摘要:

A detailed physical analysis was made of a particulate sample of limonite (1.19‐ to 2.38‐mm ASTM sieve range, Venango County, Pennsylvania) shown previously to produce the best optical and physical match to the Martian bright areas. An ASTM sieve size range as a particulate specification is an ambiguous parameter and is inadequate to describe the particulates when a subsidiary distribution of fines occurs. The 1.19‐ to 2.38‐mm limonite particles are actually flake‐shaped, with an overcoating of<0.5‐μ‐diameter platelet particles; the flake‐shaped particles have over‐all lengths, widths, and thicknesses of 3.44, 2.83, and 1.16 mm, respectively, and specific surface areas of 0.077 m2(determined from a BET calculation). This specific surface area is 70 times the projected area of the particles and could be caused by less than 1% by weight, relative to the 1‐ to 2‐mm particles, of an equivalent<1‐μ limonite powder. Scanning electron microscope photographs revealed a surface roughnes

ISSN:0148-0227    

DOI:10.1029/JB076i026p06213

年代:1971

数据来源: WILEY

摘要:

A surface‐layer representation of the lunar gravitational field has been derived dynamically from the analysis of Doppler observations on both polar and equatorial lunar orbiters. The force model contained 600 discrete masses located on the mean lunar surface between the approximate boundaries of ±60° latitude and ±95° longitude. The derived major mascons were generally in agreement with a model based on polar orbits alone. A technique for combining the discrete mass gravitational field for the front side with a spherical harmonics expansion for the back side is described. Harmonic analysis of the resultant field shows that the higher end of the power spectrum roughly follows the decay rule predicted by W. M. Kaula inNicolaides et al. [

ISSN:0148-0227    

DOI:10.1029/JB076i026p06220

年代:1971

数据来源: WILEY

摘要:

Specular reflection of an arbitrarily polarized plane electromagnetic wave incident on lunar models that consist of discrete plane layers is completely determined by two orthogonal complex reflection coefficients. These measurable quantities permit estimates of the electrical properties and vertical extents of near‐surface plane homogeneous layers and may be diagnostic of pore moisture in any layer. Substitution of a gradational lunar regolith for a homogeneous one does not alter the above comments, but at high frequencies the reflection coefficients pertain only to the properties of the very top of the debris. Reflection from layers that have magnetic properties similar to those measured in the Apollo 11 and Apollo 12 lunar samples yields fictitious estimates of dielectric constant values, significantly lower than actual, if the permeability is ignored in the interpretation process. Theoretical bistatic‐radar results presented at 136.11 MHz clearly show that large changes in reflectivity may be the result of very small changes in debris thickness. When lateral variations exist within the reflecting area, interpretations based on laterally homogeneous layering must be modif

ISSN:0148-0227    

DOI:10.1029/JB076i026p06237

年代:1971

数据来源: WILEY

摘要:

Models of the lunar near‐surface seismic velocity distribution are derived that represent hypothetical moons in which the near‐surface character is due to (1) cold accretion of particles from space and (2) steady‐state impact. Comparison of model travel times with observed travel times favors the cold‐accretio

ISSN:0148-0227    

DOI:10.1029/JB076i026p06246

年代:1971

数据来源: WILEY

摘要:

A gravity survey of the Arctic Ocean was begun in 1960. Gravity observations were obtained from aircraft landings on sea ice. Through 1969 over 800 air‐lifted gravity stations were occupied. Bathymetric, free‐air, and Bouguer gravity anomaly maps have been compiled from ice‐island and air‐lifted gravity data. In addition, the free‐air anomaly data have been averaged over 1° and 5° square areas (i.e., areas 60 and 300 nautical miles on a side, respectively). A plot of 5° free‐air gravity anomaly averages as a function of latitude shows an increasing trend with latitude, implying a gravity value for the pole that is 24 mgal higher than that calculated from the 1930 international formula and 14 mgal higher than that calculated from the 1967 g

ISSN:0148-0227    

DOI:10.1029/JB076i026p06253

年代:1971

数据来源: WILEY

摘要:

Detailed analyses of marine magnetic data that were collected between California and Hawaii south of the Murray fracture zone and north of the Molokai fracture zone reveal an apparent former center for secondary crustal spreading that trends north‐south along the 133°W meridian in this area. This secondary crustal spreading center probably originated as a flank fissure and center of volcanic eruption west of the main center of the East Pacific rise at the time of magnetic anomaly 13 (about 38 m.y. ago) and ceased to be active at about the time of anomaly 7 (about 27 m.y. ago). The center appears to have migrated west at a rate of 2.9 cm/yr during its period of activity, while new crust was spreading from the center at the same rate. This movement, in combination with the movement of 4.0 cm/yr from the primary center of crustal spreading, resulted in a total crustal movement rate of 9.8 cm/yr south of the Murray fracture zone and west of the secondary spreading center. During the 11 m.y. of its existence, the center increased the original 150‐km right lateral transform‐fault displacement along the Murray fracture zone to 680 km by right lateral strike‐slip movement along the fracture. The strike‐slip movement gave rise to such structures as apparent tension gashes across the Murray fra

ISSN:0148-0227    

DOI:10.1029/JB076i026p06265

年代:1971

数据来源: WILEY

摘要:

Results are presented from a detailed bathymetric and magnetic survey of a selected area of the southwest Indian Ocean ridge near 44°S, 38°E. The survey area is traversed by two fault zones, believed to be active transform faults. Outside these fault zones a 1000–2000 gamma peak to peak central magnetic anomaly trends roughly east‐west. Some findings (deep troughs, fault scarps, and apparent offset of magnetic anomalies) are typical of the intersection of a fracture zone with a section of oceanic ridge. Two unusual features of this particular intersection are the width of the zone of fault affected magnetics and a topographic high that is subparallel to the transform faults. The surveyed branch of ridge is currently spreading in a N 20°E direction at about 1.0 cm per year. This agrees closely with an earlier estimate by Morgan and J

ISSN:0148-0227    

DOI:10.1029/JB076i026p06276

年代:1971

数据来源: WILEY

摘要:

A simplified model is proposed that can explain the evolution of marginal offsets, ridges, and fracture zones. Submarine features of the equatorial Atlantic margins are examined and are found to be in reasonable agreement with the model. The results reaffirm the suggestion that the South Atlantic opened in two stages beginning about 140 m.y. and 80 m.y. ago. The pole of rotation describing the relative motion of the South American and African plates changed radically when the constraints imposed by adjacent continental blocks were relaxed.

ISSN:0148-0227    

DOI:10.1029/JB076i026p06283

年代:1971

数据来源: WILEY

摘要:

Several morphologic features associated with the continental margins of North America (Newfoundland fracture zone, Kelvin seamount chain, Cape Fear arch, Bahama platform) and Africa (Canary Islands, Cape Verde Islands, and Guinea fracture zone) are interpreted as structural lineaments delineating lines of crustal weakness which are related to fracture zones and the early opening of the Atlantic. The Newfoundland fracture zone, which is a prolongation of the largest offset in the continental margin, constrained the movement of the separating continents to a single rotation, making adjustments mechanically difficult until the continental edges were separated by a distance of about twice the length of the largest offset. Using the trend of the Newfoundland, Bahama, and Guinea fracture zones, a pole of early opening was computed that is located 25° away from the Newfoundland fracture zone at the northern limit of the basin and 50° from the Bahama fracture zone at the southern limit. The total opening about this pole is 46°. The other fracture zones approximately define concentric circles, and the magnetic discontinuities of Vogt et al. (1970) are isochron lines about this pole. About 80 m.y. ago, when the separated blocks no longer constrained movement, the early opening phase ended, and a complete reorganization occurred, resulting from a migration of the pole toward the north. The major physiographic differences between the mid‐Atlantic ridge and the basin provinces can be explained by the rearrangement of the relative movements of the plates 80 m.y.

ISSN:0148-0227    

DOI:10.1029/JB076i026p06294

年代:1971

数据来源: WILEY

摘要:

Chemical data recently obtained on 18 major and minor elements in 507 tektites from 205 localities are used to map the geographic distribution pattern of Australasian tektites. Areas of distinct chemical type outline a coarse structure to the pattern: an elongate zone of HCa tektites stretching northwest across Australia, a crescent zone of HMg tektites curving from Australia to Indonesia to the Philippines, and a teardrop‐shaped, zone of normal indochinites arcing northeast over Southeast Asia. Various sets of matching polygons of specific gravity and matching chemical analyses of individual specimens define a fine structure compatible with this coarse structure. The over‐all pattern is not radial, as would be required for a terrestrial origin, but is systematically curved. Numerous moon‐to‐earth trajectories were computed for ejecta leaving various lunar craters in a search for compatible places of origin. It was found that the complex tektite distribution pattern is matched by the trajectory landing pattern for ejecta leaving Tycho, and that the required heading direction for this ejecta coincides with one of Tycho's most prominent rays. From a study of visible elements comprising this and other similar rays, a ‘connate crater’ theory of tektite events is formulated which offers a reconciliation of certain tektite observations previously considered contradictory. Some implications to selenology, and comparisons with Apollo data (especially with rock 12013), are briefl

ISSN:0148-0227    

DOI:10.1029/JB076i026p06309

年代:1971

数据来源: WILEY