摘要:

We examined the distribution of surface slopes of a variety of terrestrial surfaces by field measurement, representing surfaces formed by a wide range of processes, and compared the results to planetary radar data. Slope distributions of the measured surfaces differed considerably from the distributions assumed by accepted models of radar scattering. The rms slope values for the terrestrial surfaces ranged from 0° to 16.5°, compared to a range of rms slope values of<1° to about 10° for planetary surfaces as inferred from radar observations. While the great majority of planetary surfaces observed by radar have rms slope estimates in the lower end of this range (<5°), nearly all of the terrestrial surfaces we measured have rms slope values greater than 5°. We also used Hagfors' model of radar scattering to predict the return that would be expected from surfaces where two discrete surface types were present within the radar field of view and found that the shapes of the resulting slope distributions differed from those predicted by the Hagfors model for homogeneous surfaces. Additionally, the resulting best fit rms slope was a nonlinear combination of those of the pure surfaces, emphasizing the smoother surface. Together, these results suggest that current methods of determining surface roughness from radar may significantly underestimate the roughness of planetary surfaces and that the derived rms slope can best be used as a qualitative guide to the physical interpretation of actual surface prope

ISSN:0148-0227    

DOI:10.1029/92JE02544

年代:1993

数据来源: WILEY

摘要:

Planar impact experiments were employed to induce dynamic tensile failure in Bedford limestone. Rock discs were impacted with aluminum and polymethyl methacralate flyer plates at velocities of 10 to 25 m/s. This resulted in tensile stresses in the range of ∼11 to 160 MPa. Tensile stress durations of 0.5 and 1.3 μs induced microcrack growth which in many experiments were insufficient to cause complete spalling of the samples. UltrasonicPandSwave velocities of recovered targets were compared to the velocities prior to impact. Velocity reduction, and by inference microcrack production, occurred in samples subjected to stresses above 35 MPa in the 1.3‐μs PMMA experiments and 60 MPa in the 0.5‐μs aluminum experiments. Apparent fracture toughnesses of 2.4 and 2.5 MPa m1/2are computed for the 1.3‐ and 0.5‐μs experiments. These are a factor of ∼2 to 6 greater than quasi‐static determinations. Three‐dimensional impact experiments were conducted on 20 cm‐sized blocks of Bedford limestone and San Marcos gabbro. Compressional wave velocity deficits up to 50–60% were observed in the vicinity of the crater. These damage levels correspond to O'Connell and Budiansky damage parameters of 0.4 as compared to the unshocked rock. The damage decreases as ∼r−1.5from the crater indicating a dependence on the magnitude and duration of the tensile pulse. Using the observed variation in damage with tensile stress from the one‐dimensional experiments, and estimates of the variation of peak dynamic tensile stress and tensile stress duration with distance from an impact on an elastic half‐space, the observed dependence of damage with radius in the three‐dimensional experiments are theoretically predicted and compa

ISSN:0148-0227    

DOI:10.1029/92JE02679

年代:1993

数据来源: WILEY

摘要:

This paper provides an estimate of the average number density of dust particles accumulated around Mars. As investigated earlier, submicron‐sized grains (a<1 μm) are perturbed by electromagnetic forces and solar radiation pressure so that they do not stay close to the Keplerian orbit of the ejecting satellite. The lifetime of these particles in the vicinity of Mars ranges from a few days to a few hundred days. Their trajectories are contained in an approximately spherical volume. For larger grains (1 μm

ISSN:0148-0227    

DOI:10.1029/92JE02681

年代:1993

数据来源: WILEY

摘要:

Fourier Transform Infrared Spectroscopy has been used to study homogeneously nucleated hydrazine (N2H4) aerosols formed in a variable temperature flow cell. The spectra contain both resonant absorptions and Mie scattering features which allow detailed information to be obtained on the phase of the aerosol particles and the size distribution that is formed. In addition, the relative intensities of the absorption and scattering features can be used as a qualitative probe of the density of the particles. A disordered phase is observed at cell temperatures ranging from 230 to 180 K and a transition to a crystalline phase occurs over the temperature range 180–175 K. At lower temperatures the infrared spectra become progressively narrower, indicating that the quality of the crystals improves as the temperature is lowered. The result is previously unobserved details in the hydrazine N‐H stretch vibrations of the crystalline solid. The high quality of the spectra obtained under these conditions suggests that at the high condensation rates characteristic of the present experiments, self‐annealing yields highly crystalline particles. Mie calculations, based upon the complex refractive index data for amorphous solid hydrazine, are shown to have utility in modeling the infrared spectrum of the disordered phase, which we tentatively assign as a supercooled l

ISSN:0148-0227    

DOI:10.1029/92JE02578

年代:1993

数据来源: WILEY

摘要:

Laboratory simulations have been carried out to test the possibility that interactions of SO2and silicates can produce Na‐S compounds which can account for the observed surface enrichment of Na relative to Si on Io. A wide variety of silicate compositions were heated under oxidizing conditions with a SO2/O2= 1 gas phase at a mid‐level crustal temperature for Io (1123K). All experiments produced sulfate deposits on the silicate surfaces. The nature of the sulfate changed systematically with the silicate Ca/Na ratio, with mixtures of CaSO4and Na‐rich sulfate formed by basaltic compositions having higher Ca/Na but only alkali‐rich sulfates formed for more granitic (low Ca/Na) compositions. For crystalline albite and an albite‐orthoclase eutectic glass composition, K and Al‐rich sulfates were formed. Assuming that burial of SO2to the temperatures of our experiments is plausible on Io and that somewhat less oxidizing conditions do not qualitatively change the results, SO2‐rock interactions producing Na‐rich sulfates could account for the Io surface enrichment in Na. Observations on minor elements, such as K or Ca, in the atomic cloud or in magnetospheric ion spectra could be used to identify sulfates as Io surface phases and, conceivably, even define Ionian crustal magma types. Many analyses show significant S excesses which are best interpreted as due to the presence of bisulfate (HSO4.) components, and NaHSO4deserves consideration as an Io surface mineral. The possibility of elemental S on Io can be regarded as a totally separate issue from the problem of the surfac

ISSN:0148-0227    

DOI:10.1029/92JE02509

年代:1993

数据来源: WILEY

摘要:

The Gulf Coast province provides an array of scientific dilemmas ranging from the origin of the gulf itself to the causes and effects of long‐lasting circulation of hot, deep waters throughout the thick sedimentary section. The nature of the underlying crust and superjacent sediments and their contained waters; the precise timing of rifting; depositional history and diagenesis of the sedimentary sequence; fluid dynamics; geochemistry; hydrocarbon generation and migration; thermal history, including unusually high thermal gradient; and the fluid pressure regime in the deep sedimentary section are too poorly understood to permit quantitative analysis of processes that are of enormous scientific and practical importance. The area centered on DeWitt and Victoria counties, Texas, on the southeastern extension of the San Marcos arch, is probably the best location for a deep borehole to investigate these important phenomena and problems. The arch extends southeastward from exposed Grenville‐age basement rocks of the Llano uplift and separates the deep South Texas and Houston embayment salt basins. Seaward of the Llano uplift, highly deformed and slightly metamorphosed rocks of the Ouachita‐Marathon orogen have been intersected beneath Cretaceous sediments. The inferred edge of continental crust underlies an extensive Lower Cretaceous reef trend southeast of known Ouachita orogen rocks. Rapid thickening of Tertiary and possibly of Cretaceous sediments southeast of the shelf edge, together with geophysical indications of a relatively shallow Moho, suggests that “transitional continental crust” underlies sediments basinward of the inferred edge of continental crust. This transitional crust, the ultimate objective for a proposed deep well, could be rifted Grenville basement, buried rocks of the Ouachita trend, an island arc related to the Ouachita trend, or exotic continental basement related to a proto‐South American continent. To achieve optimum results to guarantee adequate basement penetration, a borehole should be designed to penetrate a relatively thin succession of lower Mesozoic synrift, graben‐fill sediments seaward of the shelf edge. Because of expected high temperatures and pressures, new technologies will need to be developed to successfully drill and

ISSN:0148-0227    

DOI:10.1029/92JE02893

年代:1993

数据来源: WILEY