首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Both morphologic and geometric studies of the “lineated terrains” around Caloris provide evidence of several types of tectonic motions inside the ejecta blanket of the basin. These motions preferentially occurred along a preexisting pattern. In spite of several similarities to mare-filled multiring basins on the Moon, many geometric and chronologic differences suggest that the ridge pattern inside the Caloris basin may not be produced, as observed on the Moon, exclusively by subsidence of the inner basin under volcanic loading. A model of membrane stresses which yield a decrease of the radius of Caloris and the observed tectonics is proposed.  相似文献   

2.
《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   

3.
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   

4.
H.J. Melosh  D. Dzurisin 《Icarus》1978,33(1):141-144
Studies of tectonic landforms associated with Caloris Basin on Mercury suggest that isostatic adjustment has occurred in response to basin excavation, and that the smooth plains inside Caloris were emplaced significantly before isostatic equilibrium was attained. Combined with dynamical considerations, this leads us to propose that the Caloris region is characterized by a circular negative or zero free air gravity anomaly centered inside Caloris, and an annular positive anomaly which coincides with extensive tracts of young smooth plains outside the basin. This proposed gravity pattern differs markedly from that associated with mare-filled basins on the Moon.  相似文献   

5.
We present and interpret results of petrographic, mineralogical, and chemical analyses of the 1511 m deep ICDP Yaxcopoil‐1 (Yax‐1) drill core, with special emphasis on the impactite units. Using numerical model calculations of the formation, excavation, and dynamic modification of the Chicxulub crater, constrained by laboratory data, a model of the origin and emplacement of the impact formations of Yax‐1 and of the impact structure as a whole is derived. The lower part of Yax‐1 is formed by displaced Cretaceous target rocks (610 m thick), while the upper part comprises six suevite‐type allochthonous breccia units (100 m thick). From the texture and composition of these lithological units and from numerical model calculations, we were able to link the seven distinct impact‐induced units of Yax‐1 to the corresponding successive phases of the crater formation and modification, which are as follows: 1) transient cavity formation including displacement and deposition of Cretaceous “megablocks;” 2) ground surging and mixing of impact melt and lithic clasts at the base of the ejecta curtain and deposition of the lower suevite right after the formation of the transient cavity; 3) deposition of a thin veneer of melt on top of the lower suevite and lateral transport and brecciation of this melt toward the end of the collapse of the transient cavity (brecciated impact melt rock); 4) collapse of the ejecta plume and deposition of fall‐back material from the lower part of the ejecta plume to form the middle suevite near the end of the dynamic crater modification; 5) continued collapse of the ejecta plume and deposition of the upper suevite; 6) late phase of the collapse and deposition of the lower sorted suevite after interaction with the inward flowing atmosphere; 7) final phase of fall‐back from the highest part of the ejecta plume and settling of melt and solid particles through the reestablished atmosphere to form the upper sorted suevite; and 8) return of the ocean into the crater after some time and minor reworking of the uppermost suevite under aquatic conditions. Our results are compatible with: a) 180 km and 100 km for the diameters of the final crater and the transient cavity of Chicxulub, respectively, as previously proposed by several authors, and b) the interpretation of Chicxulub as a peak‐ring impact basin that is at the transition to a multi‐ring basin.  相似文献   

6.
John K. Harmon 《Icarus》2008,196(1):298-301
Radar imagery from July 2005 Arecibo observations has provided new information on surface relief over the southern portion of Caloris Basin and the smooth plains to the south of the basin. A lobe of smooth plains has been identified in the Mariner-unimaged region southwest of Mozart Crater that coincides precisely with topographically down-bowed terrain seen in earlier Arecibo radar altimetry. A 105-km-diameter crater has been found at 193.6° W, 25.6° N that appears to be the largest crater in the Caloris basin floor.  相似文献   

7.
A digital terrain model (1000-m effective spatial resolution) of the Caloris basin, the largest well-characterized impact basin on Mercury, was produced from 208 stereo images obtained by the MESSENGER narrow-angle camera. The basin rim is far from uniform and is characterized by rugged terrain or knobby plains, often disrupted by craters and radial troughs. In some sectors, the rim is represented by a single marked elevation step, where height levels drop from the surroundings toward the basin interior by approximately 2 km. Two concentric rings, with radii of 690 km and 850 km, can be discerned in the topography. Several pre-Caloris basins and craters can be identified from the terrain model, suggesting that rugged pre-impact topography may have contributed to the varying characteristics of the Caloris rim. The basin interior is relatively smooth and shallow, comparable to typical lunar mascon mare basins, supporting the idea that Caloris was partially filled with lava after formation. The model displays long-wavelength undulations in topography across the basin interior, but these undulations cannot readily be related to pre-impact topography, volcanic construction, or post-volcanic uplift. Because errors in the long-wavelength topography of the model cannot be excluded, confirmation of these undulations must await data from MESSENGER’s orbital mission phase.  相似文献   

8.
Christian Klimczak 《Icarus》2010,209(1):262-270
The origin of Pantheon Fossae, a complex structure consisting of radial graben in the center of the Caloris basin, Mercury, has been debated since the structure was first imaged by the MESSENGER spacecraft. Three different formation hypotheses have been suggested, i.e. an origin associated with the Apollodorus impact into a previously domed Caloris basin floor, graben formation as surface expressions of dike intrusions and basin-interior uplift alone. In order to test the scenarios, detailed observations from the currently available imagery were compared to the proposed formation mechanisms. We evaluate these origin hypotheses by means of detailed interpretations of the graben characteristics and patterns, by comparing to radial structures from Earth and Venus, and by mechanical analyses for each formation hypothesis. Results indicate that the formation of Pantheon Fossae as the result of doming in the central part of the Caloris basin is more likely than it having formed in association with a radially symmetric stress field centered at or near the Apollodorus crater, that would have been created by a magma chamber or been superimposed on a pre-existing dome due to impact mechanics.  相似文献   

9.
Viking images of Martian craters with rampart-bordered ejecta deposits reveal distinct impact ejecta morphology when compared to that associated with similar-sized craters on the Moon and Mercury. Topographic control of distribution, lobate and terraced margins, cross-cutting relationships, and multiple stratigraphic units are evidence for ejecta emplacement by surface flowage. It is suggested that target water explosively vaporized during impact alters initial ballistic trajectories of ejecta and produces surging flow emplacement. The dispersal of particulates during a series of controlled steam explosions generated by interaction of a thermite melt with water has been experimentally modeled. Preliminary results indicate that the mass ratio of water to melt and confining pressure control the degree of melt fragmentation (ejecta particle size) and the energy and mode of melt-ejecta dispersal. Study of terrestrial, lobate, volcanic ejecta produced by steam-blast explosions reveals that particle size and vapor to clast volume ratio are primary parameters characterizing the emplacement mechanism and deposit morphology. Martian crater ramparts are formed when ejecta surges lose fluidizing vapors and transported particles are deposited en masse. This deposition results from flow yield strength increasing above shear stress due to interparticle friction.  相似文献   

10.
Abstract— New occurrences of the Acraman impact ejecta layer were recently discovered in two South Australian drillholes, SCYW‐79 1a (Stuart Shelf) and Munta 1 (Officer Basin) using lithostratigraphy, acritarch biostratigraphy, carbon isotope stratigraphy, and biomarker anomalies to predict the stratigraphic position. The ejecta layer is conspicuous because it consists of pink, sandsized, angular fragments of volcanic rock distributed along the bedding plane surface of green marine siltstone. In SCYW‐79 1a it forms a layer 5 mm thick; in Munta 1 the ejecta layer is thin and discontinuous because of its distance (?550 km) from the impact structure. Palynological, biomarker, and carbon isotope anomalies can now be shown to coincide with the ejecta layer in SCYW‐79 1a and Munta 1 suggesting the Acraman impact event may have had far reaching influences on the rapidly evolving Ediacaran biological and geochemical cycles.  相似文献   

11.
Cryptomaria are mare basalt deposits hidden or obscured by superposed higher albedo material or variations in albedo. They represent a record of the earliest mare volcanism, and may be a significant volumetric contribution to the volcanic and magmatic history of the Moon. In order to assess their global distribution and significance, criteria for the identification of cryptomaria are developed and techniques for locating them are described. These criteria and techniques include the presence of dark halo craters, identification by spectral mixing analysis, identification by geochemical evidence, association with light plains units, location within basin topography, proximity to known mare, relation to mascons indicated by gravity anomalies, and identification of the source of an obscuring agent, such as crater ejecta. On the basis of these criteria and techniques, several types of cryptomare are recognized, depending on the nature of ejecta and mare materials. Cryptomaria may be formed when maria are obscured by coverings of proximal or distal basin ejecta, or by crater ejecta dusting, or when ejecta covers over basalts which lack a distinctive 1µm absorption band. Using these concepts we outline three case studies: 1) the Schiller-Schickard region adjacent to the Orientale basin, classified as a basin-ejecta cryptomare and grading from distal to proximal, with possible crater-ejecta covering occurring in the southwestern portion of the region, 2) the Balmer basin, classified as a crater-ejecta-dusting cryptomare, and 3) the Australe basin, in which two types of cryptomare were identified: a) crater-ejecta-dusting on old mare patches and b) possible distal-basin-ejecta covering even older mare material. These case studies provide criteria for the further global identification and classification of cryptomaria and stress the need for utilization of multiple criteria and data sets.  相似文献   

12.
Analysis of images from the Messenger MDIS narrow angle camera imply that at least part of the radial graben of the Pantheon Fossae structure, and probably the structure as a whole, predate the deformation that led to circumferential ridges on the Caloris interior plains. This follows from structural analysis and comparison with similar geological relationships on Venus and the Moon, where graben are known to both postdate and predate ridges. Observations suggest that the Pantheon Fossae radial graben (extension) formed first, pre-dating observed circumferential graben (also extension), with ridges (compression) formed in between. This scenario puts constraints on the models for the deformation of the Caloris basin and its vicinity. Our observations and analysis are consistent with Pantheon Fossae having formed in a similar manner to Venusian astra/novae, where radial dikes that propagate away from a magmatic center led to graben formation. Our results also have implications for the length of time between the emplacement of the basin volcanic fill and the onset of the compressional stresss regime that led to ridge-formation. If the Pantheon Fossae structure formed before the emplacement of ridges, as we suggest, this means that compressional stresses took some time to develop sufficiently to deform the volcanic plains. Since the Caloris interior plains had to have been already in place when Pantheon Fossae formed, and since these plains represented a significant load to the underlying lithosphere, it is striking that compression took some time to develop. These observations may provide new information about the rigidity of the basin-filling material and will help constrain models for the mechanisms and timing of events within and around the Caloris basin.  相似文献   

13.
  相似文献   

14.
Don E. Wilhelms 《Icarus》1976,28(4):551-558
The Mariner 10 television team has argued that extensive plains on Mercury were formed by volcanism and compared them with the demonstrably lunar maria. I believe, however, that in stratigraphic relations, surface morphology, and albedo contrast, the Mercurian plains more closely resemble the lunar light plains. These lunar plains were interpreted as volcanic on the basis of data comparable to that available to the Mariner 10 investigators but have been shown by the Apollo missions to be of impact origin. The plains on Mercury might also be formed of impact materials, perhaps of impact melt or other basin ejecta that behaved more like a fluid when emplaced that did lunar basin ejecta.  相似文献   

15.
New topographic data allow a reassessment of the ring structure of the Serenitatis basin and correlation with the younger Orientale basin. The northern Serenitatis basin is smaller and less well preserved than the southern Serenitatis basin. Three major rings of the main (southern) Serenitatis basin are mapped: ring 1, Linné ring, outlined by mare ridges, average diameter 420 km; ring 2, Haemus ring, outlined by basin-facing scarps and massifs with crenulated borders, 610 km; ring 3, Vitruvius ring, outlined by basin-facing linear scarps and massifs, 880 km. Ring 1 corresponds to the inner Rook Mountain ring of Orientale, ring 2 with the outer Rook ring, and ring 3 with the Cordillera Mountain ring. These ring identifications and assignments indicate that the Serenitatis basin is essentially the same size as the Orientale basin, rather than much larger, as previously proposed. The Apollo 17 site lies near the second ring, which is interpreted as the rim of the transient cavity. Apollo 15 lies at the junction of the Serenitatis and Imbrium third rings; Serenitatis ejecta should be present in significant amounts at the Apollo 15 site. The new reconstruction indicates that portions of the Serenitatis basin are better preserved than previously thought, consistent with recent stratigraphic and sample studies that suggest an age for Serenitatis which is older than, but close to, the time of formation of the Imbrium basin.  相似文献   

16.
Abstract— We have used data from the Clementine and Lunar Prospector spacecraft in conjunction with reflectance spectra collected with Earth‐based telescopes to study the geology of the Hadley‐Apennine portion of the lunar Imbrium basin. The Apennine Mountains and the Imbrium backslope are composed of Imbrium basin ejecta with a noritic or anorthositic norite composition. We find that the two major facies of Imbrium ejecta, the Apenninus material and the Alpes Formation, differ in iron and titanium content. “Pure” anorthosite has tentatively been identified in the ejecta of the crater Conon, based on low‐iron content. A difference in Th and rare earth element (REE) abundance between the northeast Apennine Mountains (lower) and the southwest Apennines (higher) is noted. Pyroclastic deposits are common in the region and are dominated by mare basalt material, probably plug rock ejected in vulcanian eruptions. The Apennine Bench Formation, which is likely to be a deposit of non‐mare volcanic material, has an Fe, Ti and Th composition consistent with that of Apollo 15 KREEP basalt samples thought to be fragments of the Bench. Aristillus crater is a Th and REE hot spot, and the stratigraphy of the impact target site has been reconstructed from knowledge of the composition of the crater interior and exterior deposits. We infer that the target consisted of highland basement, KREEP plutonics and volcanics, and both high‐ and low‐Ti mare basalt.  相似文献   

17.
N. Yan  F. Leblanc 《Icarus》2006,181(2):348-362
We have developed a 1D thermal model of Mercury's regolith, in order to simulate the heat diffusion in the upper subsurface (first 10 m). We assume in our model that the thermophysical properties of the Hermean regolith are similar to those of the lunar regolith. We apply our thermal model to the Caloris basin which slopes induce distortions of the surface temperature compared to results obtained for a perfect spherical planet. This thermal model is then coupled with a 3D Monte Carlo model of Mercury's sodium exosphere [Leblanc, F., Johnson, R.E., 2003. Icarus 164, 261-281; Leblanc, F., Delcourt, D., Johnson, R.E., 2003b. J. Geophys. Res. 108 (E12), doi:10.1029/2003JE002151/.5136], in order to describe the signatures of Caloris basin on Mercury's sodium exosphere in term of temporal and spatial variabilities. In particular, we find a motion of the maxima of sodium density in the exosphere towards the Northern hemisphere similar to the one observed by Potter et al. [Potter, A.E., Morgan, T.H., Killen, R.M., 1999. Planet. Space Sci., 47, 1441-1449] but did not reproduce the observed change of the emission brightness. The main conclusion of this study is that the Caloris basin-exosphere relations might be observable from the Earth which we hope will motivate new observations of Mercury's exosphere.  相似文献   

18.
We present the results of numerical modeling of the formation of the Ries crater utilizing the two hydrocodes SOVA and iSALE. These standard models allow us to reproduce crater shape, size, and morphology, and composition and extension of the continuous ejecta blanket. Some of these results cannot, however, be readily reconciled with observations: the impact plume above the crater consists mainly of molten and vaporized sedimentary rocks, containing very little material in comparison with the ejecta curtain; at the end of the modification stage, the crater floor is covered by a thick layer of impact melt with a total volume of 6–11 km3; the thickness of true fallback material from the plume inside the crater does not exceed a couple of meters; ejecta from all stratigraphic units of the target are transported ballistically; no separation of sedimentary and crystalline rocks—as observed between suevites and Bunte Breccia at Ries—is noted. We also present numerical results quantifying the existing geological hypotheses of Ries ejecta emplacement from an impact plume, by melt flow, or by a pyroclastic density current. The results show that none of these mechanisms is consistent with physical constraints and/or observations. Finally, we suggest a new hypothesis of suevite formation and emplacement by postimpact interaction of hot impact melt with water or volatile‐rich sedimentary rocks.  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 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   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号