Modeling diffusion advection in the mass transfer of water vapor through martian regolith

Diffusion advection is an effect in diffusive multicomponent mass transfer that occurs when the flux vectors of the individual components do not add up to zero. This can be a significant effect for the mass transfer of water vapor from subsurface ice or liquid reservoirs through porous regolith at martian temperatures and pressures. Ignoring diffusion advection and using Fick's law alone to calculate the flux under these conditions will result in an erroneously small value while using a measured flux to calculate a diffusivity will result in an erroneously high value. The inaccuracy in both cases increases with temperature. The literature contains several examples of erroneous treatment of this effect. The correct approach is well-known from other applications of mass transfer and takes diffusion advection into account in the appropriate amount regardless of the temperature and pressure and reduces to the simple Fick's law when conditions warrant. In this way, there is no need to decide under what conditions diffusion advection is or is not important. It can be used in the transition region to pure Knudsen diffusion in a fashion similar to that used with the more limited Fickian approach.     

Rain infiltration and crust formation in the extreme arid zone of the Atacama Desert, Chile

A key question in understanding life on Mars under dry(ing) conditions is how arid soils respond to small levels of liquid water. We have conducted a series of simulated rain experiments in the hyperarid core region of the Atacama Desert. Rain amounts from 0.24 to 3.55 mm were applied in the early evening to the soil. We conclude that rain events of less than 1 mm do not saturate the surface, and the soil humidity at the surface remains below 100%. Rain events of 2 mm or more generate free water in the pore space of the soil surface, which may be necessary to support biological activity in the soil. The crust on the surface of the soil is a strong barrier to the diffusion of subsurface moisture and subsequent evaporation. Our results show that once the relative humidity in hyperarid soils begins to fall below 100% the rate of decrease is quite rapid. Thus, the precise value assumed for the limits of life or water activity, do not appreciably change the time of water availability resulting from small desert rains. The Atacama Desert results may be applied to models of (H₂O) wetting in the upper soils of Mars due to light rains, melting snow and heavy precipitating fog.     

Combined experimental and theoretical studies on methane photolysis at 121.6 and 248 nm—implications on a program of laboratory simulations of Titan’s atmosphere

Methane is, together with N₂, the main precursor of Titan’s atmospheric chemistry. In our laboratory, we are currently developing a program of laboratory simulations of Titan’s atmosphere, where methane is intended to be dissociated by multiphotonic photolysis at 248 nm. A preliminary study has shown that multiphotonic absorption of methane at 248 nm is efficient and leads to the production of hydrocarbons such as C₂H₂ (Romanzin et al., 2008). Yet, at this wavelength, little is known about the branching ratios of the hydrocarbon radicals (CH₃, CH₂ and CH) and their following photochemistry. This paper thus aims at investigating methane photochemistry at 248 nm by comparing the chemical evolution observed after irradiation of CH₄ at 248 and at 121.6 nm (Ly-α). It is indeed important to see if the chemistry is driven the same way at both wavelengths in particular because, on Titan, methane photolysis mainly involves Ly-α photons. An approach combining experiments and theoretical analysis by means of a specifically adapted 0-D model has thus been developed and is presented in this paper. The results obtained clearly indicate that the chemistry is different depending on the wavelength. They also suggest that at 248 nm, methane dissociation is in competition with ionisation, which could occur through a three-photon absorption process. As a consequence, 248 nm photolysis appears to be unsuitable to study methane neutral photochemistry alone. The implications of this result on our laboratory simulation program and new experimental developments are discussed. Additional information on methane photochemistry at 121.6 nm are also obtained.     

Stability of methane clathrate hydrates under pressure: Influence on outgassing processes of methane on Titan

We have conducted high-pressure experiments in the H₂O-CH₄ and H₂O-CH₄-NH₃ systems in order to investigate the stability of methane clathrate hydrates, with an optical sapphire-anvil cell coupled to a Raman spectrometer for sample characterization. The results obtained confirm that three factors determine the stability of methane clathrate hydrates: (1) the bulk methane content of the samples; (2) the presence of additional gas compounds such as nitrogen; (3) the concentration of ammonia in the aqueous solution. We show that ammonia has a strong effect on the stability of methane clathrates. For example, a 10 wt.% NH₃ solution decreases the dissociation temperature of methane clathrates by 14-25 K at pressures above 5 MPa. Then, we apply these new results to Titan’s conditions. Dissociation of methane clathrate hydrates and subsequent outgassing can only occur in Titan’s icy crust, in presence of locally large amounts of ammonia and in a warm context. We propose a model of cryomagma chamber within the crust that provides the required conditions for methane outgassing: emplacement of an ice plume triggers the melting (if solid) or heating (if liquid) of large ammonia-water pockets trapped at shallow depth, and the generated cryomagmas dissociate surrounding methane clathrate hydrates. We show that this model may allow for the outgassing of significant amounts of methane, which would be sufficient to maintain the presence of methane in Titan’s atmosphere for several tens of thousands of years after a large cryovolcanic event.     

Elastic collisions in ion irradiation experiments: A mechanism for space weathering of silicates

Ion irradiation experiments have been performed on silicates (bulk samples) rich of olivine, pyroxene, and serpentine to simulate the effects of space weathering induced on asteroids by solar wind ions. We have used different ions (H⁺, He⁺, Ar⁺, Ar²⁺) having different energies (from 60 to 400 keV) to weather the samples, probed by Raman spectroscopy and UV-vis-NIR reflectance spectroscopy. All the irradiated materials have shown reddening and darkening of reflectance spectra in the 0.25-2.7 μm spectral range. We have found that the increase of the spectral slope of the continuum across the 1-μm band is strongly related with the number of displacements caused by colliding ions because of elastic collisions with the target nuclei. The spectral slopes have been compared, at increasing ion fluence, with those from irradiated Epinal meteorite. We show that formation of nuclear displacements by solar wind ion irradiation is a physical mechanism that reddens the asteroidal surfaces on a time-scale lower than 10⁶ years.     

Structural modeling and testing of a concrete canoe

A finite element model is set up and experimental tests are performed to help understand the behavior of a concrete canoe and subsequently optimize its design. First, the performance criteria that must be satisfied to participate at the annual ASCE/Master Builders competition are described. Then, the finite element model and the different loading cases that were studied are presented. Results from these loading cases are discussed and used to optimize the material properties as well as the thickness of the hull and the dimensions of the gunwale and reinforcing ribs. Static and dynamic experimental tests were also conducted to validate the results of the finite element analyses. The results indicate that the main stresses and strains are caused by the static load cases. The additional stresses caused during races are small.     

Scale impacts on spatial variability in reference evapotranspiration

ABSTRACT

Evapotranspiration (ET) is one of the most important components in the hydrological cycle, and a key variable in hydrological modelling and water resources management. However, understanding the impacts of spatial variability in ET and the appropriate scale at which ET data should be incorporated into hydrological models, particularly at the regional scale, is often overlooked. This is in contrast to dealing with the spatial variability in rainfall data where existing guidance is widely available. This paper assesses the impacts of scale on the estimation of reference ET (ETo) by comparing data from individual weather stations against values derived from three national datasets, at varying resolutions. These include the UK Climate Impacts Programme 50 km climatology (UKCP₅₀), the UK Met Office 5 km climatology (UKMO₅) and the regional values published in the Agricultural Climate of England and Wales (ACEW). The national datasets were compared against the individual weather station data and the UKMO₅ was shown to provide the best estimate of ETo at a given site. The potential impacts on catchment modelling were then considered by mapping variance in ETo to show how geographical location and catchment size can have a major impact, with small lowland catchments having much higher variance than those with much larger areas or in the uplands. Some important implications for catchment hydrological modelling are highlighted.

Editor D. Koutsoyiannis; Associate editor L. Ruiz     

Water balance components estimation under scenarios of land cover change in the Vea catchment,West Africa

ABSTRACT

The need for a detailed investigation of the Vea catchment water balance components cannot be overemphasized due to its accelerated land-cover dynamics and the associated impacts on the hydrological processes. This study assessed the possible consequences of land-use change scenarios (i.e. business as usual, BAU, and afforestation for the year 2025) compared to the 2016 baseline on the Vea catchment’s water balance components using the Soil and Water Assessment Tool (SWAT) model. The data used include daily climate and discharge, soil and land use/land cover maps. The results indicate that the mean annual water yield may increase by 9.1% under the BAU scenario but decrease by 2.7% under the afforestation scenario; actual evapotranspiration would decrease under BAU but increase under afforestation; and groundwater recharge may increase under both scenarios but would be more pronounced under the afforestation scenario. These outcomes highlight the significance of land-cover dynamics in water resource management and planning at the catchment.     

岩石声学参数的实验测量及研究

通过对天然岩芯及人工岩芯的测量,研究２４０ｋＨｚ-１．ＳＭＨｚ频段内的声频散、温度对岩石声速的影响、岩石含水饱和度对岩石声速和声衰减的影响,给出岩石中声衰减与声波频率间关系的最佳拟合,并讨论了裂缝对岩石中声波传播的影响。     

鄱阳湖流域农业生态环境变迁及其启示

为研究农业开发与生态环境的关系,总结不同历史时期鄱阳湖流域农业生态环境特征及其演变特征.其总体特征反映在森林、土地、水、生物资源和生态环境建设方面.生态环境变迁历史对该流域农业发展具有重要的启示.