摘要:

Numerical simulations of Martian neutron leakage flux spectra are made in order to explore the detectability of magnesium and calcium carbonate deposits using neutron spectroscopic techniques from orbit. The primary signature of such deposits is found to be an enhanced thermal amplitude. Although this enhancement is weakened by (1) partial burial beneath an aeolian regolith blanket, (2) admixture with regolith on a microscopic (centimeter) or macroscopic (tens of centimeters) scale, and (3) reduction in the areal size of the deposit, near‐surface stratigraphies of carbonates hypothesized by some authors as possible on Mars are still detectable by simple neutron sensors from orbit. However, the large variations in the magnitude of the thermal neutron enhancements caused by different carbonate deposit configurations found in this study require a combined gamma ray and neutron analysis for their unique specificatio

ISSN:0148-0227    

DOI:10.1029/91JE01489

年代:1991

数据来源: WILEY

摘要:

Finite element simulations of axisymmetric spherical shell compressible convection were carried out to investigate the effect of various surface boundary conditions in a Venusian mantle. We employed a thermal expansivity α which decreased with depth, a uniform viscosity an order of magnitude greater than the Earth's, and zero and chondritic quantities of internal heating. As long as hot plumes from the core‐mantle boundary were strong, the convection pattern was typical of that for variable α flow; that is, it was characterized by steady upflowing regions, unsteady collections of downflowtng plumes, and large aspect ratio cells. Increases in the internal heating or the temperatureT0at the lop of the convecting layer weakened the hot plumes and therefore decreased the width of the cells. A rigid surface increased the internal temperature and also decreased the widih of convection cells. Extensive regions of subadiabaticity were found in the mantle. We compare our results with those for fully three‐dimensional convection under similar conditions (Schubert et al.,

ISSN:0148-0227    

DOI:10.1029/91JE01193

年代:1991

数据来源: WILEY

摘要:

Ground‐probing radar has provided new two‐dimensional stratigraphic and structural information on Meteor Crater, Arizona. Four 50‐MHz transects inside the crater and one 100‐MHz transect on a northeast radial across the exterior ejecta blanket were carried out. The frequencies used were in relation to the desired depth of investigation: greater than 50 m at 50 MHz and ∼30 m for 100 MHz. The results obtained inside the crater show that the water table can be clearly detected at depths of ∼65 m, as can the contact between the crater wall and the infilling allochthonous breccia lens. In addition, we were able to detect (1) point reflectors from relatively coherent blocks (>1.5m) within the breccia lens, (2) a zone ∼7 m thick relatively free of point reflectors from the immediate fall back unit, (3) a series of subhorizontal reflectors a few meters apart due to bedding in the post impact crater sediments, and (4) five separate subhorizontal reflectors from what we interpret to be a series of postimpact paleo‐water table levels in the breccia lens. The ejecta blanket transect indicates a ∼4–10 m thick ejecta layer and the underlying Moenkopi Formation sandstone, as well as the Moenkopi‐Kaibab contact and bedding within the Kaibab Formation. In addition, reverse faults with displacements up to 4 m were detected in the bedrock, as well as changes from subhorizontal to ∼30° in the dip of the bedrock as the crater rim is approached from the northeast. Beneath the ejecta blanket, the total uplift observed at the crater rim is around 55 m. The radar data indicate uplift effects out to a radial distance of ∼400 m from the rim, farther than that determined from drilling (∼300 m). The results demonstrate the utility of continuous ground‐probing radar cross sections in studying the near‐subsurface (∼50 m) character at Meteor Crater. This relatively rapid, low cost technique can be extended to other structures, but confident interpretation requires sup

ISSN:0148-0227    

DOI:10.1029/91JE01114

年代:1991

数据来源: WILEY

摘要:

We have investigated the glaciological causes of large meteorite concentrations on blue ice fields west and southwest of the Allan Hills in Antarctica. A sub‐ice topography map for the area was prepared from data of a radio echo sounding survey. The map reveals a mesa‐type paleosurface. It was formed prior to and modified by glacial processes during the initial stage of glaciation of Antarctica. Ice flow toward Mawson Glacier north of the Allan Hills is largely confined to a N‐S trending depression between the Allan Hills Ice Field and the Near Western Ice Field. Blue ice at the margins of the ice stream flows over the mesas on both sides of this depression. Meteorites entrained in blue ice are uncovered by sublimation and ablation. We present arguments showing that currently almost all of the blue ice flowing into the Allan Hills Ice Field is sublimated, leaving meteorites on the ice surface. We present evidence that windblown meteorites are trapped by snow bridges across crevasses, resulting in a near‐surface meteorite concentration near the ice ridges of the Allan Hills Ice Field. Nevertheless, most meteorites are exposed for only a short period of time to the atmosphere before they are blown by the wind across the ice toward the western foot of the Allan Hills, from where they are slowly carried northward to Mawson Glacier and the R

ISSN:0148-0227    

DOI:10.1029/91JE01117

年代:1991

数据来源: WILEY

摘要:

Numerical simulations of Martian neutron leakage flux spectra are made in order to explore the detectability of magnesium and calcium carbonate deposits using neutron spectroscopic techniques from orbit. The primary signature of such deposits is found to be an enhanced thermal amplitude. Although this enhancement is weakened by (1) partial burial beneath an aeolian regolith blanket, (2) admixture with regolith on a microscopic (centimeter) or macroscopic (tens of centimeters) scale, and (3) reduction in the areal size of the deposit, near‐surface stratigraphies of carbonates hypothesized by some authors as possible on Mars are still detectable by simple neutron sensors from orbit. However, the large variations in the magnitude of the thermal neutron enhancements caused by different carbonate deposit configurations found in this study require a combined gamma ray and neutron analysis for their unique specificatio

ISSN:0148-0227    

DOI:10.1029/91JE00971

年代:1991

数据来源: WILEY

摘要:

Ridged plains are a major geologic unit on Mars and are a predominant unit on the Tharsis Plateau, a region that has undergone uplift, extension, and extensive shield and flood volcanism. These units, probably flood volcanic in origin, are characterized by landforms classed as wrinkle ridges. Wrinkle ridges are interpreted to be folds, resulting from buckling followed by reverse to thrust faulting (flexure‐fracture). A prominent characteristic of many of the wrinkle ridges on Mars, particularly those in the Tharsis ridge system, is the periodic nature of their spacing. The periodic spacing has been evaluated in six major provinces on the Tharsis Plateau with regions further divided into domains based largely on variation in ridge orientation. The average spacing of the Tharsis ridges, based on 2934 measurements, is 30 km. In an effort to account for the periodic nature of the wrinkle ridges, the ridged plains material is modeled as both a single member and a multilayer with frictionless contacts that has buckled at a critical wavelength of folding. Free slip between layers is assumed based on the possible existence of mechanically weak interbeds in the ridged plains sequence separating groups of flows. The near‐surface mechanical structure in the area of ridged plains is approximated by a strong layer or layers overlying a weak megaregolith of finite thickness overlying a strong basement. Buckling of the ridged plains is assumed to be decoupled from the basement by the weak megaregolith. Decoupling is enhanced if the megaregolith were water‐ or ice‐rich at the time of deformation. The rheologic behavior of the ridged plains and megaregolith is approximated by a linear elastic and linear viscous material. The models are examined for a range in (1) the strength contrast between the ridged plains material and the underlying megaregolith; (2) thickness of the ridged plains material; (3) thickness of the megaregolith; and (4) number of layers. The elastic model can explain the observed ridge spacing only if the ridged plains are relatively thick (≥2400 m) with multiple layers (n>1) and relatively high contrasts in Young's modulus (E/E0≥ 100). The highE/E0required is possible only if the megaregolith was water‐rich at the time of deformation. Over the same range in values of the parameters, viscous buckling is much less restricted. The viscous model can explain the observed ridge spacing over a wide range in ridged plains thickness (250 m to several kilometers), megaregolith thickness, number of layers (n= 1 to 8) and contrasts in viscosity (η/η0≥ 10). In addition, viscous buckling is viable if the megaregolith were dry, water‐rich or ice‐rich at the time of deformation. The absence of folds with a cross‐sectional geometry in the shape of a sinusoid (anticline‐syncline pairs) may be the result of initial deformation of the ridged plains material into low‐amplitude folds (in infinitesimal strain) followed by plastic yielding in the cores of the anticlines (in finite strain). Initial elastic or viscous buckling, coupled with plastic yielding confined to the hinge area followed by the development of reverse to thrust faulting, could account for the asymmetric fold geometry of

ISSN:0148-0227    

DOI:10.1029/91JE01402

年代:1991

数据来源: WILEY

摘要:

We present a new compilation of the structural and stratigraphic evolution of the Tharsis region of Mars that incorporates recent advances in understanding its stratigraphy, and we introduce a lithospheric deformation model that can account for the observations. The first period in the formation of Tharsis occurred in Late Noachian/Early Hesperian time with the deposition of volcanic plains materials throughout the surrounding highlands (e.g., Lunae Planum) and on the Tharsis rise (which includes the giant volcanoes and surrounding, elevated lava fields). Extensive radial normal faulting occurred on the rise, locally extending outward at Valles Marineris and Tempe Terra, and concentric wrinkle ridges formed along the edge of the rise. This regional deformation appears to have been modulated by a global compressional stress field due to rapid planetary cooling and contraction. The second period occurred during the Late Hesperian/Amazonian with Tharsis volcanism centered on the rise and radial extensional deformation that extended from the center of the rise for thousands of kilometers. We propose a model in which the lithosphere beneath Tharsis consists of a thin elastic crustal cap on the rise that is mechanically detached from the strong upper mantle by a volcanically thickened, hot, weak lower crust. These layers merge into a single cooler, strong lithospheric layer around the edges of the rise. This model is capable of generating large extensional hoop stresses throughout much of the western hemisphere, in agreement with observations. The tectonic interpretation of the stresses predicted by this model requires the reconciliation of extensional strain within narrow grabens and compressional strain within wrinkle ridges with (1) processes in the deeper lithosphere, (2) the sparsity of strike‐slip faults, and (3) other global or locally important stress fields. Stresses predicted by global models affect the entire thickness of the lithosphere, and they can be reconciled with narrow, closely spaced grabens that accommodate large amounts of extensional strain in the upper few kilometers of the lithosphere if the grabens are underlain and kinematically linked with dikes or other tension cracks, such as hydrofractures. Deeper levels of the lithosphere can accommodate this strain by elastic expansion if grabens are spaced far apart (many tens to hundreds of kilometers). Mechanical considerations suggest that deformation on faults beneath wrinkle ridges could extend through a significant thickness of the brittle crust. A number of factors, including stresses generated by the addition of overburden, intrusion of dikes, weakness of geologic materials under extension, and the laterally constrained nature of a single lithospheric plate, may have inhibited the formation of strike‐slip faults on Mars. Stresses generated from the removal of overburden could have augmented planetwide wrinkle ridge formation during the Late Noachian/Early Hesperian and in Kasei Valles and western Chryse Planitia during the Early Amazonian. The nonuniform distribution of tectonic features around Tharsis can be understood in terms of the concentration of regional stresses and strain near weaker volcanotectonic cent

ISSN:0148-0227    

DOI:10.1029/91JE01194

年代:1991

数据来源: WILEY

摘要:

New depth/diameter (d/D), rim, and peak height and width measurements of fresh craters on Ganymede and Callisto are utilized to explore not only crater mechanics on icy satellites but intersatellite crater scaling and crustal properties and composition. Significant systematic differences in crater morphology on icy and rocky planets are confirmed. Simple‐to‐complex transition diameters on icy satellites, including Ganymede and Callisto, are much lower than those on rocky planets. Also, complex crater depths and rim heights on Ganymede and Callisto are inherently 60–70% shallower than lunar complex craters, despite similar surface gravity. Hence viscous relaxation is not as important as is generally assumed. Central peaks on most icy satellites are 1–2 km higher and considerably wider with respect to diameter than for similar‐sized lunar craters. Terracing and rim slumping is also rare in these craters. Central peaks in craters on Ganymede (and Callisto) larger than −15 km, however, are relatively small in both width and height, however, correlating with the occurrence of rim slumping. Thus central peak dimensions are an indirect indicator of the degree of rim slumping. Transition diameters for the occurrence of central peaks and rim slumps and ford/Dcurve inflections for icy satellites scale approximately with the inverse of surface gravity, except possibly at very lowg. The transition diameters for central peaks and for rim slumping differ, however, by 5–15 km on Ganymede and Callisto but by 50 or more km on the smaller icy satellites. These large differences suggest thatd/Dcurves on icy satellites are comprised of three segments of decreasing slope: one for smaller simple craters, a second for central peak craters, and a third for larger true complex craters with both central peaks and rim slumping. The shallow slopes ofd/Dcurves on Ganymede and Callisto resemble that of the Moon, consistent with the occurrence of both central peaks and rim failure. The high slopes ofd/Dcurves on the middle‐sized icy satellites indicate that they are middle segments, consistent with the dominance of central peaks in those craters. Complex crater depths on icy satellites in general scale with the inverse of surface gravity. The depths of all the observed large basin‐scale craters on these small satellites, when scaled to Ganymede gravity, are very similar to those of complex craters on Ganymede (and Callisto). These large craters, including Odysseus (425‐km diameter) on Tethys, are probably true complex craters that have not undergone significant viscous relaxation. Ithaca Chasma on Tethys probably formed during the prompt collapse of the transient Odysseus crater. Crater morphology is clearly controlled by both gravity and by large variations in material properties (i.e., composition) and should be useful in constraining bulk crustal composition (i.e., ice‐rock ratios). The similarity of various morphological transition diameters and complex crater depths on Ganymede and Callisto, two geophysically very similar but geologically divergent large icy satellites, indicates that the crusts of both bodies are dominated by water ice with only a

ISSN:0148-0227    

DOI:10.1029/91JE00932

年代:1991

数据来源: WILEY

摘要:

Most of the major Saturnian and Uranian satellites have undergone some form of extensional tectonism and/or resurfacing early in their histories. We examine the stresses created by the early thermal evolution of these satellites and consider the geologic implications of these stresses. Starting from an initial accretional temperature profile, we calculate the satellites' temperature profiles as they evolve by conduction. Solid state convection, when it occurs, is handled using a simple parameterized convection scheme. The effects of radiogenic heating and release of heat of phase change are included. The thermal stresses built up in the satellites are then calculated, assuming the satellites to have a Maxwell viscoelastic rheology. The stress calculation takes into account both the effects of thermal stresses and of shifts in the position of the ice I/ice II phase boundary in some satellites. If accretion takes place in a cold, gas‐free environment, the stresses that build up in the smaller satellites (Mimas, Miranda, and Enceladus) are minor. In the larger satellites, however, stresses are large enough to cause fracturing and extensional tectonism under these circumstances. The stresses result largely from rapid cooling contraction of a thin outer shell; the ice II → ice I phase transition also contributes significantly for the largest satellites. If accretion occurs in a warm gaseous nebula that dissipates rapidly, stresses large enough to cause failure are attained in all of the satellites. In all cases, the material immediately underlying the zone of failure is the warmest in the satellite and hence may be buoyant and mobile enough to be extruded to the surface through fractures created by the extension. We conclude that thermal stresses following accretion led to a substantial early episode of extensional tectonism and perhaps also associated resurfacing on many of the Saturnian and Uranian satellites. If evidence of this early episode of extensional tectonism has survived subsequent impacts, then the process may have been responsible for much of the early tectonism and resurfacing observed on these satellites. If not, then it may only be responsible for the very early global resurfacing that some of the satellites are inferred to have underg

ISSN:0148-0227    

DOI:10.1029/91JE01401

年代:1991

数据来源: WILEY

摘要:

A control network for Triton has been computed using a bundle‐type analytical triangulation program. The network contains 105 points that were measured on 57 Voyager 2 pictures. The adjustment contained 1010 observation equations and 382 normal equations and resulted in a standard measurement error of 13.36 μm. We determined coordinates of the control points, the camera orientation angles at the times when the pictures were taken, and Triton's mean radius. A separate statistical analysis confirmed Triton's radius to be 1352.6±2.4 km. Attempts to tie the control network around the satellite were unsuccessful because discontinuities exist in high‐resolution coverage between 66° and 289° longitude, north of 38° latitude, and south of 78°

ISSN:0148-0227    

DOI:10.1029/91JE00976

年代:1991

数据来源: WILEY