摘要:

In a previous paper we calculated the upper limit of H2O density in the lunar atmosphere using the Explorer 35 data on the lunar perturbation of the solar wind. In this paper we search for various H2O sources on the moon and find all possible sources to be incapable of producing H2O density equal to the upper limit except possibly degassing from the lunar interior. The degassing rate is inversely proportional to the depth at which an H2O source can exist. Thus the upper limit of the atmospheric H2O density puts a lower limit on the depth (or provides a minimum depth) of a uniform H2O source in the lunar interior. We give a table of minimum depths for various subsurface H2O source temperatures and soil permeabilities. These depths are greater than the expected depths of lunar regolith; we therefore conclude that the minimum depth at which an H2O source can exist is at some depth in the bedrock. If we accept the lunar subsurface temperatures measured with the Apollo 15 heat flow experiment package and extrapolate these measured data to greater depths, it appears possible that H2O of the source, which may exist at the minimum or greater depths, is in the form of liquid.

ISSN:0148-0227    

DOI:10.1029/JB078i011p01741

年代:1973

数据来源: WILEY

摘要:

Several, mostly new, finite parameter instances of a special class of map projections are derived, illustrated, and evaluated. It is demonstrated that most of the usual evaluation measures are ordinally equivalent and thus that any one of them suffices. The new projections are shown to be at least as good as existing projections and appear otherwise advantageous.

ISSN:0148-0227    

DOI:10.1029/JB078i011p01753

年代:1973

数据来源: WILEY

摘要:

The gravitational potentials of a point mass, a spherical cap, and a spherical rectangle are presented in a spherical harmonic framework. The point mass displays the richest harmonic spectrum at all degrees. Equal mass distributed as a spherical cap or rectangle shows loss of representation at high degrees; the spectral weakening moves to progressively lower degrees as the mass is distributed more widely. The spectrum of a cap is less smooth than that of a similar rectangle with the same area. The characteristics of these three mass configurations are compared with those of other mass distributions and modeling techniques.

ISSN:0148-0227    

DOI:10.1029/JB078i011p01760

年代:1973

数据来源: WILEY

摘要:

The role of water in the electrical resistivity of rocks has long been recognized as an important one. This study examines the changes in resistivity of rock samples as induced by atmospheric moisture. An analysis of the term dry rock leads to the conclusion that oven drying and evacuation are methods that yield only relative degrees of desiccation. To obtain absolute values of the moisture present in a sample, one should probably use nuclear magnetic resonance and mass spectrometry. Experiments were performed on samples of hematitic sandstone, pyrite, and galena. The sandstone underwent a change in resistivity of 4 orders of magnitude when it was measured in a vacuum of 5 × 10−7torr and in air of 37% relative humidity. Pyrite and galena showed no variations in resistivity when they were measured under the same conditions. These results, plus others obtained elsewhere, indicate that rocks of the resistive type are affected in their electrical properties by atmospheric moisture, whereas rocks of the conductive type are not. The experimental evidence obtained is difficult to reconcile with a model of aqueous electrolytic conduction on the sample surface; it is instead suggested that adsorbed water molecules alter the surface resistivity in a manner similar to that observed in semiconductors and insulato

ISSN:0148-0227    

DOI:10.1029/JB078i011p01769

年代:1973

数据来源: WILEY

摘要:

Particle size distributions have been obtained by grain counts on electron micrographs of four samples containing submicroscopic equant magnetite particles. Blocking temperature data indicate thermally unstable (superparamagnetic, SPM) magnetic behavior in one sample whose grains range from 100 to 650 A in size. The SPM is confirmed by a large increase of saturation remanence when the hysteresis of the sample is measured at 77°K. Quantitative estimates of the SPM, single‐domain (SD), and multidomain (MD) magnetization fractions in this sample are made by comparing theoretical and observed values of the reduced saturation remanence at 77° and 300°K. The particle size distribution then yields a critical sizeds= (290 − 360) ± 50 A for the onset of SPM in magnetite at room temperature, in close agreement with Néel's SD theory. The critical sized0at which domain structure develops in equidimensional grains of magnetite is less well defined. It is certainly less than 480 ± 50 A and may even be less thands; that is, a direct MD‐SPM transition in magnetite is a distinct possibility. The extreme narrowness of the stable SD range in equidimensional magnetite particles suggests that either elongated SD grains or else small MD grains with pseudo‐SD characteristics must be the major carriers of the high‐stability natural remanence o

ISSN:0148-0227    

DOI:10.1029/JB078i011p01780

年代:1973

数据来源: WILEY

摘要:

A novel approach is presented for efficient mathematical modeling of discretely measured irregular surfaces. The technique is applicable to modeling of arbitrary surfaces; it is shown to be especially well suited for modeling of fine‐structure topographic surfaces. The macroscopic features of the method are as follows. (1) Given a set of discrete coordinate measurements, an average least squares mathematical model for the surface geometry is determined. (2) The model consists of an arbitrarily large family of locally valid surface functions that join smoothly;nth‐order continuity is satisfied everywhere. (3) Each locally valid surface function can typically be reduced to a low‐degree polynomial of two variables; thus an efficient and consistent mathematical model for local surface calculations is provided. (4) The method sequentially operates on a moderate to small subset of the measured data; it is therefore applicable to an arbitrarily large set of observed data. These features and associated computational devices are discussed in the light of numerical results obtained by using actual geodetic data sets. These results demonstrate that the method is a versatile, accurate, and efficient means for obtaining general‐purpose mathematical models of irregular s

ISSN:0148-0227    

DOI:10.1029/JB078i011p01794

年代:1973

数据来源: WILEY

摘要:

The rate of radon (222Rn) and thoron (220Rn) exhalation from the ground was measured in the yard of our laboratory to investigate effects of soil conditions on the exhalation; radon and thoron gases released from the ground were directly adsorbed on granular‐activated charcoal, and the charcoal was subjected to γ ray spectrometry to estimate the activity of adsorbed radon and thoron gases. Although the rate of radon exhalation was not appreciably changed by a light rainfall, it apparently decreased after a heavy rainfall and remained low for several days. No appreciable seasonal difference was found in the radon exhalation. On the other hand, the thoron exhalation decreased more or less after every rainfall and depended strongly on the moisture of a thin surface soil layer. The rate of thoron exhalation in summer was about twice its value in wint

ISSN:0148-0227    

DOI:10.1029/JB078i011p01804

年代:1973

数据来源: WILEY

摘要:

TheTphases from both the 5‐MT Cannikin underground explosion and the subsequent crater collapse were recorded on sofar hydrophones of the Pacific Missile Range at distances up to 3500 km. TheTphase signals were similar to those from naturally occurring earthquakes in the Aleutians, except that the onset was more rapid and the main arrival was followed by numerous reverberations from submarine topography. NoTphases from aftershocks that might have been triggered by the Cannikin shot were observed up to 36 hours afterward

ISSN:0148-0227    

DOI:10.1029/JB078i011p01809

年代:1973

数据来源: WILEY

摘要:

Rayleigh wave phase and amplitude spectra are used to determine the focal depths and mechanisms of mid‐ocean ridge earthquakes. The effect of propagation on the spectra is removed by analyzing the differential phase and the amplitude ratio of the Rayleigh waves from a pair of close events with different focal mechanisms. This analysis preserves the difference between the source spectra. By matching the observations with theoretical models, a best description of the source is derived. The two mid‐Atlantic ridge dip slip events that we have studied have a focal depth of only 3 ± 2 km beneath the ocean floor. Two dip slip events in the northeast Pacific have depths consistent with these results but with an uncertainty of 20 km. The depths of two mid‐Atlantic ridge strike slip events are 6 ± 3 km. Two strike slip events in the northeast Pacific are shallower than 25 km. The phase analysis greatly aids the amplitude analysis and, for these events, is indisp

ISSN:0148-0227    

DOI:10.1029/JB078i011p01818

年代:1973

数据来源: WILEY

摘要:

Maar‐type volcanoes and their eroded remnants, diatremes, are believed to be terrestrial counterparts of crater chain rilles and possibly of the dark halo craters observed in lunar and Martian photographs. Some, but not all, terrestrial maars and diatremes contain rock fragments (xenoliths) derived from the vent walls, some from as deep as the upper mantle. The lunar and Martian examples may provide our best means of sampling rocks representative of the shallow interiors of these planets; consequently, an understanding of the field occurrence of xenoliths in these terrestrial features is of interest. Field studies at two maars (Nanwaksjiak crater, Nunivak Island, Alaska, and Salt Lake crater, Oahu, Hawaii) and one diatreme (Moses Rock dike, Utah) show that deep‐seated xenoliths (eclogite, peridotite, and pyroxenite) occur as discrete well‐rounded fragments that increase in abundance and size with upward stratigraphic position in the ejecta (and therefore with time in the eruption sequence). The expected distribution of xenoliths around hypothetical maars on the moon, Mars, Venus, and the earth was computed by using numerical models for the flow in diatremes that take into account differences in gravity on the eruption. The mechanics of diatreme emplacement by fractive propagation from a deep mantle source and subsequent hydrodynamic flow are presented with selected numerical results. Field data on the displacement and distribution of very large blocks in and around the terrestrial features serve as constraints on the hydrodynamic models (essentially surface boundary conditions). Models were used to calculate the upward acceleration of blocks inside the vents and the ballistic history of the fragments exterior to the craters. Results show that block launch velocity is directly proportional to the planet's surface gravity. The resulting exterior ballistic range, including the effects of atmospheric drag, however, is a strong inverse function of the surface gravity and is a complicated function of fragment size and shape because of atmospheric sorting. Small fragments are decelerated in the atmosphere; very large fragments do not attain high launch velocities. It appears that there is an optimum size that attains the maximum range (when a 45° launch is assumed). On Mars, 30‐cm fragments may attain ranges of 30 km; on earth, maximum range of 8 km may be attained by 8‐meter blocks. The predicted distribution of ejecta of the size and density of typical xenoliths erupted into the terrestrial ballistic environment (atmosphere and gravity) yield results in qualitative agreement with field observations of their distribution. On the moon, because of the absence of an atmosphere, maximum range of about 250 km is attained by the smallest fragments; acceleration to lunar escape conditions by diatreme eruptions seems very unlikely. Implications for possible space missions are as follows. (1) Premission evaluation by physical observational techniques as well as photogeological methods may greatly aid in identifying craters likely to yield xenoliths. Specifically, maar‐type craters with blocky ejecta are probably the best xenolith candidates (i.e., craters with high radar backscatter cross sections or thermal IR eclipse anomalies). (2) At Salt Lake crater and Nanwaksjiak, xenoliths are largest and most abundant late in eruptions and high in the ejecta sequence, which suggests that fruitful sampling may be accomplished in ejecta exterior to the crater or just inside below the rim. (3) Although they are probably not deeply buried, xenoliths in ejecta may be coated with a thin crust of basaltic lava requiring that bombs be broken open (or kicked) to differentiate lava bombs from dense xenoliths. (4) The predicted ballistic range of ejected fragments of all sizes is greater on the moon and Mars than on the earth. Terrestrial xenoliths (commonly 1–30 cm in size) are commonly restricted to about 1 km from crater rims; similar 10‐cm fragments on the moon might attain an 80‐km range. Hence landing close to crater rims might not be required, but the areal concentration of xenoliths (number per unit area) will be lower with increasing range, and their identity as xenolithic fragments will be more difficult to establish. On Mars, as on the earth, fine ejecta will be distributed and dispersed by the wind. Best sampling is in the downwind direction at the crater rim. (5) Sampling for xenoliths should consist of taking grab samples of ash to be sorted later and specifically searching for well‐rounded fragments in the 10‐cm size range because this size is (a) abundant enough for collection when xenoliths are present, (b) identifiable from a standing position or by telemetry, (c) small enough for many samples to be collected, and (d) large enough for effective dispersal of representative sa

ISSN:0148-0227    

DOI:10.1029/JB078i011p01833

年代:1973

数据来源: WILEY