摘要:

A large number of Mars craters are nearly tangential to other craters. They occur in clusters or as isolated crater doublets. Results of probability calculations and a Monte Carlo cratering simulation model show conclusively that many of the Mars craters could not have resulted from random single‐body impact. The possibility that these craters are calderas is considered, but they could be only if calderas on Mars form by mechanisms different from those on earth. However, clusters and doublets could be caused by meteoroid breakup resulting from stresses induced in the meteoroid by the gravitational field of Mars. Calculations are provided that show the required relationships between mass, entry angle, impact velocity, and tensile strength of the meteoroid, and the separation distance between the centers of the resulting craters relative to crater sizes. It is concluded that, under certain conditions, doublets should be produced on Mars as a direct result of breakup of an impacting meteoroid. The impact process can yield nonrandom crater distributions that should be observed in different degrees of development on different planetary surfaces. If the doublets and clusters result from impact, distributions of large craters on different planetary bodies cannot be compared directly for interpretations of the geologic history of one planetary surface relative to anothe

ISSN:0148-0227    

DOI:10.1029/JB077i014p02419

年代:1972

数据来源: WILEY

摘要:

Martian craters visible on Mariner 6 and 7 imagery show a spectrum of topographic types from very fresh to highly degraded. A method of numerical scoring of rim, wall, and floor is proposed to yield a degradation number to classify each crater. Plots of degradation class versus density of large (diameter>16 km) craters are similar for all four regions studied, whereas similar plots for small (diameter 1–8 km) craters show marked differences between regions. The data suggest general continuity of crater formation and degradation, along with locally sporadic formation and/or degradation of the smallest craters classified. Deucalionis Regio, with an excess of fresh, small craters, experienced an episode of small crater formation (or nondegradation) most recently; Margaritifer Sinus was similarly disturbed at some more remote time. Meridiani Sinus and Hellespontus‐Noachis show little or no sign of excess fresh, small craters. Diameter‐density plots show crater density in the diameter range 1–128 km to be intermediate between lunar terrae and maria; an apparent deficiency of craters less than 2 km in diameter is due to difficulty in seeing highly degraded small craters. Contours of equal density of large craters transect the albedo boundary between Meridiani Sinus and Deucalionis Regio, whereas the pattern of small‐crater density suggests higher average crater density in Deucalionis Regio. No requirement is found for a great Martian episode of catastrophic crater destruction. Caution is suggested in extrapolation from the number of small, fresh craters in anomalous Deucalionis Regio to predict large numbers of Copernicus‐type fr

ISSN:0148-0227    

DOI:10.1029/JB077i014p02433

年代:1972

数据来源: WILEY

摘要:

Artificial meteor ablation was performed on natural minerals composed predominately of magnetite and hematite by using an arc‐heated plasma stream of air. Analysis indicates that most of the ablated debris was composed of two or more minerals. Wustite, a metastable mineral, was found to occur as a common product. The ‘magnetite’ sample, which was 80% magnetite, 14% hematite, 4% apatite, and 2% quartz, yielded ablated products consisting of more than 12 different minerals. Magnetite occurred in 91% of the specimens examined, hematite in 16%, and wustite in 39%. The ‘hematite’ sample, which was 96% hematite and 3% quartz, yielded ablated products consisting of more than 13 different minerals. Hematite occurred in 47% of the specimens examined, magnetite in 60%, and wustite in 28%. The more volatile elements (Si, P, and Cl) were depleted by about 50%. Also the relative abundance of Fe increased as a result of both volatile depletion (loss of Si, P, Cl, and Ca) and reduction in its oxidation state. Hematite was converted to magnetite in the ablation zone along the front face of the sample. Also quartz and apatite minerals reacted with the iron oxide melt to form an Fe‐rich glass consisting of varying amounts of Si, P, Cl, and Ca, depending on the accessory minerals available at the time of melting. These glass phases occurred as unusual myrmekiticlike intergrowths, which are unique textural indicators of the environment through which the material has survived. The chemistry and mineralogy of these phases remain the only trace of the original minerals. This study has shown that artificially created ablation products from iron oxides exibit unique properties that can be used for identification. These properties are morphologic characteristics, textural parameters, and the existence of metastable minerals and depend on the composition of the original material and the environmental conditions o

ISSN:0148-0227    

DOI:10.1029/JB077i014p02442

年代:1972

数据来源: WILEY

摘要:

Two new approaches for measuring gravity are presented. The first approach uses a gradiometer array and adds a small mass to one arm of a gravity gradiometer; the basic formula isg=m(∂g/∂z)/(Δm/Δz). The second approach uses a modified gravity meter and adds a small mass to the beam; the basic formula isg= ΔF/Δm. Both methods could achieve accuracies of about 1 mgal in total gravity, but the gravity meter approach is closer to ‘state of the art’ capability of instrumentation. Finally it seems noteworthy that the gradiometer array could, in theory, be used to measure relative gravity values at different

ISSN:0148-0227    

DOI:10.1029/JB077i014p02456

年代:1972

数据来源: WILEY

摘要:

The equationht= −f(hx,hy,h) is solved by characteristics. The results are interpreted geometrically for erosion problems, and several graphical examples are given. Shock development is examined, as well as the connection with the theory of kinematic waves. Certain effects of fluid flow are then introduced into the model, and an example is give

ISSN:0148-0227    

DOI:10.1029/JB077i014p02460

年代:1972

数据来源: WILEY

摘要:

A technique is presented for the broad‐scale mapping of gross compositional differences in silicate rocks from three medium‐width (2 to 3 μm) spectral channels of thermal infrared scanner data. Ratios of radiances in two of the channels as measured by a two‐element Hg: Cd: Te detector (approximately 8.2–10.9 and 9.4–12.1 μm at 10% responsivity points) from an altitude of 1000 meters are calculated for 25 silicate rocks. The ratios are shown to be nearly linearly correlated with the position of the centers of gravity of the reststrahlen spectral emissivity features, as measured in the laboratory. Further, the ratios are shown to be generally correlated with SiO2content of silicate rocks. A third channel is proposed for correction of temperature variations across the scene. Experimental ratios measured for a sand quarry near Mill Creek, Oklahoma, compare favorably with the predicted ratio for quartz sand, even though a third temperature‐correction channel was not available for the quarry data. Techniques of this type may prove useful for natural resource exploration, as well as for satellite exploration of extraterres

ISSN:0148-0227    

DOI:10.1029/JB077i014p02465

年代:1972

数据来源: WILEY

摘要:

On June 25, 1970, a two‐element infrared detector was flown over a sand quarry near Mill Creek, Oklahoma, as part of a detailed investigation of the emission and reflection properties of common geologic materials. Data from this flight were used to produce an analog‐processed image and a digital recognition map from which exposed quartz sand and quartz sandstone in the vicinity of the quarry are clearly identifiable. The implications of this technique are significant for both terrestrial and planetary remote sensing missi

ISSN:0148-0227    

DOI:10.1029/JB077i014p02473

年代:1972

数据来源: WILEY

摘要:

Coesite has been produced outside its stability field in highly strained samples of Dover flint. Large crystals grew in nine cylindrical samples compressed more than 50% at a strain rate of 10−4sec−1, at temperatures between 450° and 900°C, and at confining pressures of 5–20 kb. Specimens deformed under similar conditions but subjected to less strain contained no coesite. Two coesite‐bearing samples showed textural evidence of reversion to quartz associated with reduction of the local strain rate during deformation. A highly strained specimen annealed at high pressure after deformation contained no coesite. The explanation offered is that the high densities of dislocations developed in greatly compressed specimens increased the free energy of the deformed quartz to a value higher than that of unstrained coesite. In this specimen coesite could grow in preference to unstrained quartz in accordance with Ostawald's law of successive reactions. These observations indicate that care must be taken when natural or experimental environmental pressures are being inferred from polymorphic tr

ISSN:0148-0227    

DOI:10.1029/JB077i014p02478

年代:1972

数据来源: WILEY

摘要:

The electrical conductivity σ of single crystals of olivine of 0, 7.7, 8.2, 9.4, and 26.4 mole % fayalite has been measured up to 1200°C and 7.5 kb. Samples from different localities with approximately the same fayalite content and impurity levels have electrical conductivities that differ by 2–3 orders of magnitude. It is proposed that the oxidation state of the iron in the natural olivines determines σ below 1100°C. No reversible change in activation energy was noted for any sample below 1100°C. In this temperature range activation energies for different samples vary from 0.7 to 2.0 ev. At 1150°C there is a reversible increase in σ, with a poorly determined activation energy of 7–8 ev. Mantle temperatures calculated from the present data on olivines with almost the same fayalite content differ by more than 700°C at any depth. The temperature difference becomes even larger with depth if the pressure effects noted in this stu

ISSN:0148-0227    

DOI:10.1029/JB077i014p02483

年代:1972

数据来源: WILEY

摘要:

The compression of a natural Mg‐rich garnet to pressures greater than 100 kb was studied by a high‐pressure in situ X‐ray powder‐diffraction method. A quadratic least‐squares fit to the data yields a rough value for the bulk modulus of 1750 kb. The results are in agreement with the compression data found by Takahashi and Liu (1970), who used a different high‐pressure X‐ray diffraction technique, and with the results of Soga (1967), who used an ultraso

ISSN:0148-0227    

DOI:10.1029/JB077i014p02496

年代:1972

数据来源: WILEY