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1.
Abstract— The possibility of volcanism on Mercury has been a topic of discussion since Mariner 10 returned images of half the planet's surface showing widespread plains material. These plains could be volcanic or lobate crater ejecta. An assessment of the mechanics of the ascent and eruption of magma shows that it is possible to have widespread volcanism, no volcanism on the surface whatsoever, or some range in between. It is difficult to distinguish between a lava flow and lobate crater ejecta based on morphology and morphometry. No definite volcanic features have been identified on Mercury. However, known lunar volcanic features cannot be identified in images with similar resolutions and viewing geometries as the Mariner 10 dataset. Examination of high‐resolution, low Sun angle Mariner 10 images reveals several features which are interpreted to be flow fronts; it is unclear if these are volcanic flows or ejecta flows. This analysis implies that a clear assessment of volcanism on Mercury must wait for better data. MESSENGER (MErcury: Surface, Space ENvironment, GEochemistry, Ranging) will take images with viewing geometries and resolutions appropriate for the identification of such features.  相似文献   

2.
B.W. Denevi  M.S. Robinson 《Icarus》2008,197(1):239-246
Mariner 10 clear filter (490 nm) images of Mercury were recalibrated and photometrically normalized to produce a mosaic of nearly an entire hemisphere of the planet. Albedo contrasts are slightly larger than seen in the lunar highlands (excluding maria). Variegations indicative of compositional differences include diffuse low albedo units often overlain by smooth plains, the high albedo smooth plains of Borealis Planitia, and high-albedo enigmatic crater floor deposits. A higher level of contrast between immature crater ejecta and average mature material on Mercury compared to the Moon is consistent with a more intense space weathering environment on Mercury that results in a more mature regolith. Immature lunar highlands materials are ∼1.5 times higher in reflectance than analogous immature mercurian materials. Immature materials of the same composition would have the same reflectance on both bodies, thus this observation requires that Mercury's crust contains a significant darkening agent, either opaque minerals or ferrous iron bearing silicates, in abundances significantly higher than those of the lunar highlands. If the darkening agent is opaque minerals (e.g. ilmenite or ulvospinel) Mercury's crust may contain significant ferrous iron and yet not exhibit a 1-μm absorption band.  相似文献   

3.
《Icarus》1987,71(3):397-429
The results of a geological analysis of the Mariner 10 orange/UV color ratio man of Mercury (B. Hapke, C. Christman, B. Rava, and J. Mosher, Proc. Lunar Planet Sci. Conf. 11th 1980, pp. 817–822) are given. Certain errors that occured in reproducing the published version of the 1980 map are pointed out. The relationships between color and terrain are distinctly nonlunar. There is no correlation between color boundaries and the smooth plains on Mercury, in contrast to the strong correlation between color and maria-highlands contacts on the Moon. There are no large exposures of low-albedo, blue material that could be considered to be Mercurian analogs of high-FeTi lunar maria basalts on any part of Mercury imaged by Mariner 10. Three lines of evidence imply that the crust is low in Fe2+ and Ti4+: rays and ejecta blankets are bluer than most areas on Mercury; the Fe2+ band in Mercury's reflectance spectrum is very weak or nonexistent and the albedo contrasts are smaller than those on the Moon. There is no evidence in the spectral or albedo data that a lunar type of second wave of melting ever occured on Mercury; rather, the observations are most consistent with the hypothesis that the smooth plains are extrusive landforms derived from local material, possibly mobilized by the Caloris event. In several places correlations between color and topography can be explained if older, redder, higher-Fe materials underlie younger, bluer, lower-Fe surfaces. There is some evidence of late Fe-rich pyroclastic-like activity.  相似文献   

4.
Before the Apollo 16 mission, the material of the Cayley Formation (a lunar smooth plains) was theorized to be of volcanic origin. Because Apollo 16 did not verify such interpretations, various theories have been published that consider the material to be ejecta of distant multiringed basins. Results presented in this paper indicate that the material cannot be solely basin ejecta. If smoothplains are a result of formation of these basins or other distant large craters, then the plains materials are mainly ejecta of secondary craters of these basins or craters with only minor contributions of primary-crater or basin ejecta. This hypothesis is based on synthesis of knowledge of the mechanics of ejection of material from impact craters, photogeologic evidence, remote measurements of surface chemistry, and petrology of lunar samples. Observations, simulations, and calculations presented in this paper show that ejecta thrown beyond the continuous deposits of large lunar craters produce secondary-impact craters that excavate and deposit masses of local material equal to multiples of that of the primary crater ejecta deposited at the same place. Therefore, the main influence of a large cratering event on terrain at great distances from such a crater is one of deposition of more material by secondary craters, rather than deposition of ejecta from the large crater. Examples of numerous secondary craters observed in and around the Cayley Formation and other smooth plains are presented. Evidence is given for significant lateral transport of highland debris by ejection from secondary craters and by landslides triggered by secondary impact. Primary-crater ejecta can be a significant fraction of a deposit emplaced by an impact crater only if the primary crater is nearby. Other proposed mechanisms for emplacement of smooth-plains formations are discussed, and implications regarding the origin of material in the continuous aprons surrounding large lunar craters is considered. It is emphasized that the importance of secondary-impact cratering in the highlands has in general been underestimated and that this process must have been important in the evolution of the lunar surface.  相似文献   

5.
《Icarus》1987,72(3):477-491
There has been extensive debate about whether Mercury's smooth plains are volcanic features or impact ejecta deposits. We present new indirect evidence which supports a volcanic origin for two different smooth plains units. In Borealis Planitia, stratigraphic relations indicate at least two distinct stages of smooth plains formation. At least one of these stages must have had a volcanic origin. In the Hilly and Lineated Terrain, Petrarch and several other anomalously shallow craters apparently have been volcanically filled. Areally extensive smooth plains volcanism evidently occurred at these two widely separated areas on Mercury. These results, combined with work by other researchers on the circum-Caloris plains and the Tolstoi basin, show that smooth plains volcanism was a global process on Mercury. Present data suggest to us that the smooth and intercrater plains may represent two distinct episodes of volcanic activity on Mercury and that smooth plains volcanism may have been triggered by the Caloris impact. High-resolution and multispectral imaging from a future Mercury spacecraft could resolve many of the present uncertainties in our understanding of plains formation on Mercury.  相似文献   

6.
Clark R. Chapman 《Icarus》1976,29(4):523-524
It is premature to establish a chronology for Mars and Mercury, relative to the known lunar chronology, to better than an order of magnitude. Lunar evidence neither requires nor excludes a “cataclysmic” episode of bombardment about 4.0 b.y. ago. Such a cataclysm might have resulted naturally from tidal disruption by a planet or collisional fragmentation in the asteroid belt of either a Uranus/Neptune-scattered planetesimal or a large asteroid, in which case any lunar cataclysm would have occurred as well on other planets. There is no independent evidence in Mariner 10 imagery for (or against) an early episodic bombardment on Mercury. Crater densities on plains units of the Moon, Mars, and Mercury have not been shown to be “strikingly similar” and do not imply, in the absence of definitive dynamical calculations of planetary impact rates of plausible populations of planetesimals, any similarity in the geological chronologies for those planets. Photogeological studies alone cannot determine absolute chronologies for planets. In combination with dynamical analyses, they can help us date to no better than a factor of 3 to 10 the formation of the Caloris Basin or the epoch when the Martian rivers ran.  相似文献   

7.
Grooved and hilly terrains occur at the antipode of major basins on the Moon (Imbrium, Orientale) and Mercury (Caloris). Such terrains may represent extensive landslides and surface disruption produced by impact-generatedP-waves and antipodal convergence of surface waves. Order-of-magnitude calculations for an Imbrium-size impact (1034 erg) on the Moon indicateP-wave-induced surface displacements of 10 m at the basin antipode that would arrive prior to secondary ejecta. Comparable surface waves would arrive subsequent to secondary ejecta impacts beyond 103 km and would increase in magnitude as they converge at the antipode. Other seismically induced surface features include: subdued, furrowed crater walls produced by landslides and concomitant secondary impacts; emplacement and leveling of light plains units owing to seismically induced ‘fluidization’ of slide material; knobby, pitted terrain around old basins from enhancement of seismic waves in ancient ejecta blankets; and perhaps the production and enhancement of deep-seated fractures that led to the concentration of farside lunar maria in the Apollo-Ingenii region.  相似文献   

8.
MESSENGER’s Mercury Dual Imaging System (MDIS) obtained multispectral images for more than 80% of the surface of Mercury during its first two flybys. Those images have confirmed that the surface of Mercury exhibits subtle color variations, some of which can be attributed to compositional differences. In many areas, impact craters are associated with material that is spectrally distinct from the surrounding surface. These deposits can be located on the crater floor, rim, wall, or central peak or in the ejecta deposit, and represent material that originally resided at depth and was subsequently excavated during the cratering process. The resulting craters make it possible to investigate the stratigraphy of Mercury’s upper crust. Studies of laboratory, terrestrial, and lunar craters provide a means to bound the depth of origin of spectrally distinct ejecta and central peak structures. Excavated red material (RM), with comparatively steep (red) spectral slope, and low-reflectance material (LRM) stand out prominently from the surrounding terrain in enhanced-color images because they are spectral end-members in Mercury’s compositional continuum. Newly imaged examples of RM were found to be spectrally similar to the relatively red, high-reflectance plains (HRP), suggesting that they may represent deposits of HRP-like material that were subsequently covered by a thin layer (∼1 km thick) of intermediate plains. In one area, craters with diameters ranging from 30 km to 130 km have excavated and incorporated RM into their rims, suggesting that the underlying RM layer may be several kilometers thick. LRM deposits are useful as stratigraphic markers, due to their unique spectral properties. Some RM and LRM were excavated by pre-Tolstojan basins, indicating a relatively old age (>4.0 Ga) for the original emplacement of these deposits. Detailed examination of several small areas on Mercury reveals the complex nature of the local stratigraphy, including the possible presence of buried volcanic plains, and supports sequential buildup of most of the upper ∼5 km of crust by volcanic flows with compositions spanning the range of material now visible on the surface, distributed heterogeneously across the planet. This emerging picture strongly suggests that the crust of Mercury is characterized by a much more substantial component of early volcanism than represented by the phase of mare emplacement on Earth’s Moon.  相似文献   

9.
Images returned by the MESSENGER spacecraft from the Mercury flybys have been examined to search for anomalous high-albedo markings similar to lunar swirls. Several features suggested to be swirls on the basis of Mariner 10 imaging (in the craters Handel and Lermontov) are seen in higher-resolution MESSENGER images to lack the characteristic morphology of lunar swirls. Although antipodes of large impact basins on the Moon are correlated with swirls, the antipodes of the large impact basins on Mercury appear to lack unusual albedo markings. The antipodes of Mercury’s Rembrandt, Beethoven, and Tolstoj basins do not have surface textures similar to the “hilly and lineated” terrain found at the Caloris antipode, possibly because these three impacts were too small to produce obvious surface disturbances at their antipodes. Mercury does have a class of unusual high-reflectance features, the bright crater-floor deposits (BCFDs). However, the BCFDs are spectral outliers, not simply optically immature material, which implies the presence of material with an unusual composition or physical state. The BCFDs are thus not analogs to the lunar swirls. We suggest that the lack of lunar-type swirls on Mercury supports models for the formation of lunar swirls that invoke interaction between the solar wind and crustal magnetic anomalies (i.e., the solar-wind standoff model and the electrostatic dust-transport model) rather than those models of swirl formation that relate to cometary impact phenomena. If the solar-wind standoff hypothesis for lunar swirls is correct, it implies that the primary agent responsible for the optical effects of space weathering on the Moon is solar-wind ion bombardment rather than micrometeoroid impact.  相似文献   

10.
Matija ?uk  Brett J. Gladman 《Icarus》2010,207(2):590-7225
Multiple impact basins formed on the Moon about 3.8 Gyr ago in what is known as the lunar cataclysm or Late Heavy Bombardment. Many workers currently interpret the lunar cataclysm as an impact spike primarily caused by main-belt asteroids destabilized by delayed planetary migration. We show that morphologically fresh (class 1) craters on the lunar highlands were mostly formed during the brief tail of the cataclysm, as they have absolute crater number density similar to that of the Orientale basin and ejecta blanket. The connection between class 1 craters and the cataclysm is supported by the similarity of their size-frequency distribution to that of stratigraphically-identified Imbrian craters. Majority of lunar craters younger than the Imbrium basin (including class 1 craters) thus record the size-frequency distribution of the lunar cataclysm impactors. This distribution is much steeper than that of main-belt asteroids. We argue that the projectiles bombarding the Moon at the time of the cataclysm could not have been main-belt asteroids ejected by purely gravitational means.  相似文献   

11.
Abstract— ‐Mercury has widespread plains deposits proposed to be volcanic in origin. In a Mariner 10 color‐derived parameter image, sensitive to FeO and maturity, these volcanic plains have a value equivalent to, or slightly elevated above, the hemispheric average, thus implying FeO equivalent to, or slightly less than, the hemispheric average (～3 wt% FeO). Since FeO has a solid/liquid distribution coefficient ～1 during partial melting, we estimate the mantle of Mercury to have an FeO abundance equal to the lava flows. This is consistent with models that predict Mercury was assembled from planetesimals formed near the planet's current position. This new estimate of Mercury's bulk FeO (～3 wt%) is consistent with data for the other terrestrial planets that suggest there was a radial gradient in FeO in the solar nebula.  相似文献   

12.
Topographic data measured from the Mercury Laser Altimeter (MLA) and the Mercury Dual Imaging System (MDIS) aboard the MESSENGER spacecraft were used for investigations of the relationship between depth and diameter for impact craters on Mercury. Results using data from the MESSENGER flybys of the innermost planet indicate that most of the craters measured with MLA are shallower than those previously measured by using Mariner 10 images. MDIS images of these same MLA-measured craters show that they have been modified. The use of shadow measurement techniques, which were found to be accurate relative to the MLA results, indicate that both small bowl-shaped and large complex craters that are fresh possess depth-to-diameter ratios that are in good agreement with those measured from Mariner 10 images. The preliminary data also show that the depths of modified craters are shallower relative to fresh ones, and might provide quantitative estimates of crater in-filling by subsequent volcanic or impact processes. The diameter that defines the transition from simple to complex craters on Mercury based on MESSENGER data is consistent with that reported from Mariner 10 data.  相似文献   

13.
The 1300-km-diameter Caloris impact basin is surrounded by well-defined ejecta units that can be recognized from more than 1000 km, radially outward from the basin edge. A formal rock stratigraphic nomenclature is proposed for the Caloris ejecta units, which are collectively called the Caloris Group. Each of the individual formations within the Group are described and compared to similar rock units associated with the lunar Imbrium and Orientale basins. A crater degradation chronology, linked the the Caloris event, is also proposed to assist in stratigraphic correlation on a Mercury-wide basis.  相似文献   

14.
The differences between the surface structure of the near side and the far side of the Moon have been topics of interest ever since photographs of the far side have been available. One recurrent hypothesis is that a large impact on the near side has deposited ejecta on the far side, resulting in thicker crust there. Specific proposals were made by P.H. Cadogan for the Gargantuan Basin and by E.A. Whitaker for the Procellarum Basin. Despite considerable effort, no consensus has been reached on the existence of these basins. The problem of searching for such a basin is one of finding its signature in a somewhat chaotic field of basin and crater impacts. The search requires a model of the topographic shape of an impact basin and its ejecta field. Such a model is described, based on elevation data of lunar basins collected by the Lidar instrument of the Clementine mission and crustal thickness data derived from tracking Clementine and other spacecraft. The parameters of the model are scaled according to the principles of dimensional analysis and isostatic compensation in the early Moon. The orbital dynamics of the ejecta and the curvature of the Moon are also taken into account. Using such a scaled model, a search for the best fit for a large basin led to identification of a basin whose cavity covers more than half the Moon, including the area of all of the impact basins visible on the near side. The center of this basin is at 22 degrees east longitude and 8.5 degrees north latitude and its average radius is approximately 3,160 km. It is a megabasin, a basin that contains other basins (the far side South Pole-Aitken Basin also qualifies for that designation). It has been called the Near Side Megabasin. Much of the material ejected from the basin escaped the Moon, but the remainder formed an ejecta blanket that covered all of the far side beyond the basin rim to a depth of from 6 to 30 km. Isostatic compensation reduced the depth relative to the mean surface to a range of 1–5 km, but the crustal thickness data reveals the full extent of the original ejecta. The elevation profile of the ejecta deposited on the far side, together with modification for subsequent impacts by known basins (especially the far side South Pole-Aitken Basin) matches the available topographic data to a high degree. The standard deviation of the residual elevations (after subtracting the model from the measured elevations) is about one-half of the standard deviation of the measured elevations. A section on implications discusses the relations of this giant basin to known variations in the composition, mineralogy, and elevations of different lunar terranes.  相似文献   

15.
Peak-ring basins represent an impact-crater morphology that is transitional between complex craters with central peaks and large multi-ring basins. Therefore, they can provide insight into the scale dependence of the impact process. Here the transition with increasing crater diameter from complex craters to peak-ring basins on Mercury is assessed through a detailed analysis of Eminescu, a geologically recent and well-preserved peak-ring basin. Eminescu has a diameter (∼125 km) close to the minimum for such crater forms and is thus representative of the transition. Impact crater size-frequency distributions and faint rays indicate that Eminescu is Kuiperian in age, geologically younger than most other basins on Mercury. Geologic mapping of basin interior units indicates a distinction between smooth plains and peak-ring units. Our mapping and crater retention ages favor plains formation by impact melt rather than post-impact volcanism, but a volcanic origin for the plains cannot be excluded if the time interval between basin formation and volcanic emplacement was less than the uncertainty in relative ages. The high-albedo peak ring of Eminescu is composed of bright crater-floor deposits (BCFDs, a distinct crustal unit seen elsewhere on Mercury) exposed by the impact. We use our observations to assess predictions of peak-ring formation models. We interpret the characteristics of Eminescu as consistent with basin formation models in which a melt cavity forms during the impact formation of craters at the transition to peak ring morphologies. We suggest that the smooth plains were emplaced via impact melt expulsion from the central melt cavity during uplift of a peak ring composed of BCFD-type material. In this scenario the ringed cluster of peaks resulted from the early development of the melt cavity, which modified the central uplift zone.  相似文献   

16.
Abstract— The lunar surface is marked by at least 43 large and ancient impact basins, each of which ejected a large amount of material that modified the areas surrounding each basin. We present an analysis of the effects of basin formation on the entire lunar surface using a previously defined basin ejecta model. Our modeling includes several simplifying assumptions in order to quantify two aspects of basin formation across the entire lunar surface: 1) the cumulative amount of material distributed across the surface, and 2) the depth to which that basin material created a well‐mixed megaregolith. We find that the asymmetric distribution of large basins across the Moon creates a considerable nearside‐farside dichotomy in both the cumulative amount of basin ejecta and the depth of the megaregolith. Basins significantly modified a large portion of the nearside while the farside experienced relatively small degrees of basin modification following the formation of the large South Pole‐Aitken basin. The regions of the Moon with differing degrees of modification by basins correspond to regions thought to contain geochemical signatures remnant of early lunar crustal processes, indicating that the degree of basin modification of the surface directly influenced the distribution of the geochemical terranes observed today. Additionally, the modification of the lunar surface by basins suggests that the provenance of lunar highland samples currently in research collections is not representative of the entire lunar crust. Identifying locations on the lunar surface with unique modification histories will aid in selecting locations for future sample collection.  相似文献   

17.
Despite evident similarities, the Argyre basin exhibits important differences with regard to its lunar counterparts, as the Orientale basin. These differences concern both the stratigraphy of the impact related units and the tectonics of these areas. The Argyre basin is not surrounded by ejecta with radial facies, but by an annulus of structurally uplifted and faulted preimpact basement. That is different from the lunar basins which exhibit a large annulus of radial facies but only a narrow ring of uplifted terrains. The Argyre basin is surrounded by five or more outer discontinuous rings extending far away from the basis edge. That is different from the lunar basins which are surrounded by only one, continuous and closer ring. These differences could be partially explained by the external conditions, but mainly by differences in the crustal properties and lithospheres thickness which would have been thinner on Mars than on the Moon.  相似文献   

18.
Abstract— The northern lowland plains, such as those found in Acidalia and Utopia Planitia, have high percentages of impact craters with fluidized ejecta. In both regions, the analysis of crater geometry from Mars Orbiter Laser Altimeter (MOLA) data has revealed large ejecta volumes, some exceeding the volume of excavation. Moreover, some of the crater cavities and fluidized ejecta blankets of these craters are topographically perched above the surrounding plains. These perched craters are concentrated between 40 and 70°N in the northern plains. The atypical high volumes of the ejecta and the perched craters suggest that the northern lowlands have experienced one or more episodes of resurfacing that involved deposition and erosion. The removal of material, most likely caused by the sublimation of ice in the materials and their subsequent erosion and transport by the wind, is more rapid on the plains than on the ejecta blankets. The thermal inertia difference between the ejecta and the surrounding plains suggests that ejecta, characterized by a lower thermal inertia, protect the underneath terrain from sublimation. This results in a decreased elevation of the plains relative to the ejecta blankets. Sublimation and eolian erosion can be particularly high during periods of high obliquity.  相似文献   

19.
Radar imaging results for Mercury's non-polar regions are presented. The dual-polarization, delay-Doppler images were obtained from several years of observations with the upgraded Arecibo S-band (λ12.6-cm) radar telescope. The images are dominated by radar-bright features associated with fresh impact craters. As was found from earlier Goldstone-VLA and pre-upgrade Arecibo imaging, three of the most prominent crater features are located in the Mariner-unimaged hemisphere. These are: “A,” an 85-km-diameter crater (348° W, 34° S) whose radar ray system may be the most spectacular in the Solar System; “B,” a 95-km-diameter crater (343° W, 58° N) with a very bright halo but less distinct ray system; and “C,” an irregular feature with bright ejecta and rays distributed asymmetrically about a 125-km source crater (246° W, 11° N). Due south of “C” lies a “ghost” feature (242° W, 27° S) that resembles “A” but is much fainter. An even fainter such feature is associated with Bartok Crater. These may be two of the best mercurian examples of large ejecta/ray systems observed in an intermediate state of degradation. Virtually all of the bright rayed craters in the Mariner 10 images show radar rays and/or bright rim rings, with radar rays being less common than optical rays. Radar-bright craters are particularly common in the H-7 quadrangle. Some diffuse radar albedo variations are seen that have no obvious association with impact ejecta. In particular, some smooth plains regions such as the circum-Caloris plains in Tir, Budh, and Sobkou Planitiae and the interiors of Tolstoj and “Skinakas” basins show high depolarized brightness relative to their surroundings, which is the reverse of the mare/highlands contrast seen in lunar radar images. Caloris Basin, on the other hand, appears dark and featureless in the images.  相似文献   

20.
The lunar Orientale basin and its associated facies formed as a result of impact into lunar highland crustal rocks. The crater rim is shown to be closely represented by the position of the outer Rook Mountain ring, approximately 620 km in diam. The inner Rook Mountains form a central peak ring within the crater. The 900 km diam Cordillera ring is a fault scarp which formed in the terminal stages of the cratering event as a large portion of the crust collapsed inward toward the recently excavated crater, forming a mega-terrace. This collapse pushed the wall of the Orientale crater inward, distorting it and slightly decreasing its radius.A domical facies is almost exclusively developed between the Cordillera and outer Rook rings. The domical facies is interpreted to be radially textured ejecta which was disrupted and modified to a jumbled domical texture by seismic shaking associated with the formation of the mega-terrace. The plains and corrugated facies pre-date the mare fill and lie within the Orientale crater. These facies are interpreted to have been deposited contemporaneously with the cratering event as partial and total impact melts which collected on the floor of the crater during the terminal stages of the event. The plains facies, with an estimated thickness of 1 km and a volume of 75000 km3, represent the most thoroughly impact melted materials which collected and ponded in the central portion of the crater floor. The corrugated facies, with an estimated thickness of 1 km and a volume of 180000 km3, represent impact partial melts mixed with debris. A relatively small volume of mare material was subsequently deposited in the basin (probably less than 25000 km3 in Mare Orientale).There is little evidence that the basin has undergone major structural modifications subsequent to the terminal stages of the cratering event. The striking implication for the Orientale gravity anomaly is that mascon formation may be primarily related to crustal excavation and upwarping of a moho plug, rather than attributable to post-impact mare filling.The plains units on the floor of Orientale are similar to Cayley-like plains in other multi-ringed basins and on smaller crater floors. Impact melt deposits may therefore be a significant source of Cayley-like plains units.The volumes of impact melt associated with the Orientale basin and their mode of deposition have important implications for petrogenetic models. Multi-ringed basin formation provides a mechanism for instantaneously melting large volumes of shallow to intermediate depth lunar crustal material which is emplaced such that the differentiation and crystallization of a variety of igneous rock types and textures may occur.  相似文献   

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