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171.
D. N. Arnepalli S. Shanthakumar B. Hanumantha Rao D. N. Singh 《Geotechnical and Geological Engineering》2008,26(2):121-132
Characteristics of fine-grained soils primarily depend on their specific-surface area and hence, reliable determination of
this parameter is essential. In this context, researchers have employed quite sophisticated instruments (viz., a BET surface
area analyzer, the mercury intrusion porosimetry, internal reflectance spectroscopy, X-Ray diffraction and gas pycnometer
etc.) and methodologies (viz., sorption of Methylene Blue dye, Ethylene Glycol Monoethyl Ether and p-Nitrophenol) to determine specific-surface area of these soils. However, most of these methodologies are found to be quite
tedious, cost and time intensive. Apart from this, the results obtained are contentious due to the inherent limitations associated
with either the instruments employed or the basic assumptions made for computing the specific-surface area of the soil. Hence,
it becomes mandatory to evaluate the efficiency of these methodologies for determining specific-surface area of fine-grained
soils. With this in view, different types of soils were considered in this study and their specific-surface area was determined,
by following different methodologies, and the results were evaluated critically. In addition, attempts were made to develop
relationships between the basic properties of fine-grained soils (viz., liquid limit, cation-exchange capacity, activity,
and free swell index) and the specific-surface area. These relationships will be of immense help to the practicing engineers
and research fraternity. 相似文献
172.
Deborah L. Domingue Scott L. Murchie Brett W. Denevi Nancy L. Chabot David T. Blewett Nori R. Laslo Robin M. Vaughan Hong K. Kang Michael K. Shepard 《Planetary and Space Science》2011,59(15):1873-1887
During the third flyby of Mercury by the MESSENGER spacecraft, a dedicated disk-integrated photometric sequence was acquired with the wide-angle multispectral camera to observe Mercury's global photometric behavior in 11 spectral filters over as broad a range of phase angle as possible within the geometric constraints of the flyby. Extraction of disk-integrated measurements from images acquired during this sequence required careful accounting for scattered light and residual background effects. The photometric model fit to these measurements is shown to fit observed radiances at phase angles below 110°, possibly except where both solar incidence and emission angles are high (>70°). The complexity of the scattered light at wavelengths greater than 828 nm contributes to a less accurate photometric correction at these wavelengths. The model is used to correct the global imaging data set acquired at a variety of geometries to a common geometry of incidence angle=30°, emission angle=0°, and phase angle=30°, yielding a relatively seamless mosaic. The results here will be used to correct image mosaics of Mercury acquired in orbit. 相似文献
173.
Robert G. Strom Maria E. Banks Clark R. Chapman Caleb I. Fassett Jeffrey A. Forde James W. Head III William J. Merline Louise M. Prockter Sean C. Solomon 《Planetary and Space Science》2011,59(15):1960-1967
The primary crater population on Mercury has been modified by volcanism and secondary craters. Two phases of volcanism are recognized. One volcanic episode that produced widespread intercrater plains occurred during the period of the Late Heavy Bombardment and markedly altered the surface in many areas. The second episode is typified by the smooth plains interior and exterior to the Caloris basin, both of which have a different crater size-frequency distribution than the intercrater plains, consistent with a cratering record dominated by a younger population of impactors. These two phases may have overlapped as parts of a continuous period of volcanism during which the volcanic flux tended to decrease with time. The youngest age of smooth plains volcanism cannot yet be determined, but at least small expanses of plains are substantially younger than the plains associated with the Caloris basin. The spatial and temporal variations of volcanic resurfacing events can be used to reconstruct Mercury's geologic history from images and compositional and topographic data to be acquired during the orbital phase of the MESSENGER mission. 相似文献
174.
We employ Mariner 10 measurements of the interplanetary magnetic field in the vicinity of Mercury to estimate the rate of magnetic reconnection between the interplanetary magnetic field and the Hermean magnetosphere. We derive a time-series of the open magnetic flux in Mercury's magnetosphere, from which we can deduce the length of the magnetotail. The length of the magnetotail is shown to be highly variable, with open field lines stretching between 15RH and 850RH downstream of the planet (median 150RH). Scaling laws allow the tail length at perihelion to be deduced from the aphelion Mariner 10 observations. 相似文献
175.
Jürgen Oberst Stephan Elgner F. Scott Turner Mark E. Perry Robert W. Gaskell Maria T. Zuber Mark S. Robinson Sean C. Solomon 《Planetary and Space Science》2011,59(15):1918-1924
Analysis of images obtained by the MESSENGER spacecraft during its three flybys of Mercury yields a new estimate for the planet's mean radius of 2439.25±0.69 km, in agreement with results from Mariner 10 and Earth-based observations, as well as with MESSENGER altimeter and occultation data. The mean equatorial radius and polar radius are identical to within error, suggesting that rotational oblateness is negligible when compared with other sources of topography. This result is consistent with the small gravitational oblateness of the planet. Minor differences in radius obtained at different locations reflect regional variations in topography. Residual topography along three limb profiles has a dynamic range of 7.4 km and a root-mean-square roughness of 0.8 km over hemispherical scales. Following MESSENGER's entry into orbit about Mercury in March 2011, we expect considerable additional improvements to our knowledge of Mercury's size and shape. 相似文献
176.
Jiangning Lü Youshun Sun M. Nafi Toksöz Yingcai Zheng Maria T. Zuber 《Planetary and Space Science》2011,59(15):1981-1991
Striking geological features on Mercury's surface have been linked to tectonic disruption associated with the Caloris impact and have the potential to provide information on the interior structure of Mercury. The unusual disrupted terrain located directly at the antipode of the 1500-km-diameter Caloris basin could have plausibly formed as a consequence of focused seismic waves generated by the massive impact event. In this paper, we revisit the antipodal seismic focusing effects of the Caloris impact by developing physically consistent structure models for Mercury and parameterized seismic source models for the Caloris impact. If the focused seismic body waves caused the disrupted terrain, then the amplitudes of the waves and the areal extent of surface disruptions could be used for estimating the seismic energy imparted by the impact.In this study, we show that effects of direct body waves are small relative to those of focused guided waves. Two types of guided waves are produced by the Caloris impact. One is the conventional Rayleigh wave generated by the impact. The second is the mantle guided waves trapped between the core and the free surface. Mantle guided waves, not recognized in previous studies, may have played an important role in the creation of the disrupted terrain. We find that the early core state has only moderate effects on the antipodal response to the Caloris impact. The fact that the zone of predicted disruption for both fluid and solid core cases is smaller than the observed region of chaotic terrain suggests either that the antipodal response to the Caloris impact was modulated by the shallow structure of Mercury, or that the energy imparted by the impact was larger than those used in this study. 相似文献
177.
Jim M. Raines James A. Slavin Thomas H. Zurbuchen George Gloeckler Brian J. Anderson Daniel N. Baker Haje Korth Stamatios M. Krimigis Ralph L. McNutt Jr 《Planetary and Space Science》2011,59(15):2004-2015
The MESSENGER Fast Imaging Plasma Spectrometer (FIPS) measured the bulk plasma characteristics of Mercury's magnetosphere and solar wind environment during the spacecraft's first two flybys of the planet on 14 January 2008 (M1) and 6 October 2008 (M2), producing the first measurements of thermal ions in Mercury's magnetosphere. In this work, we identify major features of the Mercury magnetosphere in the FIPS proton data and describe the data analysis process used for recovery of proton density (np) and temperature (Tp) with a forward modeling technique, required because of limitations in measurement geometry. We focus on three regions where the magnetospheric flow speed is likely to be low and meets our criteria for the recovery process: the M1 plasma sheet and the M1 and M2 dayside and nightside boundary-layer regions. Interplanetary magnetic field (IMF) conditions were substantially different between the two flybys, with intense reconnection signatures observed by the Magnetometer during M2 versus a relatively quiet magnetosphere during M1. The recovered ion density and temperature values for the M1 quiet-time plasma sheet yielded np∼1–10 cm−3, Tp∼2×106 K, and plasma β∼2. The nightside boundary-layer proton densities during M1 and M2 were similar, at np∼4–5 cm−3, but the temperature during M1 (Tp∼4–8×106 K) was 50% less than during M2 (Tp∼8×106 K), presumably due to reconnection in the tail. The dayside boundary layer observed during M1 had a density of ∼16 cm−3 and temperature of 2×106 K, whereas during M2 this region was less dense and hotter (np∼8 cm−3 and Tp∼10×106 K), again, most likely due to magnetopause reconnection. Overall, the southward interplanetary magnetic field during M2 clearly produced higher Tp in the dayside and nightside magnetosphere, as well as higher plasma β in the nightside boundary, ∼20 during M2 compared with ∼2 during M1. The proton plasma pressure accounts for only a fraction (24% for M1 and 64% for M2) of the drop in magnetic pressure upon entry into the dayside boundary layer. This result suggests that heavy ions of planetary origin, not considered in this analysis, may provide the “missing” pressure. If these planetary ions were hot due to “pickup” in the magnetosheath, the required density for pressure balance would be an ion density of ∼1 cm−3 for an ion temperature of ∼108 K. 相似文献
178.
Mikhail Yu. Zolotov 《Icarus》2011,212(1):24-114
The surface of Mercury contains ancient volcanic features and signs of pyroclastic activity related to unknown magmatic volatiles. Here, the nature of possible magmatic volatiles (H, S, C, Cl, and N) is discussed in the contexts of formation and evolution of the planet, composition and redox state of its mantle, solubility in silicate melts, chemical mechanisms of magma degassing, and thermochemical equilibria in magma and volcanic gases. The low-FeO content in surface materials (<6 wt%) evaluated with remote sensing methods corresponds to less than 2.3 fO2 log units below the iron-wüstite buffer. These redox conditions imply restricted involvement of hydrous species in nebular and accretion processes, and a limited loss of S, Cl, and N during formation and evolution of the planet. Reduced conditions correspond to high solubilities of these elements in magma and do not favor degassing. Major degassing and pyroclastic activity would require oxidation of melts in near-surface conditions. Low-pressure oxidation of graphite in moderately oxidized magmas causes formation of low-solubility CO. Decompression of reduced N-saturated melts involves oxidation of nitride melt complexes and could cause N2 degassing. Putative assimilation of oxide (FeO, TiO2, and SiO2) rich rocks in magma chambers could have caused major degassing through oxidation of graphite and S-, Cl- and N-bearing melt complexes. However, crustal rocks may never have been oxidized, and sulfides, graphite, chlorides, and nitrides could remain in crystallized rocks. Chemical equilibrium models show that N2, CO, S2, CS2, S2Cl, Cl, Cl2, and COS could be among the most abundant volcanic gases on Mercury. Though, these speciation models are restricted by uncertain redox conditions, unknown volatile content in magma, and the adopted chemical degassing mechanism. 相似文献
179.
Mercury levels of Audouin's gull are amongst the highest for Mediterranean seabirds, and have been mainly attributed to its piscivorous habits in these naturally Hg rich waters. Moreover, two additional factors could enhance its mercury intake: the consumption of discarded fish (which attain higher concentrations) and/or feeding in areas receiving Hg anthropogenic inputs. In order to differentiate the relevance of both sources we analysed Hg and stable isotopes of chick feathers from different breeding locations in western Mediterranean: one in its northern part (Ebro Delta) and two southern (Chafarinas Isl. and Alborán Isl.). The results from stable isotopes indicate that consumption of discards is higher at Alborán Isl., followed by the Ebro Delta and Chafarinas Isl. Thus, the higher mercury levels found in the Ebro Delta cannot be explained uniquely by the contribution of discarded fish to diet, but local pollution caused by the river Ebro waters accounts for Hg differences observed. 相似文献
180.
Mercury speciation was performed in excess activated sewage sludge (ASS) and in marine sediments collected at the AAS disposal site off the Mediterranean coast of Israel in order to characterize the spatial and vertical distribution of different mercury species and assess their environmental impact. Total Hg (HgT) concentrations ranged between 0.19 and 1003ng/g at the polluted stations and 5.7 and 72.8ng/g at the background station, while the average concentration in ASS was 1181+/-273ng/g. Only at the polluted stations did HgT concentrations decrease exponentially with sediment depth, reaching background values at 16-20cm, the vertical distribution resulting from mixing of natural sediment with ASS solids and bioturbation by large populations of polycheates. Average Methyl Hg (MeHg) concentration in ASS was 39.7+/-7.1ng/g, ca. 3% of the HgT concentration, while the background concentrations ranged between 0.1 and 0.61ng/g. MeHg concentrations in surficial polluted sediments were 0.7-5.9ng/g (ca. 0.5% of the HgT) and decreased vertically, similar to HgT. A positive correlation between MeHg and Hg only at the polluted stations, higher MeHg concentrations at the surface of the sediment and not below the redoxline, and no seasonality in the concentrations suggest that the MeHg originated from the ASS and not from in situ methylation. By doing selective extractions, we found that ca. 80% of the total Hg in ASS and polluted sediments was strongly bound to amorphous organo-sulfur and to inorganic sulfide species that are not bioavailable. The fractions with potential bioaccessible Hg had maximal concentrations in the range in which biotic effects should be expected. Therefore, although no bioaccumulation was found in the biota in the area, the concentration in the polluted sediments are not negligible and should be carefully monitored. 相似文献