Cycloid crack sequences on Europa: Relationship to stress history and constraints on growth mechanics based on cusp angles

The original model developed to explain cycloidal cracks on Europa interprets cycloids as tensile fractures that grow in a curved path in response to the constantly rotating diurnal tidal stress field. Cusps form when a new cycloid crack segment propagates at an angle to the first in response to a rotation of the principal tidal stress orientation during a period of no crack growth. A recent revised model states that a cycloid cusp forms through the creation of a secondary fracture called a tailcrack at the tip of an existing cycloid segment during shearing motion induced by the rotating tidal stress field. As the tailcrack propagates away from the cusp, it becomes the next cycloid segment in the chain. The qualitative tailcrack model uniquely accounts for the normal and shear stresses that mechanically must resolve onto the tip of an existing cycloid segment at the instant of cusp formation. In this work, we provide a quantitative framework and test of the hitherto purely conceptual tailcrack model. We first present a relative age sequence inferred from geologic mapping of multiply cross-cutting cycloids in Europa's trailing hemisphere and place this into the context of the global stress history. The age sequence requires a cumulative minimum of 630° of shell reorientation due to nonsynchronous rotation to account for the observed range of orientations of cycloids of different ages. We determined the back-rotated longitudes of formation of two cycloid chain examples and used mathematical modeling of europan tidal stresses to show that the tailcrack model for cusp formation is not only viable, but places constraints on the overall development of a cycloid chain by controlling the timing of cusp development within Europa's orbit. For all cusps analyzed, the exact ratio of resolved shear to normal stress required to form the cusp angles by a process of tailcracking, as governed by the principles of linear elastic fracture mechanics, is produced at the tip of a shearing cycloid segment during Europa's orbit. Cusp formation occurs after the point in the orbit at which the maximum tensile principal tidal stress occurs, implying that tensile tidal stresses are not directly responsible for cusp development. Instead, cusps develop when a tailcrack forms at the tip of a cycloid segment in response to the highly perturbed stress field induced during concomitant opening and shearing at the tip of the cycloid segment.     

Stability of ice on Mars and the water vapor diurnal cycle: Experimental study of the sublimation of ice through a fine-grained basaltic regolith

In order to advance our understanding of the long-term stability of subsurface ice, the diurnal martian water cycle, and implications for liquid water, we determined diffusion coefficients and adsorption kinetics for the water vapor produced by the sublimation of ice buried beneath various layers of fine-grained (<63, 63-125, and 125-250 μm) basaltic powder under simulated martian conditions. Sublimation rates at shallower depths, <10 mm, were determined to be affected by mass transfer through the atmosphere in addition to the basalt layer. For greater depths, the measured diffusion coefficients for water vapor moving through basalt grains were 1.56±0.53×¹⁰⁻⁴, 2.05±0.82×¹⁰⁻⁴, and for the <63, 63-125, and 125-250 μm basaltic layers, respectively. Through the Brunauer, Emmett and Teller (BET) isotherm, which assumes multiple molecular layers of adsorbed water, we determined the adsorption constants of 52.6±8.3 at 270 K for <63 μm, 39.0±6.4 at 267 K for 63-125 μm, and 54.3±9.3 at 266 K for 125-250 μm, resulting in surface areas of 2.6±0.1×¹⁰⁴, 1.7±0.3×¹⁰⁴, , respectively. These results suggest that while diffusion is too rapid to explain the purported diurnal cycle in water content of the atmosphere, adsorption is efficient and rapid, and does provide an effective mechanism to explain such a cycle. The present diffusion data suggest that very thin, <50 pr μm, shallow, 10 mm, ice deposits would last for >10 h at ∼224 K, just above the freezing point of saturated CaCl₂. Temperatures can remain above ∼224 K over most of the planet, which means that water, even as saturated brine, will sublimate before the freezing point is reached and liquid could be formed. On the other hand, 1 m ice layers below 1 m of fine-grained basaltic regolith at 235 K and 10 Pa of atmospheric water could last 600 to 1300 years. At deeper depths and lower temperatures, ice could last since the last major obliquity change 400,000 years ago.     

Titan's surface from the Cassini RADAR radiometry data during SAR mode

We present initial results on the calibration and interpretation of the high-resolution radiometry data acquired during the Synthetic Aperture Radar (SAR) mode (SAR-radiometry) of the Cassini Radar Mapper during its first five flybys of Saturn's moon Titan.We construct maps of the brightness temperature at the 2-cm wavelength coincident with SAR swath imaging. A preliminary radiometry calibration shows that brightness temperature in these maps varies from 64 to 89 K. Surface features and physical properties derived from the SAR-radiometry maps and SAR imaging are strongly correlated; in general, we find that surface features with high radar reflectivity are associated with radiometrically cold regions, while surface features with low radar reflectivity correlate with radiometrically warm regions. We examined scatterplots of the normalized radar cross-section σ⁰ versus brightness temperature, outlining signatures that characterize various terrains and surface features. The results indicate that volume scattering is important in many areas of Titan's surface, particularly Xanadu, while other areas exhibit complex brightness temperature variations consistent with variable slopes or surface material and compositional properties.     

Experimental seismic investigation of Sefid‐rud concrete buttress dam model on shaking table

Owing to the devastating M7.6 earthquake of 20 June 1990 that occurred in the northern province of Iran, Sefid‐rud concrete buttress dam located near the epicenter was severely shaken. The crack penetrated throughout the dam thickness near slope discontinuity, causing severe leakage, but with no general failure. In this study, nonlinear seismic response of the highest monolith with empty reservoir is investigated experimentally through model testing. A geometric‐scaled model of 1:30 was tested on a shaking table with high‐frequency capability to study dynamic cracking of the model and serve as data for nonlinear computer model calibration. Three construction joints are set up in the model to simulate effects of construction aspects. The experimental results are then compared with smeared crack and damage mechanics finite‐element simulations using nonlinear concrete constitutive models based on fracture mechanics. The crack patterns obtained from numerical models are in good agreement with those obtained from shaking table tests for the case of including construction joint effects and rigid foundation. Copyright © 2008 John Wiley & Sons, Ltd.     

Dynamic properties and liquefaction resistance of two soil materials in an earthfill dam—Laboratory test results

Results are presented of laboratory resonant column and cyclic triaxial tests on specimens of two compacted soils (a sandy–silty clay and a sand–gravel mixture), planned to be used in the core and the shells, respectively, of a proposed earthfill dam. The values of low-amplitude shear modulus of the clayey material were found to increase with increasing confining pressure and decreasing water content, with deviations of ±20% from the predictions of the “Hardin equation”. On the other hand, the low-amplitude damping ratio was found to be at least four times higher than the values corresponding to natural undisturbed cohesive soils. The proposed G/G₀−γ_c curve for the compacted cohesive soil was found to be independent of confining pressure and small variations of the water content on either side of the optimum value and showed a remarkable agreement with recently published similar curves for natural cohesive soils. For the case of the sand–gravel mixture, normalized G/G₀–γ_c and D–γ_c curves are proposed, based on recently published results for gravelly soils and the limited data of the present study. The liquefaction resistance of the saturated sand–gravel mixture was found to be strongly dependent on its relative density, especially for high values of cyclic stress ratio. A relative density of at least 55% was found to be necessary to assure safety against earthquake-induced liquefaction of the material. The results presented herein may be used (directly or as guide) in the seismic analysis of (new or existing) earth dams constructed from similar soil materials and in addition they provide insight into the dynamic behavior of compacted soils.     

Magnetic properties of sediments from the Pearl River Delta,South China: Paleoenvironmental implications

Magnetic parameters and their environmental implications of sediments in a core (PD) from the Pearl River Delta, South China, indicate that ferrimagnetic minerals with low coercivity, such as magnetite, dominate the magnetic properties although small amounts of Fe-sulphides occur. The fraction of Fe-sulphides increases and becomes the dominant minerals determining the magnetic characteristics in grey-black organic-rich clay horizons, indicating an anoxic, sulphate-reducing swamp environment resulting from a marine regression. In the "Huaban clay" , hard magnetic minerals, such as hematite and goethite, largely control the magnetic properties of the sediments and imply a long period of exposure and weathering. Where magnetite is the main magnetic mineral, its fraction and grain size determine properties such as magnetic susceptibility (κ ) and saturation isothermal remanent magnetization (SIRM). Ratios of SIRM/κ and χarm/SIRM reflect changes in sea level with high SIRM/κ and χarm/SIRM correlating with a smaller magnetic mineral grain size and rising sea level. Based on downcore variations of these environmental magnetic parameters along with sediment characteristics and microfauna, the sedimentary environment of the Pearl River Delta area can be divided into two main cycles of transgression and regression during the late Pleistocene and Holocene with more sub-cycles of sea level fluctuation during each transgression.     

腾格里沙漠南缘沙漠化逆转过程中的土壤物理性质变化特征

在腾格里沙漠南缘,应用空间代替时间的方法,选择了流动沙丘、封育5 a、封育15 a和封育25 a的恢复序列,研究了沙漠化逆转过程中的土壤物理性质变化特征。结果表明,腾格里沙漠南缘沙漠化逆转过程中,土壤含水率先降低后增大;土壤沙粒含量逐渐减少,土壤粉粒和粘粒含量逐渐增加;土壤容重逐渐降低,土壤孔隙度逐渐增大,表层土壤容重在封育恢复过程中逐渐减小,表层土壤孔隙度在封育恢复15 a后趋于稳定。研究认为,在腾格里沙漠南缘的沙漠化逆转过程中,沙化土地土壤性状不断提高、土壤结构不断改善;沙漠化逆转过程也是土壤的形成过程。     

The porosity and engineering properties of vesicular basalt in Saudi Arabia

The presence of non-connected vesicles in extensive basalt flows in western Saudi Arabia greatly affects their engineering properties such as compressive strength, modulus of elasticity, Poisson's ratio and sonic velocities. In order to evaluate some of these properties, the density and/or the porosity of the material must be known. Using conventional methods to determine the porosity or the density of irregular lumps proved to be erroneous. An image analysis technique is suggested to estimate the porosity of the vesicular basalt. The estimated values were correlated against the calculated porosity values using core samples, and were found to have excellent correlation (R=0.99). The estimated porosity values were also correlated against the static and dynamic properties of the basalt and a good level of correlation (R=0.77) was obtained.     

渤海湾盆地晚古生代煤系烃源岩及其生烃性评价

在对渤海湾盆地晚古生代煤系烃源岩特征进行研究的基础上,分别从烃源岩的主要生烃组分、有机质类型及生烃门限等方面对其生烃性进行了评价,认为太原组烃夺生烃性要好于山西组;渤海地晚古生代煤系烃源岩不但具有良好的生烃物质基础,较高的生烃能力和热成熟条件,而且具有良好的天然气勘探前景。