摘要:

Relic tektites are associated with a Pt‐group metal abundance anomaly and shocked minerals in a thin marl bed that marks the K‐T boundary on Haiti. The presence of these three impact‐produced materials at the precise K‐T boundary enormously strengthens the Alvarez impact extinction hypothesis. The tektites occur in smectite spherules that have external shapes typical of tektites. Their chemical and physical properties are broadly similar to those of other tektite groups, except that the Haitian tektites have lower Si and higher Fe, Ca, and Na. On average, they contain more Sc, V, Cu, Zn, Ga, Sr, Sn, and Ba and less Cr, Ni, Co, B, Mn, and Hf than Other tektite groups. Amounts of rare earth elements (REE) in the tektites indicate that their progenitor materials were not melted mafic or ultramafic oceanic crust; rather they were sedimentary deposits having a bulk composition of andesite. Rare tektites contain unusually high amounts of CaO (∼20%) and S (0.4%), and these data suggest that some target materials consisted of CaSO4. Anhydrite beds occur in the subsurface at two candidate impact sites (Chicxulub and Manson). Sm‐Nd isotopic data for the tektites indicate that the melted precursor sediments were most likely deposited less than −400 m.y. between the Late Cretaceous and Silurian periods. Major chemical changes accompanied the diagenetic change of glass to smectite. The Haitian tektites are the first datable impact products in K‐T boundary rocks, and40Ar‐39Ar ages of the glass show that the K‐T boundary and impact event are co

ISSN:0148-0227    

DOI:10.1029/91JE02249

年代:1991

数据来源: WILEY

摘要:

Photoclinometry has become a popular technique for the extraction of topography from digital images of planetary surfaces. The technique, however, is subject to a number of error sources that can degrade results significantly. We present here a quantitative analysis of error sources in one‐dimensional planetary photoclinometry. The technique is affected by at least seven error sources, which can be broken down into three categories: those arising from (1) the spacecraft image, (2) the planetary body, and (3) the scan line orientation. Spacecraft image error sources include noise, background offset, and quantization. Errors deriving from the planetary body include those induced by variations in photometric properties, such as albedo, by incorrectly compensated atmospheric effects, and by an incorrectly determined photometric function. Finally, errors will result for scan lines which do not lie perpendicular to topographic strike. We calculate slope errors for each of these sources, using the examples of Voyager imaging of Ganymede and Viking orbiter imaging of Mars. Slope errors are investigated for a variety of viewing and lighting geometries, slope angles, and slope orientations. The results can be broken down into nonsystematic and systematic errors. Nonsystematic errors are introduced by image noise and quantization and affect the slope calculation for each picture element independently. Other error sources are systematic; these errors are more insidious, since they may retain the general appearance of the topography while approximately scaling all relief by a multiplicative constant. We present derivations that allow the calculation of photoclinometric slope errors for any photometric function and also briefly discuss the implications of our results for two‐dimensional photoclinometric techniq

ISSN:0148-0227    

DOI:10.1029/91JE02209

年代:1991

数据来源: WILEY

摘要:

Widths of slump terraces of complex craters can be used to determine the effective cohesion of the cratered region during crater collapse. We have measured terrace widths for complex craters on Mercury: these generally increase outward toward the rim for a given crater, and the width of the outermost major terrace is generally an increasing function of crater diameter. Similar observations apply to lunar complex craters, but the widths of the outermost slump terraces of Mercurian complex craters are less than those of similarly sized lunar complex craters. Using the terrace widths on Mercury and a gravity‐driven slump model, we estimate the strength of the cratered region immediately after impact (specifically, during the modification stage of crater formation) to be ∼1–2 MPa. Comparison with the previous study of lunar complex craters by Pearce and Melosh indicates that the transient strength of cratered Mercurian crust is no greater than that of the Moon. The strength estimates only vary slightly with the geometric model used to restore the outermost major terrace to its precollapse configuration and are consistent with independent strength estimates from the simple‐to‐complex crater depth/diameter transition, in particular, the most recent depth/diameter study of Mercurian craters by Pike. Thus, contrary to previous work, the difference in “target properties” between Mercury and the Moon may be small, and systematic morphological differences between craters on the two worlds may be largely caused by the factor of 2 difference in su

ISSN:0148-0227    

DOI:10.1029/91JE02248

年代:1991

数据来源: WILEY

摘要:

Coronae on Venus are circular to elongate structures with maximum widths of 150–1000 km characterized by annuli of concentric ridges surrounding complex interiors. The features have raised topography relative to the surroundings, they are associated with volcanic activity, and most are partially surrounded by a peripheral trough. Variations in morphology between individual coronae are due to differences in their stage of evolution and/or differences in the relative significance of the geologic processes that occur in each stage. We examine models for three processes that may be involved in corona origin and evolution: (1) a hotspot or rising mantle diapir model, (2) a sinking mantle diapir model, and (3) gravitational relaxation of topography. Rising mantle diapirs are caused by heating at depth (e.g., hotspot), while sinking mantle diapirs may result from cooling or a phase change causing increased density and negative buoyancy at the base of the lithosphere. The hotspot model is most consistent with the major characteristics of coronae, with gravitational relaxation occurring as a modificational process. The sinking mantle diapir would produce dominant central compression that has not been observed at coronae; however, higher‐resolution image and altimetry data from Magellan can be used to distinguish more fully between the two models. Coronae in various states of formation and degradation can be identified in the Venera 15/16 data, suggesting that the process may be continuing to

ISSN:0148-0227    

DOI:10.1029/91JE02218

年代:1991

数据来源: WILEY

摘要:

The Equatorial Highlands of Venus consist of four main structures, Atla, Beta, Ovda, and Thetis regiones. Each has a circular to oval‐shaped planform and rises 4–5 km above the mean planetary radius. These highlands are associated with long‐wavelength geoid highs, with amplitudes ranging from 35 m at Ovda to 120 m at Atla. They also contain topographic valleys, interpreted as extensional rift zones, and Beta is known to contain shield volcanoes. These characteristics are all consistent with the Equatorial Highlands being formed by mantle plumes. An alternative model, in which Ovda and Thetis are interpreted as spreading centers analogous to terrestrial mid‐ocean ridges, fails to explain most of the observed geoid anomalies and topography in these regions. Some smaller highlands, such as Bell Regio, Eistla Regio, and the Hathor/Innini/Ushas region, may also be plume related, but most coronae are unlikely to be the direct result of plume activity. We have modeled plumes using a cylindrical, axisymmetric finite element code and a depth‐dependent, Newtonian rheology. We compare our model results with profiles of geoid and topography across Atla, Beta, Ovda, and Thetis; our best model fits are for Beta and Atla. Assuming whole mantle convection and that Earth and Venus have similar mantle heat flows, Venus must lack an Earth‐like low‐viscosity zone in its upper mantle in order satisfy the observed geoid and topography for these features. This conclusion is consistent with the long‐wavelength admittance spectrum of Venus and with the observed differences in the slopes of the geoid spectra for the two planets. One explanation for the different viscosity structures of the two planets could be that the mantle of Venus is drier than

ISSN:0148-0227    

DOI:10.1029/91JE02221

年代:1991

数据来源: WILEY

摘要:

The Ishtar Terra region contains the highest topography known on Venus, over 10 km above mean planetary radius, as well as abundant tectonic features, many of apparently compressional origin. These characteristics suggest that Ishtar is a crustal convergence zone overlying downwelling mantle. In order to explore quantitatively the implications of this hypothesis for Ishtar's origin, we present models of viscous crustal flow driven by gradients in topography. Assuming a free‐slip surface boundary condition, we find that if the crustal convergence hypothesis is correct, then the crustal thickness in the plains surrounding Ishtar can be no more than about 25 km thick. This upper bound assumes a cold (10 K km−1) geotherm and the stiffest available diabase flow law. If the geothermal gradient is larger or the rheology is weaker, the crust must be even thinner for net crustal convergence to be possible. This upper bound is in good agreement with several independent estimates of crustal thickness of 15–30 km in the plains of Venus based on modeling of the spacing of tectonic features and of impact crater relaxation. If the surface layer of Venus provides a no‐slip boundary, then our models allow the crustal thickness in the plains to be up to 50 km, but the likely existence of faults that cut through the crust makes a no‐slip surface layer unlikely. Our upper bound on crustal thickness is much less than that derived from an Airy isostasy model of Ishtar's gravity anomaly. Much of the observed gravity anomaly must be due to density anomalies in the mantle beneath Ishtar. Although we treat Ishtar as a crustal convergence zone, our crustal flow model shows that under some circumstances near‐surface material may actually flow away from Ishtar, providing a possible explanation for grabenlike structures in Fort

ISSN:0148-0227    

DOI:10.1029/91JE02219

年代:1991

数据来源: WILEY

摘要:

Using the width of concentric grabens formed during the relaxation of the Asgard basin on Callisto, in combination with rheological flow laws of ice and laws concerning the variations in strain rate around a relaxing crater, the surface thermal gradient of Callisto at −4 b.y. is estimated to be near 0.5°K km−1. It is assumed that the graben width depends on the depth of the brittle/ductile transition zone. Around the Asgard basin, the depth of this brittle/ductile transition zone varies from 23 km (at 200 km from the crater rim) to 4 km (at 800 km from the crater rim). It is assumed that the thermal characteristics were constant over the extended zone and that the difference in the depth of the brittle/ductile transition zone is only due to differences in the strain rate around the basin during the relaxation of the initial cavity. Calculations show that Newtonian flow law for ice gives a strain rate of external relaxation at the crater rim of about 10−14s−1, while a power law gives a strain rate of about 10−6s−1, assuming a surface temperature between 100 and 150°K. The order of magnitude of the calculated thermal gradient is independent of the chosen flow law. This calculated gradient is, however, one order of magnitude lower than those calculated theoretically with a chondritic abundance of radioactive elements in the silicate par

ISSN:0148-0227    

DOI:10.1029/91JE02220

年代:1991

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/91JE02290

年代:1991

数据来源: WILEY