The role of collisions with interplanetary particles in the physical evolution of comets     

Julio A. Fernández 《Earth, Moon, and Planets》1981,25(4):507-519

Effects of collisions with interplanetary particles are investigated. To this purpose, collision probabilities for comets with different orbital elements are computed. It is found that collisions may have a non-negligible effect on the physical evolution of comets. In this connection, it is shown that under certain conditions collisional lifetimes may be shorter than dynamical or vaporization lifetimes. In particular, collisional lifetimes are on average shorter for comets in retrograde orbits than those for direct ones. It is further suggested that catastrophic collisions may contribute to prevent long-period comets in retrograde orbits from reaching short-period orbits by orbital diffusion. Collisions may also produce irregularities of the nucleus brightness by leaving exposed regions of fresh volatile material and may in this way lead to a rejuvenation of old dusty short-period comets. Catastrophic collision probabilities are too low to account for the observed comet splittings, so other trigger mechanisms should be at work. However, it is shown that collisional mini-bursts (increases in brightness of one magnitude or so) caused by decimeter-sized bodies may occur rather frequently on short-period comets when they pass through the asteroid belt. The burst observed in comet Tempel-2 at 3 AU in December, 1978 could be an example of such collisional mini-bursts. The systematic observation of periodic comets when they pass through the asteroid belt could give valuable information about the spatial density of decimeter and meter-sized bodies. In particular, collisional effects for comet Halley, for which a continuous surveillance is planned, are evaluated.  相似文献   

Climate variability in the Spanish Pyrenees during the last 30,000 yr revealed by the El Portalet sequence     

P. González-Sampériz  B.L. Valero-Garcés  G. Jalut  C. Martí-Bono  A. Navas  J.J. Dedoubat 《Quaternary Research》2006,66(1):38-52

Palynological, sedimentological and stable isotopic analyses of carbonates and organic matter performed on the El Portalet sequence (1802 m a.s.l., 42°48′00?N, 0°23′52?W) reflect the paleoclimatic evolution and vegetation history in the central-western Spanish Pyrenees over the last 30,000 yr, and provide a high-resolution record for the late glacial period. Our results confirm previous observations that deglaciation occurred earlier in the Pyrenees than in northern European and Alpine sites and point to a glacial readvance from 22,500 to 18,000 cal yr BP, coinciding with the global last glacial maximum. The patterns shown by the new, high-resolution pollen data from this continental sequence, chronologically constrained by 13 AMS ¹⁴C dates, seem to correlate with the rapid climate changes recorded in Greenland ice cores during the last glacial-interglacial transition. Abrupt events observed in northern latitudes (Heinrich events 3 to 1, Oldest and Older Dryas stades, Intra-Allerød Cold Period, and 8200 cal yr BP event) were also identified for the first time in a lacustrine sequence from the central-western Pyrenees as cold and arid periods. The coherent response of the vegetation and the lake system to abrupt climate changes implies an efficient translation of climate variability from the North Atlantic to mid latitudes.  相似文献   

Study of geothermal water intrusion due to groundwater exploitation in the Puebla Valley aquifer system, Mexico   总被引：1，自引：1，他引：1  

E. Leticia Flores-Márquez  Gabriel Jiménez-Suárez  Raymundo G. Martínez-Serrano  René E. Chávez  Daniel Silva Pérez 《Hydrogeology Journal》2006,14(7):1216-1230

Significant intrusion of geothermal water into fresh groundwater takes place in the Puebla Valley aquifer system, Mexico. The decline in the potentiometric surface due to the overexploitation of the groundwater induces this intrusion. This hydrological system comprises three aquifers located in Plio-Quaternary volcanic sediments and Mesozoic calcareous rocks. The hydraulic balance of the aquifer shows that the annual output exceeds the natural inputs by 12 million m³. Between 1973 and 2002, a drop in the potentiometric surface, with an 80 m cone of depression, was identified in a 5-km-wide area located southwest of the city of Puebla. Chemical analyses performed on water samples since 1990 have shown an increase in total dissolved solids (TDS) of more than 500 mg/L, coinciding with the region showing a cone of depression in the potentiometric surface. A three-dimensional flow and transport model, based on the hydrogeological and geophysical studies, was computed by using the MODFLOW and MT3D software. This model reproduces the evolution of the aquifer system during the last 30 years and predicts for 2010 an additional drawdown in the potentiometric surface of 15 m, and an increase in the geothermal water intrusion.  相似文献   

A Practical Procedure for the Measurement of Fragmentation by Blasting by Image Analysis   总被引：1，自引：0，他引：1  

J. A. Sanchidrián  P. Segarra  L. M. López 《Rock Mechanics and Rock Engineering》2006,39(4):359-382

Summary. The operation of a digital image analysis system in a limestone quarry is described. The calibration of the system, required in order to obtain moderately reliable fragmentation values, is done from muckpile sieving data by tuning the image analysis software settings so that the fragmentation curve obtained matches as close as possible the sieving. The sieving data have also been used to extend the fragment size distribution curves measured to sizes below the system’s optical resolution and to process the results in terms of fragmented rock, discounting the material coming from a loose overburden (natural fines) that is cast together with the fragmented rock. Automatic and manual operation modes of the image analysis are compared. The total fragmentation of a blast is obtained from the analysis of twenty photographs; a criterion for the elimination of outlier photographs has been adopted using a robust statistic. The limitations of the measurement system due to sampling, image processing and fines corrections are discussed and the errors estimated whenever possible. An analysis of consistency of the results based on the known amount of natural fines is made. Blasts with large differences in the amount of fines require a differentiated treatment, as the fine sizes tend to be the more underestimated in the image analysis as they become more abundant; this has been accomplished by means of a variable fines adjustment factor. Despite of the unavoidable errors and the large dispersion always associated with large-scale rock blasting data, the system is sensitive to relative changes in fragmentation.  相似文献   

Alteration, evaluation and use of extremaduran granite residues     

C. Albarrán-Liso  M. M. Jordán-Vidal  T. Sanfeliu-Montolio  M. J. Liso-Rubio 《Environmental Geology》2006,49(7):1043-1050

The necessity of eliminating debris from a granite quarry has awakened an interest in applications of by-products, called “marginal arids”, in different fields, like construction and foundations for roadways, restoration, material for the manufacture of artificial rocks, and artesian products etc. Conclusions obtained from the results of tests carried out by X-ray diffraction of granite quarry by-products in Extremadura, Spain, submitted to different treatments, are established. Test pieces from two quarries are analyzed and compared generally and specifically, for commercial use. Finally, conclusions relating to essays in test pieces and mineral dynamics of marginal arid granite are exposed.  相似文献   

Risk assessment of groundwater pollution using sensitivity analysis and a worst-case scenario analysis   总被引：5，自引：0，他引：5  

Marijke Huysmans  Tamás Madarász  Alain Dassargues 《Environmental Geology》2006,50(2):180-193

This paper illustrates how sensitivity analysis and a worst-case scenario analysis can be useful tools in risk assessment of groundwater pollution. The approach is applied to a study area in Hungary with several known groundwater pollution sources and nearby drinking water production wells. The main concern is whether the contamination sources threaten the drinking water wells of the area. A groundwater flow and transport model is set up to answer this question. Due to limited data availability, the results of this model are associated with large uncertainty. Sensitivity analysis and a worst-case scenario analysis are applied to estimate this uncertainty and build confidence in the model results.  相似文献   

P–T evolution and reaction textures in retrogressed eclogites from Svetlik, the Moldanubian Zone (Czech Republic)     

S. W. Faryad  M. Perraki  S. Vrána 《Mineralogy and Petrology》2006,88(1-2):297-319

Summary Lenses and pods of mafic rocks from the Monotonous Unit near Svetlik are characterized by eclogite facies mineral assemblages; however some inclusion patterns (oriented quartz rods in clinopyroxene and cuboids of disordered graphite in garnet) that are usually known from ultra-high pressure rocks were also observed in one sample. Conventional thermobarometry yielded maximum P–T conditions of 2.0–2.5 GPa and 750 °C. Decompression and heating at amphibolite and granulite facies conditions resulted in the formation of at least five distinct types of symplectites. These include symplectitic intergrowth of ilmenite and clinopyroxene after titanite, described here for the first time from the Moldanubian Zone. In addition, symplectites of plagioclase and biotite with accessory amounts of spinel after tabular pseudomorphs (after phengite?) are also reported here. Mass balance relations indicate that symplectites of diopside + plagioclase after omphacite and plagioclase + spinel (sapphirine) after kyanite + garnet, formed by nearly isochemical reactions. All other symplectite-forming reactions were allochemical and were accelerated by the presence of fluid in the primary phases. Preserved zoning pattern in garnet with high compositional gradient in some samples suggests that the rocks were affected briefly by granulite facies overprint.  相似文献   

Reconstructing hydrological variability from testate amoebae analysis in Carpathian peatlands     

C. Schnitchen  D. J. Charman  E. Magyari  M. Braun  I. Grigorszky  B. Tóthmérész  M. Molnár  Zs. Szántó 《Journal of Paleolimnology》2006,36(1):1-17

Peatlands offer the potential for high resolution records of water balance over Holocene timescales, yet this potential is under-exploited in many areas of the world. Within Europe, peatlands are mostly confined to areas north of 55° N, but several areas of southern and eastern Europe contain small peatlands which may be suitable for palaeoclimatic reconstruction. In this paper we test the potential of peatlands in the Carpathian region for deriving quantified estimates of water table changes using testate amoebae analysis. A training set for palaeohydrological reconstruction from testate amoebae assemblages was obtained by collecting surface samples from 13 peatlands, including 9 from Hungary and 4 from Transylvania (Romania). Using a simple measure of mean annual water tables estimated from staining of PVC tape, we found that some peatlands were heavily influenced by runoff and groundwater, and were therefore not suitable as modern analogues of ombrotrophic climatically sensitive sites. The relationship between the testate amoebae assemblages in the modern samples and the environmental variables was explored using CCA. The CCA biplot showed that the most important variables are depth to water table and moisture content, confirming that hydrology is a key control on taxon distribution. pH was a secondary gradient. A transfer function for % moisture and depth to water table was established and applied to fossil assemblages from a sequence from Fenyves-tető, Transylvania, Romania. The reconstructed water table shows a number of variations which have parallels with other palaeoclimatic records from Europe and the North Atlantic prominent phases of higher water tables are associated with the periods 8000–8300 cal BP, 3000–2500 cal BP and after 600 cal BP. We suggest that these were periods of particular intensification of westerly airflow which affected eastern Europe as well as western and central Europe.  相似文献   

The effect of thermal treatment on the 57Fe Mössbauer spectrum of beryl     

Geraldo Magela da Costa  Gabriel de Oliveira Polli  Márcio A. Kahwage  Eddy de Grave  Antônio Claret Soares Sabioni  Júlio Cesar Mendes 《Physics and Chemistry of Minerals》2006,33(3):161-166

Mössbauer spectra (MS) of blue, green and yellow beryl (ideally Be₃Al₂Si₆O₁₈) containing approximately 1% of iron were obtained at 295 and 500 K. Room temperature (RT) spectra of both blue and green samples showed the presence of an asymmetric Fe²⁺ doublet (ΔE _Q~2.7 mm/s, δ~1.1 mm/s), with a very broad low-velocity peak. There is no clear evidence for the presence of a ferric component. The MS of the yellow sample at RT consists of an intense central absorption with parameters typical for Fe³⁺ (ΔE _Q~0.4 mm/s, δ~0.29 mm/s), plus an apparently symmetrical Fe²⁺ doublet. This sample acquires a light-blue shade upon heating in air at about 620 K. Thermal treatments at high temperatures caused no significant changes in the MS, but the green and yellow beryl acquire a blue colour. All these results are interpreted in relation to the existence of channel water and the distribution of iron among the available crystallographic sites.  相似文献   

Environmental and health risk assessment in abandoned mining area, Zlata Idka, Slovakia   总被引：1，自引：0，他引：1  

S. Rapant  Z. Dietzová  S. Cicmanová 《Environmental Geology》2006,51(3):387-397

The Zlata Idka village is a typical mountainous settlement. As a consequence of more than 500 years of mining activity, its environment has been extensively affected by pollution from potentially toxic elements. This paper presents the results of an environmental-geochemical and health research in the Zlata Idka village, Slovakia. Geochemical analysis indicates that arsenic (As) and antimony (Sb) are enriched in soils, groundwater, surface water and stream sediments. The average As and Sb contents are 892 mg/kg and 818 mg/kg in soils, 195 mg/kg and 249 mg/kg in stream sediments, 0.028 mg/l and 0.021 mg/l in groundwater and 0.024 mg/l and 0.034 mg/l in surface water. Arsenic and Sb concentrations exceed upper permissible limits in locally grown vegetables. Within the epidemiological research the As and Sb contents in human tissues and fluids have been observed (blood, urine, nails and hair) in approximately one third of the village’s population (120 respondents). The average As and Sb concentrations were 16.3 μg/l and 3.8 μg/l in blood, 15.8 μg/l and 18.8 μg/l in urine, 3,179 μg/kg and 1,140 μg/kg in nails and 379 μg/kg and 357 μg/kg in hair. These concentrations are comparatively much higher than the average population. Health risk calculations for the ingestion of soil, water, and vegetables indicates a very high carcinogenic risk (>1/1,000) for as content in soil and water. The hazard quotient [HQ=average daily dose (ADD)/reference dose (RfD)] calculation method indicates a HQ>1 for groundwater As and Sb concentrations.  相似文献