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1.
Aphrodite Terra is the largest highland area on Venus of the size of Africa. It is traversed by the Aphrodite-Beta belt of troughs with a length of 21 000 km. There are two other large belts of troughs on Venus: Themis-Atla, 14 000 km long, and Beta-Phoebe, 8000 km long. In this paper, four gravity profiles across Aphrodite Terra are studied and compared with the morphology.Western Aphrodite and Niobe Planitia to the north seem to be in isostatic equilibrium under the assumption of Airy compensation with a mean crustal thickness of T = 50 km. The graben area in the middle part of Aphrodite Terra shows negative isostatic gravity anomalies indicating deficit masses. The adjacent Atla Regio to the east is regionally Airy compensated with T = 50 km, and the mountains Nokomis, Maat and Ozza Montes are locally undercompensated, i.e. they are associated with surplus masses in the depth. Ulfrun Regio, a hilly terrain just east of Atla Regio is Airy compensated with T = 30 km. These results give a mean crustal thickness around 50 km for Aphrodite Terra. The isostatic disturbed zones in the middle of Aphrodite (grabens) and Atla Regio as well as the undercompensated Beta Regio have been associated with recent volcanism from the observation of the concentrations of electrical discharges in these areas. Atla and Beta Regiones are both located at intersections of the systems of troughs described above.Contribution No. 308, Institut für Geophysik der Universität Kiel, F.R.G.  相似文献   

2.
Abstract— Nine seismic-reflection profiles and four continuous core holes define the gross structural and stratigraphic framework of the outer rim of the Chesapeake Bay impact crater. The rim is manifested as a 90 km diameter ring of terraced normal-fault blocks, which forms a ～320 m–1200 m high rim escarpment. The top of the rim escarpment is covered by a 20 m–30 m thick ejecta blanket. The escarpment encircles a flat-floored annular trough, which is partly filled with an ～250 m thick breccia lens (Exmore breccia). The Exmore breccia overlies a 200 m–800 m thick interval of slumped sedimentary megablocks, which, in turn, rests on crystalline basement rocks. All postimpact strata (upper Eocene to Quaternary) sag structurally into the annular trough, and most units also thicken as they cross the rim into the crater. Postimpact compaction and subsidence of the Exmore breccia have created extensive normal faulting in overlying strata.  相似文献   

3.
Eastern Aphrodite Terra and Western Aphrodite form an altimetrically prominent 14,000 km long part of the equatorial highlands on Venus. Several parallel linear discontinuities striking northwest across the general east-west regional strike of the highlands are mapped in the altimetric and radar image data of Eastern Aphrodite and identified on the basis of abrupt termination of rift-like central chasma, offset and segmentation of the center of the highlands, and radar image discontinuities in the lowlands to the north. These characteristics are similar to those of linear discontinuities previously mapped in Western Aphrodite in terms of length, orientation, and influence on the central highlands and adjacent lowlands.Altimetric profiles in directions parallel to the discontinuities are regionally symmetric, more ridge-like in Eastern Aphrodite compared to the plateau-dominated form of topography in Western Aphrodite, and are characterized by alternating paired ridge-and-trough forms near their crests and on their flanks. By mapping the center of symmetry in multiple profiles, the prominent segmentation of the highland is shown to be imparted by an offset of the regional symmetry along the mapped discontinuities. These characteristics are morphologically similar to several of the large-scale characteristics of divergent plate boundaries of Earth, including mid-ocean rise crests and rifts, offset at fracture zones and transform faults, and symmetric thermal boundary layer topography.The altitude of the surface in profiles parallel to the discontinuities decreases as the square root of distance from the symmetry axes and with a slope similar to that predicted for thermal boundary layer topography associated with rates of divergence on Venus of ~ 1 ± 0.5 cm/yr. In order to test the hypothesis that the linear discontinuities are analogous to fracture zones, the predicted altitude of the surface at great distance from the centers of symmetry of the central highland and in directions across the discontinuities was calculated on the basis of a thermal boundary layer topography model with offset of altimetric symmetry at each discontinuity. Similarity of observed Arecibo high-resolution altimetric profiles across the discontinuities with that calculated for thermal boundary layer topography offset by transform faults reveals that in terms of the sense and magnitude of regional steps in altimetry across discontinuities and the altitude of the surface, Eastern Aphrodite is similar to the known characteristics of crustal spreading at divergent boundaries. The plateau-like form of Western Aphrodite and the ridge-like form of Eastern Aphrodite are analogous respectively to the difference between areas of anomalous (Iceland) and normal crustal production along rise crests on Earth. Estimates of volumetric differences in crustal production in the environment of Venus and as it would be influenced by differences in mantle temperature beneath Western and Eastern Aphrodite imply that Eastern Aphrodite represents normal crustal production. On this basis, Western Aphrodite may be characterized by a mantle temperature that is warmer than the mantle beneath Eastern Aphrodite Terra, perhaps in association with deep convective mantle upwelling.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).  相似文献   

4.
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5.
The composition of small areas within three large lunar highland craters are compared using near-infrared, Earth-based telescopic data. Spectra from many areas in the walls and central peaks of the craters indicate the presence of crystalline components which include olivine, two types of pyroxene, and sometimes Fe-bearing feldspar. Spectra from other regions suspected of being impact melt deposits, located on the floors and walls of the three craters, have remarkably similar, yet anomalous features. These features are interpreted to indicate the presence of pyroxene, Fe-bearing glass, and Fe-bearing feldspar. Pyroxene and feldspar are believed to occur primarily in the form of lithic clasts and rapidly recrystallized impact melt. The Fe-bearing glass is interpreted as impact melt glass.  相似文献   

6.
Bell Regio is a highland fragment south of Ishtar Terra, extending 1300 km in N-S direction and 900 km in E-W direction. South of this region Eisila Regio is located with an E-W extension of 8000 km and a width of 2000 km. Bell Regio consists of two large massifs: a northern massif with maximum altitudes of 2.5 to 3.0 km above the 6051 km datum and with a semi-corona (other coronae on Venus are associated with volcanic-tectonic processes) and a southern massif with a maximum of 4 to 4.5 km above the datum. The possible shield volcano Tepev Mons of 250 km in diameter is superimposed on the southern massif. It shows a radar dark crater of 40 km diameter on its eastern flank, a crater-like feature of 15 km diameter on the top and a radar bright area extending from the dark crater across the summit. South of Tepev Mons are several volcanic structures with summit depressions. The crest of Bell Regio exhibits a N-S extending fossa system. The whole fresh appearing plain-like area has been classified as rather young compared to other units. Gravity data show a maximum of 33 mGal at Bell Regio and 35 mGal at eastern Eisila Regio. The basins north and south of the highland fragments are associated with gravity lows.Density models have been calculated along the gravity profile Rev. 163 of Pioneer Venus Orbiter across Bell and Eisila Regiones assuming Airy isostatic compensation of the topography and considering several boundary conditions (e.g. mean crustal thickness T<- 100 km). There are two groups of density models in the case of Airy compensation. In the first group global total compensation is assumed along the profile and regional partial compensation for Bell and Eisila Regiones. This solution gives a range of possible models with 10 km <- T <- 100 km and a partial compensation for Bell and Eisila Regiones between 12% and 55%. Thus these two highland fragments show subsurface surplus masses.The second group of models considers for the whole profile total compensation with a global T <- 100 km and a regional very large depth of compensation for Bell and Eisila Regiones, i.e. T > 100 km.The highland of Beta Regio has, like Bell Regio, a N-S rifting system, volcanic structures, a fresh appearing plain-like surface and either deep-seating compensating masses or near surface surplus masses. Bell can be considered as little sister of Beta. The geological and geophysical results imply a volcanic-tectonic uplift over a hot spot. The conditions of Atla Regio in eastern Aphrodite Terra are similar. Thus the existence of volcanic-tectonic uplifts support the important role of hot spot volcanism on Venus.Contribution Nr. 343, Institut für Geophysik der Universität Kiel, F.R.G.  相似文献   

7.
The early history of Mars included two large-scale events of great significance: (1) the lowering and resurfacing of one-third of the crust, followed closely by (2) evolution of the Tharsis bulge. Tharsis development apparently involved two stages: (1) an initial rapid topographic rise accompanied by the development of a vast radial fault system, and (2) an extremely long-lived volcanic stage apparently continuing to the geologic present. A deformational model is proposed whereby a first-order mantle convection cell caused early subcrustal erosion and foundering of the low third of the planet. Underplating and deep intrusion by the eroded materials beneath Tharsis caused isostatic doming. Minor radial gravity motions of surficial layers off the dome produced the radial fault system. The hot underplate eventually affected the surface to cause the very long-lived volcanic second stage. Deep crustal anisotropy associated with the locally NE-trending boundary between the highland two-thirds and the lowland one-third caused the NE elongation of many features of Tharsis.  相似文献   

8.
The intensity distribution of lunar radar echoes has been mapped for two-thirds of the earth-visible lunar surface at a wavelength of 70 cm. The depolarizing effects of the lunar surface were observed by simultaneously receiving the radar echoes in opposite polarizations. These echoes were mapped with areal resolutions of 25–100 km2. Mappings with this resolution confirmed that the young craters have enhanced returns. A few craters were found to have enhanced echoes only from their rims. Backscattering differences were also observed between various areas within a mare, between different highland areas, and between maria and adjacent highlands. These scattering differences were interpreted with a simple model, which assumed that the surface backscattered with varying amounts of quasi-specular and diffuse power. Only an increase in the diffuse power was needed to give the numerical values of the enhancements.  相似文献   

9.
The Taurus-Littrow region (Apollo 17 landing area) is located in the northeastern quadrant of the Moon in the mountainous area on the southeastern rim of the Serenitatis basin. The highlands in the Taurus-Littrow region can be divided into three broad terrain types. (1)Littrow massifs - massive, 10-20 km diam, steep-sloped (20°–30°), highland blocks often bordered by linear graben-like valleys. (2)Littrow sculptured hills - a series of closely spaced 1-5 km diam domical hills occupying broad highland plateaus which have been cratered and block faulted. Sculptured hill units stretch along the eastern edge of Serenitatis from the Apollo 17 area north to Posidonius. (3)Vitruvius front and plateau - a long irregular but generally north-trending scarp (occasionally rising over 2 km above the surrounding terrain) and its associated uplifted plateau to the east. This terrain is composed of hills ranging from 2-7 km diam, whose morphology is intermediate between the sculptured hills and the massifs. It is concluded that the highland units in the Taurus-Littrow region are primarily related to the origin of the Serenitatis basin because of their marked similarity to more well-preserved basin-related deposits in the younger Imbrium and Orientale basins: (1) the massifs and sculptured terra are morphologically similar to the Imbrium basin-related Montes Alpes and Alpes Formation, (2) the relative geographic position of the Taurus-Littrow highlands and Montes Alpes/Alpes Formation is the same, forming the second ring and spreading distally, and (3) the structures are similar in orientation and development (e.g., massifs are related to radial and concentric structure; Alpes Formation/sculptured terra are not). Interpretation of the massifs and sculptured hills as Serenitatis impact-related deposits lessens the possible role of highland volcanism in the origin and evolution of the Taurus-Littrow terrain, although extensive pre-Serenitatis volcanism cannot be ruled out. The preserved morphology of the sculptured hills suggests that the thickness of post-Serenitatis large basin ejecta (from Imbrium, for instance) is small, compared to the total highland section. This implies that the primary contributions to the highland stratigraphy are from Serenitatis and pre-Serenitatis events. The highland surface, however, may be dominated by ejecta from the latest nearby large event (formation of the Imbrium basin). Structural elements mapped in the Taurus-Littrow area include lineaments, the Vitruvius structural front, two types of grabens, and scarps. The majority of lineaments, as well as some grabens, appear to be related to a dominant NW trend and subordinate N and NE trends. These trends are interpreted to be related to a more regional lunar grid pattern which formed in the area prior to the origin of the Serenitatis basin, causing distinct structural inhomogeneities in the highland terrain. The Serenitatis event produced radial and concentric structures predominantly influenced by this pre-existing trend. Younger grabens are generally circumferential to the Serenitatis basin and appear to be related to readjustment of Serenitatis-produced structures; those that are oblique to Serenitatis follow the pre-Serenitatis structural grain. No obvious structural elements can be correlated with the post-Serenitatis, Nectaris and Crisium basins. It is believed that the origin and hence the geographic concentration of the Littrow massifs is related to the fact that Serenitatis radials in the massif area coincide with lines of pre-existing structural weakness along a general lunar grid direction (NW). Pre-existing structurally weak lunar grid trends seem to have been structurally reactivated by Serenitatis radials, causing preferential uplift of large blocks in this area. Elsewhere in the region radials would be oblique to this direction. Since Serenitatis and Imbrium radials coincide in the massif area, the post-Serenitatis Imbrium event may have reactivated Serenitatis radial fractures, possibly rejuvenating the massif terrain. The geologic and tectonic history of the Taurus-Littrow highlands began prior to the origin of Serenitatis in Tectonic Interval I. The strong NW trending structural elements are believed to have formed as part of a global stress pattern (possibly shear) sometime during this period of probable crustal formation and fragmentation. Tectonic Interval II was initiated by the origin of the Serenitatis basin. The basic topography and morphology of the region and most large grabens resulted from this event and their orientations show that they were controlled at least in part by the pre-existing grid. No other large basins forming during this interval appear to have had a major effect on the area. Tectonic Interval III is dominated by the formation of narrow grabens following structural patterns circumferential to the Serenitatis basin and tangential to it where they coincide with pre-existing grid directions. Serenitatis isostatic rebound or early mare fill may have produced this stress system. The scarp in the vicinity of the Apollo 17 landing site is the youngest obvious structural element.  相似文献   

10.
Geochemical profiles of surface units, impact, and volcanic features are studied in detail to determine the underlying structure in an area of extensive mare/highland interface, Sinus Amoris. This study region includes and surrounds the northeastern embayment of Mare Tranquillitatis. The concentrations of two major rock-forming elements (Mg and Al), which were derived from the Apollo 15 orbital geochemical measurements, were used in this study. Mapped units and deposits associated with craters in the northwestern part of the region tend to have correlated low Mg and Al concentrations, indicating the presence of KREEP-enriched basalt. Found along the northeastern rim of Tranquillitatis were areas with correlated high Mg and Al concentration, indicating the presence of troctolite. Distinctive west/east and north/south trends were observed in the concentrations of Mg and Al, and, by implication, in the distribution of major rock components on the surface. Evidence for a systematic geochemical transition in highland or basin-forming units may be observed here in the form of distinctive differences in chemistry in otherwise similar units in the western and eastern portions of the study region.  相似文献   

11.
Maxima of calculated topographical line-of-sight (LOS) gravity attractions caused by Ishtar Terra are shifted to the north with respect to the measured LOS free air gravity maxima south of the highland. This implies a tendency to isostatic compensation of central Ishtar and mass surpluses at the continental border and the southern forelands.The following scenario is compatible with the interpretation of the gravity anomalies and morphological features. Relative motions of the lowland Sedna Planitia against continental Ishtar Terra have caused buckling and flat subduction of the lowland lithospheric material. (Deep subduction can be ruled out by thermal reasons). The free air gravity high is modelled by surplus masses of the buckling and of the high density subducting plate. Evidence for this is given by several compressional features like Ut and Vesta Rupes at the southern continental border and ridges at the SW-flanks of Maxwell Montes. It is further supported by several possible volcanic-tectonic depressions located in the southern part of Ishtar. This local interpretation does not necessarily imply the existence of global plate tectonics on Venus like on Earth, but at least limited horizontal movements of the Venusian lithosphere seem to be likely. This result shows that plate recycling must be considered for heat transfer through the lithosphere beside conduction and hot spot volcanism.Contribution No. 273, Institut für Geophysik der Universität Kiel, F.R.G.  相似文献   

12.
The surface topography of Asteroid 25143 Itokawa is explored using the LIght Detection And Ranging instrument (LIDAR). The data confirm the presence of a rough highland and a smooth lowland. The highland is dominated by boulders, but also possesses topography associated with surface lineaments and broad surface facets. The boulders ensure that the roughness of the highlands over short distances is typically greater relative to most surfaces on 433 Eros. Over larger distances, Itokawa is always smoother than Eros possibly because of its smaller size and weak rubble-pile structure. The lowlands of Itokawa are very smooth, and are typically devoid of boulders. Some transitional regions midway between the highlands and lowlands also exist. In these areas, craters that retain their regolith fill possess flat floors and resemble “ponds” seen on 433 Eros. Analyses of surface elevation, imagery and a quantitative measure of surface roughness are consistent with regolith flowing downhill from the highlands to fill in the low areas of Itokawa, probably covering up any pre-existing rough terrain. Using this interpretation, we find a minimum 2.3±0.4 m thick layer of regolith in the lowlands, which, if spread evenly across the entire asteroid, corresponds to a 42±1 cm thick layer. It is very difficult to generate this amount of regolith with the population of craters seen on Itokawa. However, an Itokawa composed of several large masses may have retained this regolith during its formation. The presence of such large masses could account for the observed lineaments and what appear to be exposures of bedrock on the largest steep slope observed.  相似文献   

13.
Material is ejected from impact craters in ballastic trajectories; it impacts first near the crater rim and then at progressively greater ranges. Ejecta from craters smaller than approximately 1 km is laid predominantly on top of the surrounding surface. With increasing crater size, however, more and more surrounding surface will be penetrated by secondary cratering action and these preexisting materials will be mixed with primary crater ejecta. Ejecta from large craters and especially basin forming events not only excavate preexisting, local materials, but also are capable of moving large amounts of material away from the crater. Thus mixing and lateral transport give rise to continuous deposits that contain materials from within and outside the primary crater. As a consequence ejecta of basins and large highland craters have eroded and mixed highland materials throughout geologic time and deposited them in depressions inside and between older crater structures.Because lunar mare surfaces contain few large craters, the mare regolith is built up by successive layers of predominantly primary ejecta. In contrast, the lunar highlands are dominated by the effects of large scale craters formed early in lunar history. These effects lead to thick fragmental deposits which are a mixture of primary crater material and local components. These deposits may also properly be named regolith though the term has been traditionally applied only to the relatively thin fine grained surficial deposit on mare and highland terranes generated during the past few billion year. We believe that the surficial highland regolith - generated over long periods of time - rests on massive fragmental units that have been produced during the early lunar history.  相似文献   

14.
We studied a data set of 28 well‐preserved lunar craters in the transitional (simple‐to‐complex) regime with the aim of investigating the underlying cause(s) for morphological differences of these craters in mare versus highland terrains. These transitional craters range from 15 to 42 km in diameter, demonstrating that the transition from simple to complex craters is not abrupt and occurs over a broad diameter range. We examined and measured the following crater attributes: depth (d), diameter (D), floor diameter (Df), rim height (h), and wall width (w), as well as the number and onset of terraces and rock slides. The number of terraces increases with increasing crater size and, in general, mare craters possess more terraces than highland craters of the same diameter. There are also clear differences in the d/D ratio of mare versus highland craters, with transitional craters in mare targets being noticeably shallower than similarly sized highland craters. We propose that layering in mare targets is a major driver for these differences. Layering provides pre‐existing planes of weakness that facilitate crater collapse, thus explaining the overall shallower depths of mare craters and the onset of crater collapse (i.e., the transition from simple to complex crater morphology) at smaller diameters as compared to highland craters. This suggests that layering and its interplay with target strength and porosity may play a more significant role than previously considered.  相似文献   

15.
We use Viking and new MGS and Odyssey data to characterize the lobate deposits superimposed on aureole deposits along the west and northwest flanks of Olympus Mons, Mars. These features have previously been interpreted variously as landslide, pyroclastic, lava flow or glacial features on the basis of Viking images. The advent of multiple high-resolution image and topography data sets from recent spacecraft missions allow us to revisit these features and assess their origins. On the basis of these new data, we interpret these features as glacial deposits and the remnants of cold-based debris-covered glaciers that underwent multiple episodes of advance and retreat, occasionally interacting with extrusive volcanism from higher on the slopes of Olympus Mons. We subdivide the deposits into fifteen distinctive lobes. Typical lobes begin at a theater-like alcove in the escarpment at the base of Olympus Mons, interpreted to be former ice-accumulation zones, and extend outward as a tongue-shaped or fan-shaped deposit. The surface of a typical lobe contains (moving outward from the basal escarpment): a chaotic facies of disorganized hillocks, interpreted as sublimation till in the accumulation zone; arcuate-ridged facies characterized by regular, subparallel ridges and interpreted as the ridges of surface debris formed by the flow of underlying ice; and marginal ridges interpreted as local terminal moraines. Several lobes also contain a hummocky facies toward their margins that is interpreted as a distinctive type of sublimation till shaped by structural dislocations and preferential loss of ice. Blocky units are found extending from the escarpment onto several lobes; these units are interpreted as evidence of lava-ice interaction and imply that ice was present at a time of eruptive volcanic activity higher on the slopes of Olympus Mons. Other than minor channel-like features in association with lava-ice interactions, we find no evidence for the flow of liquid water in association with these lobate features that might suggest: (1) near-surface groundwater as a source for ice in the alcoves in the lobe source region at the base of the scarp, or (2) basal melting and drainage emanating from the lobes that might indicate wet-based glacial conditions. Instead, the array of features is consistent with cold-based glacial processes. The glacial interpretations outlined here are consistent with recent geological evidence for low-latitude ice-rich features at similar positions on the Tharsis Montes as well as with orbital dynamic and climate models indicating extensive snow and ice accumulation associated with episodes of increased obliquity during the Late Amazonian period of the history of Mars.  相似文献   

16.
Global acquisition of infrared spectra and high-resolution visible and infrared imagery has enabled the placement of compositional information within stratigraphic and geologic context. Mare Serpentis, a low albedo region located northwest of Hellas Basin, is rich in spectral and thermophysical diversity and host to numerous isolated exposures of in situ rocky material. Most martian surfaces are dominated by fine-grained particulate materials that bear an uncertain compositional and spatial relationship to their source. Thus location and characterization of in situ rock exposures is important for understanding the origin of highland materials and the processes which have modified those materials. Using spectral, thermophysical and morphologic information, we assess the local and regional stratigraphy of the Mare Serpentis surface in an effort to reconstruct the geologic history of the region. The martian highlands in Mare Serpentis are dominated by two interspersed surface units, which have distinct compositional and thermophysical properties: (1) rock-dominated surfaces relatively enriched in olivine and pyroxene, and depleted in high-silica phases, and (2) sediment or indurated material depleted in olivine and pyroxene, with relatively higher abundance of high-silica phases. This is a major, previously unrecognized trend which appears to be pervasive in the Mare Serpentis region and possibly in other highland areas. The detailed observations have led us to form two hypotheses for the relationship between these two units: either (1) they are related through a widespread mechanical and/or chemical alteration process, where less-mafic plains materials are derived from the mafic bedrock, but have been compositionally altered in the process of regolith formation, or (2) they are stratigraphically distinct units representing separate episodes of upper crust formation. Existing observations suggest that the second scenario is more likely. In this scenario, plains materials represent older, degraded, and possibly altered, “basement” rock, whereas the rocky exposures represent later additions to the crust and are probably volcanic in origin. These hypotheses should be further testable with decimeter-resolution imagery and meter-resolution short wavelength infrared spectra.  相似文献   

17.
Reflectance Spectral Characteristics of Lunar Surface Materials   总被引：2，自引：0，他引：2  
Based on a comprehensive analysis of the mineral composition of major lunar rocks (highland anorthosite, lunar mare basalt and KREEP rock), we investigate the reflectance spectral characteristics of the lunar rock-forming minerals, including feldspar, pyroxene and olivine. The affecting factors, the variation of the intensity of solar radiation with wavelength and the reflectance spectra of the lunar rocks are studied. We also calculate the reflectivity of lunar mare basalt and highland anorthosite at 300 nm, 415 nm, 750 nm, 900 nm, 950 nm and 1000 nm. It is considered that the difference in composition between lunar mare basalt and highland anorthosite is so large that separate analyses are needed in the study of the reflectivity of lunar surface materials in the two regions covered by mare basalt and highland anorthosite, and especially in the region with high Th contents, which may be the KREEP-distributed region.  相似文献   

18.
Wind erosion seems to be the dominant process eroding crater ejecta deposits and sorrounding materials on Mars. In the equatorial zone, ejecta deposits are eroded back by scarp recession, where scarp heights appear to be approximately equivalent to ejecta thickness. In mantled areas, escarpments develop by relatively rapid deflation of sorrounding aeolian debris, leaving the ejecta deposit (continuous deposit and zone of high density of secondary craters) standing high above sorrounding terrain. If the rate of scarp recession is controlled by the rate of aeolian undercutting of escarpment bases, then recession rates may scale roughly as the inverse with respect to scarp height. Thus, preferential preservation of ejecta deposits emplaced in thickest aeolian debris may occur. An empirical model developed for wind erosion of ejecta deposits in nonmantled areas suggests that removal of ejecta materials on the average is exceedingly slow (～10?5m/yr for 10m high scarp). On the other hand, rapid deflation of aeolian debris around crater ejecta is implied. Results suggest high differential aeolian erosion rates that are a function of both grain sizes and large-scale surface roughness. Aeolian activity on Mars has probably been dominated by rapid recycling of fine-grained debris, the bulk of which formed under more erosive conditions prevalent in the early history of Mars.  相似文献   

19.
David H. Scott 《Icarus》1978,34(3):479-485
Stratigraphic relations between lowland plains and highlands, two major types of Martian geologic-terrain units, were not directly distinguishable on Mariner-9 images. Morphologic characteristics and crater densities suggested that the lava plains beneath their eolian cover were younger than adjacent highland rocks, which form a plateau bounded in many places by highly dissected escarpments. Alternatively, the lowland plains could be the older unit and represent a broad erosional surface exhumed by southward retreat of the highlands along their frontal scarp. Viking photos across five areas of the highland-lowland boundary, however, tend to confirm the younger age of the plains-forming lava flows. A time interval of several hundred million years probably occurred between the retreat of the highland scarp and its latest embayment by lava extrusions in the lowlands.  相似文献   

20.
《Planetary and Space Science》2007,55(13):2015-2024
The Descent Imager/Spectral Radiometer (DISR) aboard the Huygens Probe took several hundred visible-light images with its three cameras on approach to the surface of Titan. Several sets of stereo image pairs were collected during the descent. The digital terrain models constructed from those images show rugged topography, in places approaching the angle of repose, adjacent to flatter darker plains. Brighter regions north of the landing site display two styles of drainage patterns: (1) bright highlands with rough topography and deeply incised branching dendritic drainage networks (up to fourth order) with dark-floored valleys that are suggestive of erosion by methane rainfall and (2) short, stubby low-order drainages that follow linear fault patterns forming canyon-like features suggestive of methane spring-sapping. The topographic data show that the bright highland terrains are extremely rugged; slopes of order of 30° appear common. These systems drain into adjacent relatively flat, dark lowland terrains. A stereo model for part of the dark plains region to the east of the landing site suggests surface scour across this plain flowing from west to east leaving ∼100-m-high bright ridges. Tectonic patterns are evident in (1) controlling the rectilinear, low-order, stubby drainages and (2) the “coastline” at the highland–lowland boundary with numerous straight and angular margins. In addition to flow from the highlands drainages, the lowland area shows evidence for more prolific flow parallel to the highland–lowland boundary leaving bright outliers resembling terrestrial sandbars. This implies major west to east floods across the plains where the probe landed with flow parallel to the highland–lowland boundary; the primary source of these flows is evidently not the dendritic channels in the bright highlands to the north.  相似文献   

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