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.     

Differences between the impact regimes of the terrestrial planets: Implications for primordial D:H ratios

It is often assumed that the terrestrial worlds have experienced identical impact regimes over the course of their formation and evolution, and, as a result, would have started life with identical volatile budgets. In this work, through illustrative dynamical simulations of the impact flux on Venus, the Earth, and Mars, we show that these planets can actually experience greatly different rates of impact from objects injected from different reservoirs. For example, we show scenarios in which Mars experiences far more asteroidal impacts, per cometary impactor, than Venus, with the Earth being intermediate in value between the two. This difference is significant, and is apparent in simulations of both quiescent and highly stirred asteroid belts (such as could be produced by a mutual mean-motion resonance crossing between Jupiter and Saturn, as proposed in the Nice model of the Late Heavy Bombardment). We consider the effects; such differences would have on the initial volatilisation of the terrestrial planets in a variety of scenarios of both endogenous and exogenous hydration, with particular focus on the key question of the initial level of deuteration in each planet's water budget. We conclude that each of the terrestrial worlds will have experienced a significantly different distribution of impactors from various reservoirs, and that the assumption that each planet has the same initial volatile budget is, at the very least, a gross over-simplification.     

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.     

Intense fracturing and fracture sealing induced by mineral growth in porous rocks

Mineral precipitation in the pores of a rock may exert a force, which is called crystallization pressure. This process has been studied experimentally and results bring a new look on the way fractures may develop and seal in natural systems. Cylindrical core samples of porous limestone and sandstone were left for several weeks in contact with an aqueous solution saturated with sodium chloride, at 30 or 45 °C, under axial normal stress in the range 0.02–0.26 MPa. The fluid was allowed to rise in the core samples by capillary forces, up to a controlled height where evaporation and precipitation occurred. The uniaxial deformation of the samples was measured using high-resolution displacement sensors. The samples were characterized using computed X-ray tomography, allowing therefore imaging in 3D the intensity and localization of the damage. Two kinds of damage could be observed. Firstly, small rock fragments were peeled from the sample surface. Secondly, and more interestingly, fracture networks developed, by nucleation of microcracks at the interface where evaporation occurred, and propagation to the free surface. Two families of fractures could be identified. A first set of sealed fracture parallel to the evaporation front is directly induced by crystallization pressure. A second fracture network, perpendicular to the evaporation front, accommodates the first set of fractures. An analytical model where fluid flow is coupled to evaporation, vapour transport, and localization of mineral precipitation explains the shape of this fracture network.     

Troposphere-to-stratosphere transport in the tropics

The analysis of the data collected over Brazil, Northern Australia and Africa from balloons, high altitude aircraft and satellites during the recent HIBISCUS, TROCCINOX, SCOUT-O3 and AMMA European campaigns, has led to significant revision in the understanding of troposphere-to-stratosphere transport. Repeated observations of strong updrafts of adiabatically cooled and washed-out tropospheric air rich in chemical and greenhouse gases by convective overshooting over the three continents, demonstrate the high frequency of occurrence of such events in contrast to their generally assumed scarcity. Moreover, global scale information provided by ODIN and CALIPSO satellite observations suggests that the mechanism could play a major, if not dominant, role in troposphere-to-stratosphere transport in contrast to the generally evoked slow ascent by radiative heating. Ignored by global scale models because of their limited extension and duration, convective overshootings might have a significant impact on the chemistry and climate of the stratosphere.     

Multi-criteria spatialization of soil organic carbon sequestration potential from agricultural intensification in Senegal

On the eve of the 15th climate negotiations conference in Copenhagen, the pressure to assess all climate mitigation options is mounting. In this study, a bio-physic model and a socio-economic model were designed and coupled to assess the carbon sequestration potential of agricultural intensification in Senegal. The biophysical model is a multiple linear regression, calibrated and tested on a dataset of long-term agricultural trials established in West Africa. The socio-economic model integrates both financial and environmental costs related to considered practice changes. Both models are spatially explicit and the resulting spatial patterns were computed and displayed over Senegal with a geographic information system. The national potential from large-scale intensification was assessed at 0.65–0.83 MtC. With regards to local-scaled intensification as local projects, the most profitable areas were identified in agricultural expansion regions (especially Casamance), while the areas that meet the current financial additionality criteria of the Clean Development Mechanism were located in the northern part of the Peanut Basin. Using the current relevant mode of carbon valuation (Certified Emission Reductions), environmental benefits are small compared to financial benefits. This picture is radically changed if “avoided deforestation”, a likely consequence of agricultural intensification, is accounted for as the greenhouse gases sink capacity of projects increases by an average of a hundred-fold over Senegal.     

Physical modelling of gelifluction and frost creep: Some results of a large-scale laboratory experiment

An experimental slope was constructed in a 5 m × 5 m square refrigerated tank. The slope was formed of four sections, each consisting of regolith (soil) collected from a distinct bedrock lithology. The four lithologies utilized were granite, limestone, mudstone and slate. The slope was subjected to freezing and thawing from the surface downwards. Water was supplied at the base of the soil during freezing. Frost heaving and surface downslope soil movement were determined after each of 15 freezing cycles, and the profiles of soil movement with depth for each soil type were measured at the end of the 15th cycle. The experimental soils were non-cohesive; those derived from granite and limestone were respectively sandy and gravelly in texture, while those derived from mudstone and slate were silt-rich. Mass movement in the granite and limestone soils was due mainly to frost creep and was associated with the growth of needle ice. In the mudstone and slate soils, gelifluction was dominant as a result of high moisture contents caused by the melting of segregation ice. Mean per cycle rates of downslope soil transport for the granite, limestone, mudstone and slate soils were 5·8 cm³ cm^?1, 6·9 cm³ cm^?1, 21·2 cm³ cm^?1 and 31·2 cm³ cm^?1 respectively, units referring to the volume of soil passing a unit width of slope per cycle. Mass movement rates were shown to be strongly related to the silt content of the soils.     

Radiation-induced paramagnetic defects in natural kaolinites: Alpha dosimetry with ion beam irradiation

Three types of paramagnetic radiation-induced defects (RID), namely A, A′ (Si-O^?-centers) and B (Al-O^?-Al center), had been identified in natural kaolinites by means of electron paramagnetic resonance spectroscopy. The A-center, stable at the scale of geological periods, was thought to be of particular relevance to quantify past transits of radionuclides in the geosphere. Alpha radiation being likely the main source of RID in kaolinite, the objective of this paper is to define the role of α-particles on the creation of RID and to test the use of A-centers for an α-dosimetry. Three kaolinites with different crystalline order and containing other clays as impurities were irradiated with He⁺ ion beams. The radiation dose range (0–750 MGy) was consistent with natural radioactivity in environments from the Earth's surface. Irradiation drastically enhanced the original signals due to RID. An important increase of concentration of the unstable B-center, partly due to unrealistic dose rate provided by accelerator ion beam, was observed from the lower doses. The most stable defects remained of the Atype all along experimental irradiations. The contribution of ancillary phyllosilicates to EPR spectra was negligible. The concentration of the three types of RID was related to radiation doses up to 30 MGy. Dosimetry growth curves for the A-centers exhibited variable efficiencies and saturation levels that were related to the structural order and the chemical purity of the kaolinites: the more ordered and purer the kaolinite, the higher the efficiency and the lower the saturation plateau. Moreover, these results are of geochemical significance: dosimetry based on A-centers could be directly used to quantify past migrations of radioelements in the geosphere, by determination of the naturally-cumulated doses (paleodose) of kaolinites.     

Pressure-induced structural modifications in Mg2GeO4-olivine: A Raman spectroscopic study

Raman spectra of Mg₂GeO₄-olivine were obtained from ambient pressure up to 34 GPa at ambient temperature. Under quasi-hydrostatic pressure conditions, the following modifications in the Raman spectra occur as pressure increases: 1) near 11 GPa, two sharp extra bands appear in the 600–700 cm^?1 frequency range, and increase in intensity with respect to the olivine bands; 2) above 22 GPa, these two bands become very intense, and the number, position and relative intensity of the other vibrational bands drastically change; 3) the intensity of sharp bands progressively decreases above 25 GPa. The transformation occurs at lower pressures under non-hydrostatic conditions. During decompression to atmospheric pressure, the high-pressure phase partially reverts to olivine. These observations can be interpreted as the progressive metastable transformation from the olivine structure to a crystalline phase with four-fold coordinated Ge, in which the GeO₄ tetrahedra are polymerized. We propose that the metastable high-pressure phase is a structurally disordered spinelloid close to the hypothethical ω- or ?^*-phase, and forms by a shear mechanism assisted by the development of a dynamical instability in the olivine structure. Implications for the transformations undergone by olivines under far-from-equilibrium conditions (e.g. in subducting lithospheric slabs and in shocks) are discussed.