beginning of observations on each orbit reveals that the photometric response of the C channel (1.0–2.5 μm) has been very stable since orbit insertion in January 2004. On the contrary the L channel (2.5–5.1 μm) response has varied significantly during the mission, and only orbits for which the response is close to nominal could be used with confidence. The spatial coverage of ice-free surfaces in this wavelength range is consequently limited to only ～30%, mainly during northern spring and summer. This paper presents the empirical method used to derive new instrumental transfer functions (ITF) for the non-nominal orbits. This method consists of analyzing the variation of the signal between several observations of a same region acquired at nominal and non-nominal calibration states. In the cases where the mineralogy and the atmospheric conditions between the two observations are the same, the variation in reflectance spectra is only due to the ITF variation, which provides a new ITF. We then associate these new ITFs with their corresponding OBCs to model a relationship between both. The resulting model enables us to provide a new ITF for each orbit for which the OBC is available. The new ITFs derived for the entire dataset have been validated (1) through a comparison of the C and L channel global albedo trends and (2) through a comparison of the surface temperatures derived from the L channel with those calculated from the General Circulation Model (GCM) numerical simulation of the LMD released in the Martian Climate Database. The non-nominal data processed with adapted ITFs for orbits up to 3050 increase the non-icy surface coverage of Mars to ～70% including all seasons.     

Time evolution of the mineralogical composition of Mississippi Valley loess over the last 10 kyr: Climate and geochemical modeling

Anthropogenic and natural climate change affect processes in the atmosphere, biosphere, hydrosphere, and pedosphere. The impact of climate on soil evolution has not been well-explored, largely due to slow rates and the complexity of coupled processes that must be observed and simulated. The rates of mineral weathering in loess deposited 23 kyr ago and experiencing soil formation for 13 kyr are explored here using the WITCH model for weathering and the GENESIS model for climate simulation. The WITCH model, which uses rigorous kinetic parameters and laws with provision for the effect on rates of deviation from equilibrium, can successfully simulate the depletion profiles in the soil for dolomite and albite if soil CO₂ is assumed to rise over the last 10 kyr up to about 30-40× the present atmospheric pressure, and if the solubility product of the Ca-smectite is assumed equal to that of an Fe(III)-rich Ca-montmorillonite. Such simulations document that dissolution behavior for silicates and carbonates are strongly coupled through pH, and for Ca-smectite and feldspars through dissolved silica. Such coupling is not incorporated in simple geometric and analytical models describing mineral dissolution, and therefore probably contributes to the long-standing observation of discrepancies among laboratory and field mineral dissolution rates.     

Synthetic stellar and SSP libraries as templates for Gaia simulations

ESA’s Gaia mission will collect low resolution spectroscopy in the optical range for ～10⁹ objects. Complete and up-to-date libraries of synthetic stellar spectra are needed to built algorithms aimed to automatically derive the classification and the parametrization of this huge amount of data. In addition, libraries of stellar spectra are one of the main ingredients of stellar population synthesis models, aiming to derive the properties of unresolved stellar populations from their integrated light. We present (a) the newly computed libraries of synthetic spectra built by the Gaia community, covering the whole optical range (300–1100 nm) at medium-high resolution of (0.3 nm) for stars spanning the most different types, from M to O, from A-peculiar to Emission lines to White Dwarfs, and (b) the implementation of those libraries in our SSP code (Tantalo in The Initial Mass Function 50 Years Later, 327:235 2005), exploring different stellar evolution models.     

An improved methodology for filling missing values in spatiotemporal climate data set

In this paper, an improved methodology for the determination of missing values in a spatiotemporal database is presented. This methodology performs denoising projection in order to accurately fill the missing values in the database. The improved methodology is called empirical orthogonal functions (EOF) pruning, and it is based on an original linear projection method called empirical orthogonal functions (EOF). The experiments demonstrate the performance of the improved methodology and present a comparison with the original EOF and with a widely used optimal interpolation method called objective analysis.     

Physical properties of fault zones within a granite body: Example of the Soultz-sous-Forêts geothermal site

In EGS projects, fault zones are considered as the structures controlling deep flow at the reservoir scale. Using a large set of petrophysical properties (porosity, density, permeability, thermal conductivity [TC]) measured on cores collected along the EPS-1 borehole, a model of fault zone is proposed to describe them. A fault zone is a complex structure, showing different parts with different kinds of deformations and/or materials that could explain chemical and physical processes observed during fluid-rock interactions. The different parts composing the fault zone are: (1) the fault core or gauge zone; (2) the damage zone; (3) and the protolith. They are usually heterogeneous and show different physical properties. The damage zone is a potential high permeability channel and could become the main pathway for fluids if secondary minerals seal the fault core. Porosity is the lowest within the protolith, between 0.5 and 1%, but can go up to 15% in the fault zone. Permeability ranges from 10^?20 m² in the fresh granite to, at least, 10^?15 m² in the fault core, and TC ranges from 2.5 W K^?1m^?1 to 3.7 W K^?1m^?1. Finally, variations in specific surface are set over two orders of magnitude. If the lowest values usually characterize the fresh granite far from fault zones, physical properties could show variations spread over their whole respective ranges within these fault zones.     

Associations of very high energy gamma-ray sources discovered by H.E.S.S. with pulsar wind nebulae

The H.E.S.S. array of imaging Cherenkov telescopes has discovered a number of previously unknown γ-ray sources in the very high energy (VHE) domain above 100 GeV. The good angular resolution of H.E.S.S. (∼0.1° per event), as well as its sensitivity (a few percent of the Crab Nebula flux) and wide 5° field of view, allow a much better constrained search for counterparts in comparison to previous instruments. In several cases, the association of the VHE sources revealed by H.E.S.S. with pulsar wind nebulae (PWNe) is supported by a combination of positional and morphological evidence, multi-wavelength observations, and plausible PWN model parameters. These include the plerions in the composite supernova remnants G 0.9+0.1 and MSH 15–52, the recently discovered Vela X nebula, two new sources in the Kookaburra complex, and the association of HESS J1825–137 with PSR B1823–13. The properties of these better-established associations are reviewed. A number of other sources discovered by H.E.S.S. are located near high spin-down power pulsars, but the evidence for association is less complete. These possible associations are also discussed, in the context of the available multi-wavelength data and plausible PWN scenarios. For the H.E.S.S. Collaboration     

Towards an understanding of the Of?p star HD 191612: optical spectroscopy

We present extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f?pe–O8fp). The Balmer and He  i lines show strongly variable emission which is highly reproducible on a well-determined 538-d period. He  ii absorptions and metal lines (including many selective emission lines but excluding He  ii λ4686 Å emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with   P _orb= 1542 d, e = 0.45  . We conduct a model-atmosphere analysis of the spectrum, and find that the system is consistent with a ∼O8 giant with a ∼B1 main-sequence secondary. Since the periodic 538-d changes are unrelated to orbital motion, rotational modulation of a magnetically constrained plasma is strongly favoured as the most likely underlying 'clock'. An upper limit on the equatorial rotation is consistent with this hypothesis, but is too weak to provide a strong constraint.     

Experimental and numerical behaviour of reinforced mortar plates subjected to accelerated corrosion

Most reinforced concrete structures are damaged due to corrosion of reinforcements in concrete. In normal conditions the pH near the reinforcements is around 12–13 which means that steel is in a passive state. But aggressive species, such as chloride ions or carbon dioxide, may penetrate into concrete and promote active corrosion. As a consequence (hydro)oxides are produced leading to degradation of concrete structures. For instance cracking of the concrete is generated due to the pressure induced by rust. In this paper, we study the inception and the propagation of cracking on reinforced mortar plates with rebars located either in the middle or at the corner. Additional experiments have been performed on cylindrical specimens to determine the local effect of rust pressure at the interface rust/mortar. The specimens have been subjected to imposed current density in order to enhance the corrosion and digital image intercorrelation has been used to determine displacement fields. The experiments have been compared to numerical modelling. Copyright © 2010 John Wiley & Sons, Ltd.     

Lake‐level fluctuations at Lake Bourget (eastern France) around 4500–3500 cal. a BP and their palaeoclimatic and archaeological implications

On the basis of sedimentological analysis of two cores taken at Chatillon, Lake Le Bourget (northern French Pre‐Alps), and well dated by radiocarbon dates in addition to tree ring dates obtained from an archaeological layer, this paper presents a high‐resolution lake‐level record for the period 4500–3500 cal. a BP. The collected data provide evidence of a complex palaeohydrological (climatic) oscillation spanning the ca. 4300–3850 cal. BP time interval, with major lake‐level maxima at ca. 4200 and 4050–3850 cal. a BP separated by a lowering episode around 4100 cal. a BP. The lake‐level highstands observed at Chatillon between 4300 and 3850 cal. BP appear to be synchronous with (i) a major flooding period recorded in deep cores from the large lakes Le Bourget and Bodensee, and (ii) glacier advance and tree line decline in the Alps. Such wetter and cooler climatic conditions in west‐central Europe around 4000 cal. a BP may have been a nonlinear response to decrease and seasonal changes in insolation. They may also provide a possible explanation for the general abandonment of prehistoric lake dwellings north of the Alps between 4360 and 3750 cal. a BP. Copyright © 2012 John Wiley & Sons, Ltd.