摘要:

Upon impact of a meteorite with a planetary surface the resulting shock wave both ‘processes’ the material in the vicinity of the impact and sets a larger volume of material than was subjected to high pressure into motion. Most of the volume which is excavated by the impact leaves the crater after the shock wave has decayed. The kinetic energy which has been deposited in the planetary surface is converted into reversible and irreversible work, carried out against the planetary gravity field and against the strength of the impacted material, respectively. By using the results of compressible flow calculations prescribing the initial stages of the impact interaction (obtained with finite difference techniques) the final stages of cratering flow along the symmetry axis are described, using the incompressible flow formalism proposed by Maxwell. The fundamental assumption in this description is that the amplitude of the particle velocity field decreases with time as kinetic energy is converted into heat and gravitational potential energy. At a given time in a spherical coordinate system the radial velocity is proportional toR−z, whereRis the radius (normalized by projectile velocity) andzis a constant shape factor for the duration of flow and a weak function of angle. The azimuthal velocity, as well as the streamlines, is prescribed by the incompressibility condition. The final crater depth (for fixed strengthY) is found to be proportional toR0[2(z+ 1)uor²/g]1/(z+1), whereuoris the initial radial particle velocity at (projectile normalized) radiusR0,gis planetary gravity, andz(which varied from 2 to 3) is the shape factor. The final crater depth (for fixed gravity) is also found to be proportional to [ρuor2/Yz]1/(z+1), whereρandYare planetary density and yield strength, respectively. By using a Mohr‐Coulomb yield criterion the effect of varying strength on transient crater depth and on crater formation time in the gravity field of the moon is investigated for 5‐km/s impactors with radii in the 10‐ to 107‐cm range. Comparison of crater formation time and maximum transient crater depth as a function of gravity yields dependencies proportional tog−0.58andg−0.19, respectively, compared tog−0.618andg−0.165observed by Gault and Wedekind for hypervelocity impact craters in the 16‐ to 26‐cm‐diameter range in a quartz sand (with Mohr‐Coulomb type behavior) carried out over an effecti

ISSN:8755-1209    

DOI:10.1029/RG019i001p00001

年代:1981

数据来源: WILEY

摘要:

More than 15 years of planetary exploration of Mars have given insight into the geologic processes that have shaped its surface. The newly acquired Viking data have shown that volcanism is one of the most important geologic processes operating on Mars throughout its history. In situ chemical analyses of Martian soil by the Viking lander spacecraft indicate mafic to ultramafic source rocks. This is consistent both with available remote sensing data, which indicate the presence of mafic minerals such as pyroxene and olivine, and with petrologic modeling, based on available geophysical data which suggest that Martian lavas are probably iron rich and ultramafic. These data strongly suggest that basaltic volcanism is widespread on Mars, and much of the photogeological data may be studied in this context. Photogeological analysis of the Martian surface has shown two main types of volcanic morphologies: the first type is central volcanoes, which are volcanic landforms developed by continued and prolonged eruption from a point source vent. This category includes (1) shields, the classic low‐profile volcanic mountains of which Olympus Mons is the most spectacular example, (2) domes, steep‐sided constructs, such as Tharsis Tholus, that may represent lower rates of eruption than the shields or, possibly, more silicic lava compositions, (3) highland patera, radially textured low‐profile volcanoes that occur in the cratered terrain and are interpreted as ash shields, (4) Alba Patera, an apparently unique volcanic landform consisting of a vast volcanic center over 1500 km across with flank slopes of less than a tenth of a degree, and (5) various small features such as cinder cones. The second major category is volcanic plains, which are units recognized by several criteria, of which the presence of mare ridges and flow lobes are the most useful. Volcanic plains are subdivided into four main groups: (1) simple flows, broad, smooth to rolling plains that contain numerous mare‐type ridges but no flow lobes, interpreted as being composed of thick, single‐cooling units, (2) complex flows, displaying multiple overlapping flow lobes interpreted to be indicative of thin, multiple‐cooling units, (3) undifierentiated flows, plains that typically lack any morphologic identifying feature but are considered to be volcanic partly on the basis of their association with large volcanic centers, and (4) questionable plains, volcanic(?) units heavily modified by other processes (erosion, tectonism, etc.) so that their origins are uncertain. When these categories of volcanic morphologies are combined with relative age data provided by crater statistics, a volcanic history for Mars can be derived as follows: Early heavy bombardment of Mars was accompanied and followed by small‐scale fluvial channeling, extensive flood volcanism (the plateau plains), and ash shield volcanism in the cratered terrain. Shortly after this time, less extensive flood volcanism continued to resurface the planet during formation of the northern/southern hemisphere dichotomy. Central volcanism became more prominent with the development of the Alba Patera center as well as the older shields and domes of the northern hemisphere (early Tharsis and Elysium regions). The development of the Tharsis and Elysium uplifts may have triggered the release of large‐scale catastrophic floods, producing large channels. Continued uplift and lithospheric thinning concentrated volcanic activity in the Tharsis region, producing large shield volcanoes and extensive lava plains. Both central vent and plains volcanism have been active throughout Martian history, but the volumes of extrusion have gradually decreased with time. This is consistent with a moonlike thermal history involving a lithosphere of increasing thickness with time, gradually ‘turning off’ the volcanism. Although many questions remain regarding Martian volcanism, the Viking data have provided a remarkable, detailed overview of the probable nature of the volca

ISSN:8755-1209    

DOI:10.1029/RG019i001p00013

年代:1981

数据来源: WILEY

摘要:

This is a review of recent research results pertaining to the atmospheres of the outer planets and satellites. It includes the atmospheres of Pluto, Triton, Neptune, Uranus, Titan, Saturn, and Io. Io’s neutral torus and plasma torus are covered, but Jupiter and Jupiter’s magnetosphere are not. Jupiter is not included because of the number of specialized reviews which have recently become available for Jupiter. This review includes earlier Voyager 1 and 2 results for Io’s atmosphere and plasma torus as well as early Pioneer 11 results for Saturn and Titan. Neither ring systems nor atmosphereless bodies are reviewed

ISSN:8755-1209    

DOI:10.1029/RG019i001p00043

年代:1981

数据来源: WILEY

摘要:

An introductory survey of the global energy balance climate models is presented with an emphasis on analytical results. A sequence of increasingly complicated models involving ice cap and radiative feedback processes are solved, and the solutions and parameter sensitivities are studied. The model parameterizations are examined critically in light of many current uncertainties. A simple seasonal model is used to study the effects of changes in orbital elements on the temperature field. A linear stability theorem and a complete nonlinear stability analysis for the models are developed. Analytical solutions are also obtained for the linearized models driven by stochastic forcing elements. In this context the relation between natural fluctuation statistics and climate sensitivity is stressed.

ISSN:8755-1209    

DOI:10.1029/RG019i001p00091

年代:1981

数据来源: WILEY

摘要:

Extant data from measurements of halogens in the atmosphere are reviewed in the following categories: gaseous chlorine compounds (inorganic and organic), particulate chloride and chloride in precipitation, gaseous bromine compounds (inorganic and organic), particulate bromide and bromide in precipitation, gaseous iodine compounds (inorganic and organic), iodine in particles and in precipitation, gaseous fluorine compounds (inorganic and organic), and fluoride in particles and precipitation. The roles that these data and other unavailable data play in defining global cycles of the halogens are discussed. Speciation of the halogen gases in the troposphere is very uncertain: the only inorganic species detected by species‐specific methods are HCl and SF6. More specific data are available on organic forms that contain halogens. Key species of gaseous halogens, either established or suspected to be important, are listed along with key processes that need investigation. Heterogeneous reactions, both gas‐to‐particle and particle‐to‐gas processes, precipitation removal, and sea‐salt aerosol generation and fractionation processes need quantitative investigation to allow progress in estimating halogen sources and sinks. Where practical, as with stratospheric inorganic chlorine gases, quantitative comparisons are made between measured and predicted con

ISSN:8755-1209    

DOI:10.1029/RG019i001p00123

年代:1981

数据来源: WILEY

摘要:

The structure of the inertial peak in deep ocean kinetic energy spectra is studied here. Records were obtained from Polymode arrays deployed in the Western North Atlantic Ocean (40°W to 70°W, 15°N to 42°N). The results are interpreted both in terms of local sources and of turning point effects on internal waves generated at lower latitudes. In most of the data, there is a prominent inertial peak slightly abovef; however, the peak height above the background continuum varies with depth and geographical environment. Three classes of environment and their corresponding spectra emerge from peak height variations: class 1 is the 1500‐m level near the Mid‐Atlantic Ridge, with the greatest peak height of 18 dB; class 2 includes (a) the upper ocean (depth less than 2000 m), (b) the deep ocean (depth greater than 2000 m) over rough topography, and (c) the deep ocean underneath the Gulf Stream, with intermediate peak height of 11.5 dB; class 3 is the deep ocean over smooth topography, with the lowest peak height of 7.5 dB. Nearf, the horizontal coherence scale is 0 (60 km) at depths from 200 m to 600 m, and the vertical coherence scale is 0 (200 m) in the lower part of the main thermocline and 0 (1000 m) in the deep water; the phase difference suggests a downward energy propagation in the lower thermocline and standing waves in the deep water. A one‐turning‐point model is developed to describe inertial waves at mid‐latitudes, based on the assumption that inertial waves are randomly generated at lower latitudes (global generation) where their frequency‐wave number spectrum is given by the model of Garrett and Munk (1972, 1975). Using the globally valid wave functions obtained by Munk and Phillips (1968), various frequency spectra nearfare calculated numerically. The model yields a prominent inertial peak of 7 dB in the horizontal velocity spectrum but no peaks in the temperature spectrum. The model is latitudinally dependent: the frequency shift and bandwidth of the inertial peak decrease with latitude; energy level nearfis minimum at about 30° and higher at low and high latitudes. The observations of class 3 can be well described by the model; a low zonal wave number cutoff is required to produce the observed frequency shift of the inertial peak. The differences between the global generation model and the observations of class 1 and class 2 are interpreted as the effects of local sources. A locally forced model is developed based on the latitudinal modal decomposition of a localized source function. Asymptotic eigensolutions of Laplace’s tidal equation are therefore derived and used as a set of expansion functions. The forcing is through a vertical velocity field specified at the top or bottom boundaries of the ocean. For white noise forcing, the horizontal velocity spectrum of the response has an inertial peak which diminishes in the far field. With the forcing located at either the surface or the bottom, several properties of the class 2 observations can be described qualitatively by a combination of the global and local models. The reflection of inertial waves from a turbulent benthic boundary layer is studied by a slab model of given depth. Frictional effects are confined to the boundary layer and modeled by a quadratic drag law. For given incident waves, reflection coefficients are found to be greater than 0.9 for the long waves which contain most of the energy. This result suggests that energy‐containing inertial waves can propagate over great distance as is required by the validity of the model

ISSN:8755-1209    

DOI:10.1029/RG019i001p00141

年代:1981

数据来源: WILEY

摘要:

The general conditions for wave generation (γ>0) or wave absorption (γ0 or γ<0) can be easily determined by considering the relative position of three curves in the υ∥, υ⊥plane (where υ∥and υ⊥are the parallel and perpendicular velocity of the particle): the isodensity curve, the constant energy curve, and the diffusion curve. For ‘regular’ distribution functions (i.e., distributions which are concave around the origin and for which there are less particles of high energy than particles of low energy) a necessary and sufficient condition for positive growth rate is that the third curve be situated within the area delineated by the other two curves. This geometrical formalism is extended to other kinds of distributions (field‐aligned beams or ring distributions). It is also applied to wave‐particle interactions in multicomponent plasmas. The conclusions which we arrive at are in agreement with previous findings. The analytical demonstration of this geometrical property is given. It is linked with the fact that the equations which govern the wave amplification and the particle diffusion involve the same operator, which is the derivative of the distribution function taken along the diffusion curve. The origin of this property lies in the special form of the Bo

ISSN:8755-1209    

DOI:10.1029/RG019i001p00171

年代:1981

数据来源: WILEY

摘要:

Theories for and observations relating to auroral precipitation are reviewed for purposes of assessing our understanding of the aurora and of identifying those processes which play the most important role in auroral precipitation. For purposes of the review, four categories of auroral forms are defined: (1) the diffuse aurora, which usually forms the equatorward boundary of the auroral oval, (2) auroral patches and pulsations, which appear most commonly within the morning sector of the diffuse aurora, (3) the discrete aurora, most prominently observed in the evening and poleward sectors of the auroral oval, and (4) the ‘inverted‐V’ aurora, which appears in the same regions as discrete aurora but with much larger size scales. One principal conclusion is that the diffuse aurora, likely caused by both proton and electron precipitation, is the result of pitch angle scattering by electrostatic cyclotron waves. Another is that the auroral pulsations and patches are the result of pitch angle scattering of more energetic electrons by electromagnetic whistler mode waves. The patches may represent flux tubes of enhanced ionization tied to the lower ionosphere. The discrete aurora is the result of electron acceleration parallel to the magnetic field by the quasi‐static electric field of a current‐driven laminar electrostatic V shock. The theory for inverted‐V precipitation is not well developed, but observations strongly suggest that it is related to ion cyclotron anomalous

ISSN:8755-1209    

DOI:10.1029/RG019i001p00185

年代:1981

数据来源: WILEY

摘要:

Currently, two types of methods are used for the detailed representation of the earth’s gravity potential, namely, integral formulae and approximation techniques. The theoretical advantages and disadvantages of the methods (or of a combination of the methods) are discussed, and the results obtained are compared. If the methods are judged according to their general applicability, flexibility, and possibilities for obtaining error estimates, then approximation techniques like least squares or minimum norm collocation must be preferred. Integral formulae techniques are, on the other hand, well suited in areas with a homogeneous data coverag

ISSN:8755-1209    

DOI:10.1029/RG019i001p00213

年代:1981

数据来源: WILEY

摘要:

The fundamental equations of magnetospheric plasma electrodynamics are considered from a theoretical standpoint that stresses the basic equivalence of various seemingly different formal representations. The mathematical properties of vector fields are reviewed, and their implications in electrodynamics are studied. The irrotational and solenoidal parts of the electric field are associated with two physically distinct types of sources. Relativistic covariance and gauge invariance in electromagnetic theory are reviewed and discussed in the context of an approach in which the mathematical properties of vector fields are taken as primary concepts. Special attention is given to the use and interpretation of the Coulomb gauge potential functions. This choice of gauge is sometimes regarded with undue suspicion, possibly because of a certain paradox concerning causality. The paradox is discussed and resolved. Useful properties of the Coulomb gauge are identified. These need not be limited to the case of slow time variations and can extend beyond the limits of validity of ideal MHD theory.

ISSN:8755-1209    

DOI:10.1029/RG019i001p00223

年代:1981

数据来源: WILEY