Mercury: the enigmatic innermost planet     

Sean C Solomon ^{Author Vitae} 《Earth and Planetary Science Letters》2003,216(4):441-455

The planet Mercury, a difficult object for study by astronomical observation and spacecraft exploration alike, poses multiple challenges to our general understanding of the inner planets. Mercury’s anomalously high uncompressed density implies a metal fraction of 60% or more by mass, an extreme outcome of planetary formational processes common to the inner solar system. Whether that outcome was the result of chemical gradients in the early solar nebula or removal by impact or vaporization of most of the silicate shell from a differentiated protoplanet can potentially be distinguished on the basis of the chemical composition of the present crust. Our understanding of the geological evolution of Mercury and how it fits within the known histories of the other terrestrial planets is restricted by the limited coverage and resolution of imaging by the only spacecraft to have visited the planet. The role of volcanism in Mercury’s geological history remains uncertain, and the dominant tectonic structures are lobate scarps interpreted as recording an extended episode of planetary contraction, issues that require global imaging to be fully examined. That Mercury has retained a global magnetic field when larger terrestrial planets have not stretches the limits of standard hydromagnetic dynamo theory and has led to proposals for a fossil field or for exotic dynamo scenarios. Hypotheses for field generation can be distinguished on the basis of the geometry of Mercury’s internal field, and the existence and size of a fluid outer core on Mercury can be ascertained from measurements of the planet’s spin axis orientation and gravity field and the amplitude of Mercury’s forced librations. The nature of Mercury’s polar deposits, suggested to consist of volatile material cold-trapped on the permanently shadowed floors of high-latitude impact craters, can be tested by remote sensing of the composition of Mercury’s surface and polar atmosphere. The extremely dynamic exosphere, which includes a number of species derived from Mercury’s surface, offers a novel laboratory for exploring the nature of the complex and changing interactions among the solar wind, a small magnetosphere, and a solid planet. Recent ground-based astronomical measurements and several new theoretical developments set the stage for the in-depth exploration of Mercury by two spacecraft missions within the coming decade.  相似文献   

Factors governing subaqueous siliceous sinter precipitation in hot springs: examples from Yellowstone National Park, USA   总被引：4，自引：0，他引：4  

Sean A. Guidry  Henry S. Chafetz 《Sedimentology》2002,49(6):1253-1267

Abstract Siliceous sinter precipitation within hot spring systems has been attributed to a variety of mechanisms: evaporative concentration, cooling, changes in pH and cation effects. Repetitive in situ (T, pH, alkalinity, etc.) and laboratory (major, minor and trace elemental, stable isotopic) analyses of the waters plus observations of silica precipitation on natural (e.g. twigs, pine cones) as well as artificial substrates (glass slides and copper plates) in the waters substantiate that subaqueous precipitation is occurring throughout the vent to distal end of flow in both Cistern Spring (Norris Geyser Basin) and Deerbone Spring (Lower Geyser Basin), Yellowstone National Park, Wyoming, USA. Quartz and sodium–potassium geothermometers indicate that Cistern Spring is fed by a subsurface reservoir that is between 232 and 272 °C. Calculated reservoir temperatures are significantly lower at Deerbone Spring (182–197 °C). Based on a suite of measured and theoretical saturation indices, downflow changes in the system resulting from evaporative concentration (e.g. Cl increases 10%), changes in pH (e.g. 5·6–7·1) and cation effects (Al and Fe) are of negligible importance in the subaqueous precipitation of hot spring opal‐A. Similarly, at the macroenvironmental scale, potential biotic effects on opal‐A precipitation appear to be minimal. Modelling of the two active siliceous sinter precipitating systems indicates that cooling (e.g. 80–17 °C) is the predominant process governing subaqueous mineral precipitation.  相似文献   

The etiquette of state-building and modernisation in dependent states: performing stateness and the normalisation of separate development in South Africa     

Peris Sean Jones 《Geoforum》2002,33(1):25-40

The paper reflects upon the continuing allure of state-building and modernisation in peripheral, `new' states and regions. The central argument is to suggest that our understanding of the conflicts surrounding `nation-building' can be improved upon by looking at one of the most powerful symbols of the neo-colonial, economic and institutional dependency and territorial artificiality of the `nation-state'- the South African Bantustan. As enforced state-hood projects, their `newness' and illegality in the eyes of the international community suggest that the Bantustans provide a particularly striking evocation of both the limits of state-building and, above all, the significance of recognition for `newer' states. Efforts to overturn the illegal status and non-recognition of one `independent' Bantustan, namely Bophuthatswana, therefore, illustrate what are widely held to be the modern attributes and evolutionary etiquette (`growth', `building', `development') that need to be `performed' in order to legitimise or normalise a state internationally. To be given a `voice' internationally, states are seemingly required to enact state-hood and sovereignty through a range of normalising rituals associated with state-building. In this case- study, strategies promoting neo-liberal development policies and also clandestine diplomatic and economic manoeuvres were believed by the regime to deliver international recognition. Directed at locating Bophuthatswana within its place in the world, these strenuous efforts also show how crude territorial `divide and rule' tactics alone were not responsible for underpinning the apartheid project. Rather, spaces were provided for discourses and practices of state-led `national' development in the Bantustan periphery. Thought to deliver normality for the renegade Bophuthatswana state, these international and local policies exacerbated and induced further a range of crises at the heart of the Bantustan strategy and apartheid modernisation more generally. These debilitating, and in Bophuthatswana's case, disastrous, tensions between the internal and external realms of state-building point to a more general need to rethink alternatives both to neo-liberalism as well as the rituals associated with state- building.  相似文献   

Effectiveness of vertical drains in mitigation of liquefaction     

A. J. Brennan  S. P. G. Madabhushi 《Soil Dynamics and Earthquake Engineering》2002,22(9-12):1059-1065

One method of mitigating the damaging effects of earthquake-induced liquefaction is to provide rapid dissipation of excess pore pressures by the use of vertical drains through the liquefiable material. Drain systems are currently designed using a chart-based approach. Field experience suggests that the performance of these installations cannot yet be accurately predicted.

In this paper, high quality centrifuge testing is used to help clarify drain behaviour. It will be established, supported by centrifuge test data, that the pore water from a radially expanding zone of soil contributing to drainage through the drains is developed. Naturally, the geometry of this expanding zone changes with time. It will be shown that fluid from deeper strata is drained first, reducing the effectiveness of the drain for near-surface soil layers. It is concluded that these zones are useful in analysing more complicated drain system geometries.  相似文献   

Entrainment of Glaciomarine Sediments and Formation of Thrust-Block Moraines at the Margin of Sørsdale Glacier,East Antarctica     

Sean J. Fitzsimons 《地球表面变化过程与地形》1997,22(2):175-187

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Chert provenance analysis at the Mack Canyon Site,Sherman County,Oregon: An evaluative study     

Sean C. Hess 《Geoarchaeology》1996,11(1):51-81

Chert is the most frequent lithic raw material in the Columbia Plateau, but archaeologists have made few attempts to link chert artifacts to their quarries in this area. The Mack Canyon Site in north-central Oregon is near a chert source, providing an opportunity to compare provenance analysis techniques. Macroscopic and microscopic visual attributes and element concentrations were determined for 26 samples from six sources in the Columbia Plateau. Additional concentration data from two other sources were included for comparison. Four artifacts were selected from the Mack Canyon Site for comparison to the nearby outcrop. Although all of the artifacts were macroscopically indistinguishable from the local material, microscopic attributes and element concentrations indicate that only one of the artifacts came from the outcrop. Linking chert artifacts and quarries through statistical analysis of element concentrations holds promise for the Columbia Plateau. © 1996 John Wiley & Sons, Inc.  相似文献   

Effectiveness of landscape‐based green infrastructure for stormwater management in suburban catchments          下载免费PDF全文

Sean A. Woznicki  Kelly L. Hondula  S. Taylor Jarnagin 《水文研究》2018,32(15):2346-2361

Land cover changes associated with urbanization have negative effects on downstream ecosystems. Contemporary urban development attempts to mitigate these effects by designing stormwater infrastructure to mimic predevelopment hydrology, but their performance is highly variable. This study used in situ monitoring of recently built neighbourhoods to evaluate the catchment‐scale effectiveness of landscape decentralized stormwater control measures (SCMs) in the form of street connected vegetated swales for reducing runoff volumes and flow rates relative to curb‐and‐gutter infrastructure. Effectiveness of the SCMs was quantified by monitoring runoff for 8 months at the outlets of 4 suburban catchments (0.76–5.25 ha) in Maryland, USA. Three “grey” catchments installed curb‐and‐gutter stormwater conveyances, whereas the fourth “green” catchment built parcel‐level vegetated swales. The catchment with decentralized SCMs reduced runoff, runoff ratio, and peak runoff compared with the grey infrastructure catchments. In addition, the green catchment delayed runoff, resulting in longer precipitation–runoff lag times. Runoff ratios across the monitoring period were 0.13 at the green catchment and 0.37, 0.35, and 0.18 at the 3 grey catchments. Runoff only commenced after 6 mm of precipitation at the decentralized SCM catchment, whereas runoff occurred even during the smallest events at the grey catchments. However, as precipitation magnitudes reached 20 mm, the green catchment runoff characteristics were similar to those at the grey catchments, which made up 37% of the total precipitation in only 10 of 72 events. Therefore, volume‐based reduction goals for stormwater using decentralized SCMs such as vegetated swales require additional redundant SCMs in a treatment train as source control and/or end‐of‐pipe detention to capture a larger fraction of runoff and more effectively mimic predevelopment hydrology for the relatively rare but larger precipitation events.  相似文献   

Proportion of Sewage Sludge to Soil Influences the Survival of Salmonella Dublin and Escherichia coli          下载免费PDF全文

Stephanie Ellis  Sean Tyrrel  Emma O'Leary  Karl Richards  Bryan Griffiths  Karl Ritz 《洁净——土壤、空气、水》2018,46(4)

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Investigating a time‐shift extended imaging condition in a Kirchhoff pre‐stack depth migration algorithm          下载免费PDF全文

Gareth Shane O'Brien  Sean Joseph Delaney  Michael Igoe  John Doherty  Andrew Colhoun 《Geophysical Prospecting》2018,66(4):688-706

Extracting true amplitude versus angle common image gathers is one of the key objectives in seismic processing and imaging. This is achievable to different degrees using different migration techniques (e.g., Kirchhoff, wavefield extrapolation, and reverse time migration techniques) and is a common tool in exploration, but the costs can vary depending on the selected migration algorithm and the desired accuracy. Here, we investigate the possibility of combining the local‐shift imaging condition, specifically the time‐shift extended imaging condition, for angle gathers with a Kirchhoff migration. The aims are not to replace the more accurate full‐wavefield migration but to offer a cheaper alternative where ray‐based methods are applicable and to use Kirchhoff time‐lag common image gathers to help bridge the gap between the traditional offset common image gathers and reverse time migration angle gathers; finally, given the higher level of summation inside the extended imaging migration, we wish to understand the impact on the amplitude versus angle response. The implementation of the time‐shift imaging condition along with the computational cost is discussed, and results of four different datasets are presented. The four example datasets, two synthetic, one land acquisition, and a marine dataset, have been migrated using a Kirchhoff offset method, a Kirchhoff time‐shift method, and, for comparison, a reverse time migration algorithm. The results show that the time‐shift imaging condition at zero time lag is equivalent to the full offset stack as expected. The output gathers are cleaner and more consistent in the time‐lag‐derived angle gathers, but the conversion from time lag to angle can be considered a post‐processing step. The main difference arises in the amplitude versus offset/angle distribution where the responses are different and dramatically so for the land data. The results from the synthetics and real data show that a Kirchhoff migration with an extended imaging condition is capable of generating subsurface angle gathers. The same disadvantages with a ray‐based approach will apply using the extended imaging condition relative to a wave equation angle gather solution. Nevertheless, using this approach allows one to explore the relationship between the velocity model and focusing of the reflected energy, to use the Radon transformation to remove noise and multiples, and to generate consistent products from a ray‐based migration and a full‐wave equation migration, which can then be interchanged depending on the process under study.  相似文献   

Development of push moraines in deeply frozen sediment adjacent to a cold-based glacier in the McMurdo Dry Valleys,Antarctica     

Sean Fitzsimons  Jamie Howarth 《地球表面变化过程与地形》2020,45(3):622-637

Previous studies of push moraines have concluded that structural architecture is controlled by a combination of stress exerted by flowing ice and the rheology of the deforming sediment. However, the relationship between deformation processes and the thermal/hydrologic conditions within the sediment remains ambiguous. Using a combination of surface mapping, logging exposures and ground penetrating radar we examine the relationships between surface morphology, material properties and development of deformation structures in deeply frozen sediment that has been deformed by Joyce Glacier, a cold-based glacier in the McMurdo Dry Valleys. The structural architecture of the push moraines is characterized by a combination of brittle and ductile deformation structures that have produced a structurally complex pattern of thrust faults, low angle listric thrusts and recumbent folds that extend 400 m beyond the glacier margin. Deformation is ductile where the ice concentration exceeds c. 65% volume and predominantly brittle where the ice concentration is less than c. 65%. The change in rheology reflects transition in behaviour from the material having a predominantly frictional character when the ice is limited to pore spaces to a non-frictional character in which strength is primarily determined by the cohesive strength of the ice. This work shows that glaciotectonic deformation can occur in deeply frozen permafrost where there is no liquid pore water. We conclude that the presence of liquid porewater is not a necessary condition for the development of glaciotectonic deformation or for the formation of push moraines. © 2019 John Wiley & Sons, Ltd.  相似文献