Showing papers in "Journal of Geophysical Research in 1977"

An analysis of the variation of ocean floor bathymetry and heat flow with age

Abstract: Two models, a simple cooling model and the plate model, have been advanced to account for the variation in depth and heat flow with increasing age of the ocean floor. The simple cooling model predicts a linear relation between depth and t½, and heat flow and 1/t½, where t is the age of the ocean floor. We show that the same t½ dependence is implicit in the solutions for the plate model for sufficiently young ocean floor. For larger ages these relations break down, and depth and heat flow decay exponentially to constant values. The two forms of the solution are developed to provide a simple method of inverting the data to give the model parameters. The empirical depth versus age relation for the North Pacific and North Atlantic has been extended out to 160 m.y. B.P. The depth initially increases as t½, but between 60 and 80 m.y. B.P. the variation of depth with age departs from this simple relation. For older ocean floor the depth decays exponentially with age toward a constant asymptotic value. Such characteristics would be produced by a thermal structure close to that of the plate model. Inverting the data gives a plate thickness of 125±10 km, a bottom boundary temperature of 1350°±275°C, and a thermal expansion coefficient of (3.2±1.1) × 10−5°C−1. Between 0 and 70 m.y. B.P. the depth can be represented by the relation d(t) = 2500 + 350t½ m, with t in m.y. B.P., and for regions older than 20 m.y. B.P. by the relation d(t) = 6400 - 3200 exp (−t/62.8) m. The heat flow data were treated in a similar, but less extensive manner. Although the data are compatible with the same model that accounts for the topography, their scatter prevents their use in the same quantitative fashion. Our analysis shows that the heat flow only responds to the bottom boundary at approximately twice the age at which the depth does. Within the scatter of the data, from 0 to 120 m.y. B.P., the heat flow pan be represented by the relation q(t) = 11.3/t½ μcal cm−2s−1. The previously accepted view that the heat flow observations approach a constant asymptotic value in the old ocean basins needs to be tested more stringently. The above results imply that a mechanism is required to supply heat at the base of the plate.

The energy release in great earthquakes

Abstract: The conventional magnitude scale M suffers saturation when the rupture dimension of the earthquake exceeds the wavelength of the seismic waves used for the magnitude determination (usually 5–50 km). This saturation leads to an inaccurate estimate of energy released in great earthquakes. To circumvent this problem the strain energy drop W (difference in strain energy before and after an earthquake) in great earthquakes is estimated from the seismic moment M_0. If the stress drop Δσ is complete, W = W_0 = (Δσ/2μ)M_0 ∼ M_0/(2×10^4), where μ is the rigidity; if it is partial, W_0 gives the minimum estimate of the strain energy drop. Furthermore, if Orowan's condition, i.e., that frictional stress equal final stress, is met, W_0 represents the seismic wave energy. A new magnitude scale M_w is defined in terms of W_0 through the standard energy-magnitude relation log W_0 = 1.5M_w + 11.8. M_w is as large as 9.5 for the 1960 Chilean earthquake and connects smoothly to M_s (surface wave magnitude) for earthquakes with a rupture dimension of about 100 km or less. The M_w scale does not suffer saturation and is a more adequate magnitude scale for great earthquakes. The seismic energy release curve defined by W_0 is entirely different from that previously estimated from Ms. During the 15-year period from 1950 to 1965 the annual average of W_0 is more than 1 order of magnitude larger than that during the periods from 1920 to 1950 and from 1965 to 1976. The temporal variation of the amplitude of the Chandler wobble correlates very well with the variation of W_0, with a slight indication of the former preceding the latter. In contrast, the number N of moderate to large earthquakes increased very sharply as the Chandler wobble amplitude increased but decreased very sharply during the period from 1945 to 1965, when W_0 was largest. One possible explanation for these correlations is that the increase in the wobble amplitude triggers worldwide seismic activity and accelerates plate motion which eventually leads to great decoupling earthquakes. This decoupling causes the decline of moderate to large earthquake activity. Changes in the rotation rate of the earth may be an important element in this mechanism.

Cenozoic evolution of Antarctic glaciation the Circum-Antarctic Ocean and their impact on global paleoceanography

Abstract: Deep-sea drilling in the Antarctic region (Deep-Sea Drilling Project legs 28, 29, 35, and 36) has provided many new data about the development of circum-Antarctic circulation and the closely related glacial evolution of Antarctica. The Antarctic continent has been in a high-latitude position since the middle to late Mesozoic. Glaciation commenced much later, in the middle Tertiary, demonstrating that near-polar position is not sufficient for glacial development. Instead, continental glaciation developed as the present-day Southern Ocean circulation system became established when obstructing land masses moved aside. During the Paleocene (t = ∼65 to 55 m.y. ago), Australia and Antarctica were joined. In the early Eocene (t = ∼55 m.y. ago), Australia began to drift northward from Antarctica, forming an ocean, although circum-Antarctic flow was blocked by the continental South Tasman Rise and Tasmania. During the Eocene (t = 55 to 38 m.y. ago) the Southern Ocean was relatively warm and the continent largely nonglaciated. Cool temperate vegetation existed in some regions. By the late Eocene (t = ∼39 m.y. ago) a shallow water connection had developed between the southern Indian and Pacific oceans over the South Tasman Rise. The first major climatic-glacial threshold was crossed 38 m.y. ago near the Eocene-Oligocene boundary, when substantial Antarctic sea ice began to form. This resulted in a rapid temperature drop in bottom waters of about 5°C and a major crisis in deep-sea faunas. Thermohaline oceanic circulation was initiated at this time much like that of the present day. The resulting change in climatic regime increased bottom water activity over wide areas of the deep ocean basins, creating much sediment erosion, especially in western parts of oceans. A major (∼2000 m) and apparently rapid deepening also occurred in the calcium carbonate compensation depth (CCD). This climatic threshold was crossed as a result of the gradual isolation of Antarctica from Australia and perhaps the opening of the Drake Passage. During the Oligocene (t = 38 to 22 m.y. ago), widespread glaciation probably occurred throughout Antarctica, although no ice cap existed. By the middle to late Oligocene (t = ∼30 to 25 m.y. ago), deep-seated circum-Antarctic flow had developed south of the South Tasman Rise, as this had separated sufficiently from Victoria Land, Antarctica. Major reorganization resulted in southern hemisphere deep-sea sediment distribution patterns. The next principal climatic threshold was crossed during the middle Miocene (t = 14 to 11 m.y. ago) when the Antarctic ice cap formed. This occurred at about the time of closure of the Australian-Indonesian deep-sea passage. During the early Miocene, calcareous biogenic sediments began to be displaced northward by siliceous biogenic sediments with higher rates of sedimentation reflecting the beginning of circulation related to the development of the Antarctic Convergence. Since the middle Miocene the East Antarctic ice cap has remained a semipermanent feature exhibiting some changes in volume. The most important of these occurred during the latest Miocene (t = ∼5 m.y. ago) when ice volumes increased beyond those of the present day. This event was related to global climatic cooling, a rapid northward movement of about 300 km of the Antarctic Convergence, and a eustatic sea level drop that may have been partly responsible for the isolation of the Mediterranean basin. Northern hemisphere ice sheet development began about 2.5–3 m.y. ago, representing the next major global climatic threshold, and was followed by the well-known major oscillations in northern ice sheets. In the Southern Ocean the Quaternary marks a peak in activity of oceanic circulation as reflected by widespread deep-sea erosion, very high biogenic productivity at the Antarctic Convergence and resulting high rates of biogenic sedimentation, and maximum northward distribution of ice-rafted debris.

Spatially averaged flow over a wavy surface

Abstract: An understanding of the mechanics of nonuniform flow is important in a variety of ecological and geophysical fluid mechanical problems. Moreover, the ability to predict local boundary shear stress on an uneven bed is essential in erosion and sediment transport problems. In order to elucidate the important fluid mechanical phenomena active over natural quasi-two-dimensional bed forms a series of detailed flow measurements were made above 60- to 100-m-long 1- to 3-m-high dunes in the Columbia River. In this paper, velocity profiles obtained by averaging these flow data along lines of constant distance above the riverbed are examined, and it is shown that they can be constructed from well-known uniform flow results used in conjunction with a hypothesis about the structure of internal boundary layers. This approach permits skin friction as well as total boundary shear stress, averaged over one wavelength of the bed form, to be determined from spatially averaged velocity profiles and, conversely, provides a mechanism whereby zero-order velocity profiles can be constructed for two-dimensional nonuniform channel flows. Corrections for changes in the bottom roughness parameter caused by bed load transport and for flow stratification induced by suspended load transport are derived and applied in order to make the results consistent with the measured spatially averaged shear stress field.

Determination of the three‐dimensional seismic structure of the lithosphere

Abstract: A new three-dimensional earth modeling is proposed as a framework to obtain more detailed and accurate information about the earth's interior. We start with a layered medium of classic seismology but divide each layer into many blocks and assign a parameter to each block which describes the velocity fluctuation from the average for the layer. Our data are the teleseismic P travel time residuals observed at an array of seismographs distributed on the surface above the earth's volume we are modeling. By isolating various sources of errors and biases we arrive at a system of equations to determine the model parameters. The solution was obtained by the use of generalized inverse and stochastic inverse methods. Our method also gives a lower limit of the true rms slowness fluctuation in the earth under the assumption of ray theory. Using P wave residual data from the Norwegian Seismic Array (Norsar), we have obtained the map of velocity anomalies at various depths up to a depth of 126 km. The rms slowness fluctuation was found to be at least 3.1%. This is in agreement with estimates obtained from statistical analysis of P time fluctuations based on the Chernov theory. The three-dimensional velocity anomalies are presented both by the generalized inverse and by the stochastic inverse solutions. We prefer the dual presentation because it gives the reader greater freedom in judging the results than a single ‘optimal’ solution. Both methods gave essentially the same results. The discrepancies, when they existed, were always explainable in terms of differences in the smoothing procedure which is explicitly given in the resolution matrix. The dominant features in the obtained three-dimensional velocity image of the lithosphere beneath the Norsar array are low velocities to the west and high velocities to the east. The latter feature may be attributed to rocks of the Baltic shield which are undisturbed by the Caledonian orogeny or by Permian volcanism. Our result conclusively demonstrates the existence of strong small-scale inhomogeneities to the bottom of the lithosphere. More theoretical and experimental works are needed to relate these velocity anomalies with the magma ascent mechanism which caused the Oslo graben.

Viscoelastic properties of fluid-saturated cracked solids

Abstract: The effective elastic moduli of a fluid-saturated solid containing thin cracks depend on the degree of interconnection between the cracks. Three separate regimes may be identified: (1) dry (drained), in which fluid in cracks can flow out of bulk regions of compression, (2) saturated isobaric, in which fluid may flow from one crack to another but no bulk flow takes place, and (3) saturated isolated, in which there is no communication of fluid between cracks. Transitions between these cases involve fluid flow, resulting in dissipation of energy. Relaxation of shear stresses in viscous fluid inclusions also results in dissipation. Viscoelastic moduli are derived, by using a self-consistent approximation, that describe the complete range of behavior. There are two characteristic frequencies near which dissipation is largest and the moduli change rapidly with frequency. The first corresponds to fluid flow between cracks, and its value can be estimated from the crack geometry or permeability. The second corresponds to the relaxation of shear stress in an isolated viscous fluid inclusion; its value may also be estimated. Variations of crack geometry result in a distribution of characteristic frequencies and cause Q to be relatively constant over many decades of frequency. Fluid flow between cracks accounts for attenuation of seismic waves in water-saturated rocks and attenuation observed in laboratory measurements on water-saturated rocks and partially molten aggregates. Attenuation in a partially molten upper mantle is probably due to fluid flow between cracks, although grain boundary relaxation in an unmelted upper mantle could also account for the seismic low-velocity zone. Grain boundary relaxation in the mantle may cause the long-term shear modulus to be around 20% less than that measured from seismic observations.

Free convection about a vertical flat plate embedded in a porous medium with application to heat transfer from a dike

Abstract: An analysis is made for steady free convection about a vertical flat plate embedded in a saturated porous medium at high Rayleigh numbers. Within the framework of boundary layer approximations, similarity solutions are obtained for a class of problems where wall temperature varies as xλ, i.e., a power function of distance from the origin where wall temperature begins to deviate from that of the surrounding fluids. Analytical expressions are obtained for boundary layer thickness, local and overall surface heat flux, and local and average heat transfer coefficients. Application to convective heat transfer about an isothermal dike intruded in an aquifer is discussed.

Thermal and albedo mapping of Mars during the Viking primary mission

Abstract: The behavior of Mars as observed by the Viking infrared thermal mapper (IRTM) is considered. The IRTM is a 28-channel, 4-telescope radiometer that operated in six spectral bands. The studies considered include observations from the interplanetary phase through data collection on November 7, 1976. During this interval, thermal mapping of the whole Martian surface has been possible. Attention is given to polar temperatures, global albedos, predawn temperatures, a thermal inertia contour map, geometry considerations, clouds, aspects of predawn warming, and observations of earth.

The composition of the atmosphere at the surface of Mars

Abstract: The current status is summarized of investigations of the composition of the Martian atmosphere, in which use was made of the mass spectrometers that function as the analytical component of the molecular analysis experiments on the two Viking landers. The following points seem well established: N2, Ar-40, Ne, Kr, Xe, and the primordial isotope of Ar are present. The present atmosphere of Mars represents only a small fraction of the total amount of volatiles outgassed by the planet, so that high surface pressure and abundant water may have been present. The noble gases in the Martian atmosphere exhibit a relative abundance pattern similar to that in the earth's atmosphere and (except for Xe) to that in the primordial component of meteorites. The existence of a 'planetary component' is thus proven, supporting the arguments of those who favor a fractionation of noble gases prior to the formation of the planets. In spite of these similarities, the isotopic ratios of nitrogen, argon, and xenon indicate that the histories of the Martian and the earth's atmospheres have been very different.

A model for earthquake swarms

Abstract: A model for earthquake swarms in volcanic regions consists of the following concepts: (1) clusters of magma-filled dikes exist within brittle volumes of the crust, (2) dikes within a cluster are systematically oriented with their long dimension in the direction of the regional greatest principal stress, and (3) a sequence of shear failures (an earthquake swarm) occurs along a system of conjugate fault planes joining en echelon offset dike tips at oblique angles. This model accounts for commonly observed geometric relations between surface faulting patterns, the hypocentral distribution of swarm earthquakes, and fault plane solutions in a variety of situations. Swarm areas dominated by strike-slip faulting, however, provide the most compelling examples of the utility of the model. Specific examples considered here include a swarm on the east rift zone of Kilauea volcano, Hawaii, and swarms in the Imperial Valley, California, and the Reykjanes Peninsula, Iceland, which represent transitional zones between spreading centers and transform faults.

The search for organic substances and inorganic volatile compounds in the surface of Mars

Abstract: A total of four Martian samples, one surface and one subsurface sample at each of the two Viking landing sites, Chryse Planitia and Utopia Planitia, have been analyzed for organic compounds by a gas chromatograph-mass spectrometer. In none of these experiments could organic material of Martian origin be detected at detection limits generally of the order of parts per billion and for a few substances closer to parts per million. The evolution of water and carbon dioxide, but not of other inorganic gases, was observed upon heating the sample to temperatures of up to 500 C. The absence of organic compounds seems to preclude their production on the planet at rates that exceed the rate of their destruction. It also makes it unlikely that living systems that behave in a manner similar to terrestrial biota exist, at least at the two Viking landing sites.

Fault plane with barriers: A versatile earthquake model

Abstract: Shear cracks with finite cohesive forces can propagate by skipping past barriers. The barriers left behind may remain unbroken or may eventually break because of subsequent increase in dynamic stress depending on the ratio of barrier strength to tectonic stress. This model can explain a variety of observations on rupture in the earth, including (1) segmentation of the fault or ruptured zone in earthquakes and rock bursts, (2) ripples in seismograms which cannot be explained by path effect, and (3) departure of the scaling law of the seismic spectrum from that based upon the similarity assumption. The model also explains why the simple uniform dislocation model sometimes works better than the crack model without barriers. It also predicts, contrary to common belief, that an earthquake with low average stress drop may generate relatively greater amounts of high-frequency waves than an earthquake with high average stress drop. One important consequence of our barrier model is the possibility of predicting the occurrence of aftershocks by analyzing the source spectrum of the main shock.

The Martian ionosphere as observed by the Viking retarding potential analyzers

Abstract: The first in situ measurements of the ionosphere of another planet were obtained by retarding potential analyzers of the Viking landers. These results are presented with attention to: (1) the determination of the peak ion concentration in the ionosphere layer at several altitudes, (2) the measurement of ion temperatures, and (3) an equatorward horizontal ion velocity observed at various heights. Both landers entered the ionosphere layer at solar zenith angles near 44 deg, and more structure was observed in the height profiles of ionospheric quantities on Viking 2, although the profiles were similar in shape to those of Viking 1.

A global thermospheric model based on mass spectrometer and incoherent scatter data MSIS, 1. N2 density and temperature

Abstract: Measurements of neutral nitrogen density from mass spectrometers on five satellites (AE-B, Ogo 6, San Marco 3, Aeros A, and AE-C) and neutral temperatures inferred from incoherent scatter measurements at four ground stations are combined to produce a model of thermospheric neutral temperatures and nitrogen densities similar to the Ogo 6 empirical model (Hedin et al., 1974). This global model is designated MSIS (mass spectrometer and incoherent scatter). The global average temperature, the annual temperature variation, lower bound density, and lower bound temperature are discussed. The data set covers the time period from the end of 1965 to mid-1975 and also a wide range of solar activities. Diurnal and semidiurnal variations in lower bound density and temperature are considered, as is magnetic activity.

Composition and structure of Mars' Upper atmosphere: Results from the neutral mass spectrometers on Viking 1 and 2

Abstract: The upper atmospheric mass spectrometers flown on Viking 1 and 2 are described, and results obtained for the composition and structure of Mars' upper atmosphere are summarized. Carbon dioxide is the major constituent of the atmosphere at all heights below 180 km. The thermal structure of the upper atmosphere is complex and variable with average temperatures below 200°K for both Viking 1 and 2. The atmosphere is mixed to heights in excess of 120 km. The isotopic composition of carbon and oxygen in the Martian atmosphere is similar to that in the terrestrial atmosphere: 15N is enriched in Mars' atmosphere by a factor of 1.62±0.16.

Empirical models of high latitude electric fields

Abstract: Model cross sections of the high latitude dawn-dusk electric field based on OGO-6 data are presented for the signature profiles, most frequently encountered for both + and -Y orientations of the interplanetary magnetic field. Line integrals give a total potential of 76 keV in each case. To illustrate extremes, examples of model cross-sections with total potentials of 23 keV and 140 keV are also given. Model convection patterns are also presented utilizing OGO-6 data on boundary locations at other magnetic local times. When this information is combined with characteristic field geometries in the region of the Harang discontinuity, and is supplemented by data from Ba+ cloud motions in the polar cap, it becomes possible to construct realistic convection patterns on the nightside which deviate from the usual sun-aligned patterns. The observational models presented are of limited applicability as a consequence of the variability of observed distributions. These limitations are emphasized with particular attention given to several types of recurrent deviations which have not previously been discussed.

Two types of magnetospheric ELF chorus and their substorm dependences

Abstract: The distribution of extremely low frequency (10-1500 Hz) magnetospheric chorus to all local times and latitudes is investigated in order to determine dependence on substorms, and to evaluate the conditions under which chorus is generated. The analysis carefully separates space and time effects by an investigation of data obtained by the OGO 5 search coil magnetometer. A study of spatial dependencies shows that chorus occurs in two magnetic regions: equatorial chorus is located near the equator, and high-latitude chorus is located above 15 degrees. An analysis of chorus in each of the regions illustrates that equatorial chorus is definitely related to substorm, whereas high-latitude chorus often occurs within magnetically quiet intervals.

Large-scale heterogeneities in the lower mantle

Abstract: Coefficients of a spherical harmonic expansion, up to angular order 3, of velocity anomalies in five shells within the earth's mantle were obtained from an analysis of nearly 700,000 P wave travel time residuals. The results for depths less than 1100 km are unreliable; on the basis of tests and numerical experiments we infer that lateral heterogeneities in this depth interval are dominated by velocity perturbations of lateral dimensions less than 5000 km. Relatively large wavelength features were resolved below 1500-km depth; the average perturbation level increases in the lowermost mantle. The region between 1100 and 1500 km may represent a transition zone with respect to the dimensions of anomalies. We present statistical evidence for a negative correlation between the long wavelength gravity anomalies observed at the surface and those computed from velocity anomalies at depths greater than 1100 km under the assumption of proportionality between velocity and density perturbations. The proportionality coefficient Δυ/Δρ has been determined by using two different methods: the values are −4.45 and −6.02 (km/s)/(g/cm3). Only minor changes in the velocity distribution are needed to satisfy the long wavelength gravity field exactly. Possible origins of the correlation are (1) sinking of eclogite-rich material into the lower mantle from regions of lithospheric subduction, (2) chemical plumes of light high-velocity material originating near the core-mantle boundary, (3) temperature differences and perturbations of the core-mantle boundary and the earth's surface associated with mantle-wide convection, or (4) static chemical heterogeneities in a nonconvecting mantle. The first three alternatives, all involving flow in the lower mantle, may be complementary but act on a different time scale. There appears to be a westward or northwestward translation of some anomalies with respect to the pattern obtained for the innermost shell. In particular, the direction of translation of a large negative anomaly under the Pacific is in agreement with the sense of motion of the Pacific plate. We must caution the reader, however, that this is a highly speculative interpretation. If correct, it would favor the convective hypotheses.

A global thermospheric model based on mass spectrometer and incoherent scatter data MSIS, 2. Composition

Abstract: Measurements of O, He, and Ar from neutral gas mass spectrometers on four satellites (Ogo 6, San Marco 3, Aeros A, and AEC-C) and inferred oxygen and hydrogen densities from an ion mass spectrometer on AE-C have been combined with a neutral temperature and nitrogen density model to produce a global model of thermospheric composition in terms of inferred variations at 120 km. The data set covers the time period from mid-1969 to mid-1975. The MSIS (mass spectrometer and incoherent scatter data) model is compared with the Ogo 6 model (Hedin et al., 1974). Ar variations at 120 km tend to be in phase with temperature variations and inverse to the He, O, and H variations.

Experimental deformation of dry westerly granite

Abstract: The deformation behavior of quartz and feldspar has been studied in Westerly granite deformed dry at a constant strain rate of 10−6/s, confining pressures of 1.5–15 kbar, and temperatures of 25°–1000°C. Samples deformed at lower temperatures and pressures show throughgoing faults; those deformed at intermediate conditions show a combination of grain-scale faults and plastic deformation; and those deformed at higher temperatures and pressures show plastic deformation with no faults of any scale. On a grain scale the deformation is inhomogeneous at all conditions because of the polyphase nature of the material. Detailed petrographic and transmission electron microscope (TEM) observations have been made of the deformed specimens. The fault gouge consists of very fine grained material which verges on being amorphous, but no evidence of melt was seen. In the regions away from fault zones, there is a transition from dominantly microcracking to dominantly dislocation glide and climb; this transition is primarily a function of temperature. Dislocation motion is thermally activated and is probably almost unaffected by pressure over the range investigated. Thus at low temperature the strain rate that can be produced by dislocation motion is limited, and the difference between this and the imposed strain rate must occur by microcracking. The way in which the microcracking accomplishes the deformation depends on pressure. At low pressures (<5 kbar) the microcracks link up to form a throughgoing fault after very low strain; at higher pressures (7.5–15 kbar) they produce only grain-scale faults, ‘deformation bands,’ and undulatory extinction, and no throughgoing faults are formed after 15–20% shortening. At the laboratory strain rate of 10−6/s the transition from dominantly microcracking to dominantly dislocation motion occurs at approximately 300°–400°C for quartz and 550°–650°C for feldspar. When one allows for the slower natural strain rates and the presence of water, this grain-scale brittle-ductile transition may correspond to the limiting depth of earthquakes on strike slip faults.

Plasma bubbles and irregularities in the equatorial ionosphere

Abstract: The Atmosphere Explorer satellite observed large-scale (10 to 200-km) irregular biteouts of up to three orders of magnitude in the ion concentration in the nighttime equatorial F region associated with small-scale inhomogeneities in the ion concentration. Simultaneous plasma velocity observations show irregular upward and westward motion of the order of 150 m/s associated with some of these 'bubbles', while others move more slowly or move with approximately the velocity of the background plasma. The plasma composition signatures indicate that most of the bubbles observed have recently moved upward. Several features of recent VHF radar observations can be understood as resulting from these plasma bubbles, e.g., the 'plume' features and very high apparent velocities seen on range-time-intensity spread F maps and the very complex and/or wide spectral features observed using such radars.

Geochemical and mineralogical interpretation of the Viking inorganic chemical results

Abstract: The current status of geochemical, mineralogical, petrological interpretation of refined Viking Lander data is reviewed, and inferences that can be drawn from data on the composition of Martian surface materials are presented. The materials are dominantly fine silicate particles admixed with, or including, iron oxide particles. Both major element and trace element abundances in all samples are indicative of mafic source rocks (rather than more highly differentiated salic materials). The surface fines are nearly identical in composition at the two widely separated Lander sites, except for some lithologic diversity at the 100-m scale. This implies that some agency (presumably aeolian processes) has thoroughly homogenized them on a planetary scale. The most plausible model for the mineralogical constitution of the fine-grained surface materials at the two Lander sites is a fine-grained mixture dominated by iron-rich smectites, or their degradation products, with ferric oxides, probably including maghemite and carbonates (such as calcite), but not such less stable phases as magnesite or siderite.

Solar wind plasma injection at the dayside magnetospheric cusp

Abstract: Two mechanisms have been proposed for solar wind particle injection at the dayside magnetospheric cusps: magnetic merging and cross-field diffusion. These two mechanisms are experimentally distinguishable in that they produce different latitudinal distributions of particles penetrating to the low-altitude cusp. An examination of proton and electron measurements obtained by the AE-C satellite in the low-altitude dayside cusp reveals evidence of both types of injection processes. A majority of the injection events, especially the more intense fluxes, are best explained by a merging injection model in which cusp particles are confined to the poleward side of the last closed field line and have a characteristic energy that decreases with increasing latitudinal distance from the last closed field line. Less frequent and less intense injection events are better explained in terms of a diffusive injection of cusp particles onto closed dayside field lines with a characteristic energy that increases with increasing latitudinal distance from the last closed field line. Although diffusion appears to be quantitatively less important than merging in terms of the instantaneous particle injection rate, cross-field diffusion nevertheless appears to proceed at an unexpectedly fast rate, possibly exceeding the Bohm diffusion limit.

Observation of an ionospheric acceleration mechanism producing energetic (keV) ions primarily normal to the geomagnetic field direction

Abstract: O/sup +/ ions with energies of approximately 1 keV have been observed flowing upward out of the ionosphere with a pitch angle distribution having a minimum along the magnetic field direction and maxima in about 130/sup 0/--140/sup 0/ range. The measurements were obtained with an energetic ion mass spectrometer experiment on the satellite 1976-65B at an altitude of about 7600 km in the northern dayside polar cusp. The data are interpreted as resulting from a mechanism which accelerates ambient ionospheric ions in a direction perpendicular to the geomagnetic field.

Structure of the atmosphere of Mars in summer at mid-latitudes

Abstract: Instruments onboard Viking 1 and 2 landers were used to measure the structure of Mars' atmosphere in situ from near the surface to an altitude of 120 km. Atmospheric structure was found to be well defined by the instruments and relatively similar at the two sites. Viking 1 and 2 surface pressures were 7.62 and 7.81 mbar, and temperatures were 238 K and 236 K, respectively, with pressures at the elevation of the reference ellipsoid of 6.74 and 6.30 mbar. Mean temperature was found to decrease with a lapse rate of about 1.6 K/km (significantly subadiabatic) from above the boundary layer to about 40 km. The temperature was then near isothermal with a large-amplitude wave superimposed (attributed to the diurnal thermal tide). It is suggested that the mean profile is governed by radiative equilibrium. The obtained density data are found to merge well with those obtained by an upper-atmosphere spectrometer (at 200 km). The temperature wave is found to continue above 100 km while increasing in wavelength and amplitude.

Resonant interaction of oceanic internal waves

Abstract: Three classes of nonlinearly interacting triads, whose wave numbers and frequencies satisfy K1 ± K2 = K3 and ω1 ± ω2 = ω3, dominate the numerically computed transfer of energy in the Garrett and Munk models of the oceanic internal wave spectrum. We designate these triads as induced diffusion, elastic scattering, and parametric subharmonic instability. Special characteristics of wave number and frequency among the components identify each triad class. The concepts represented by these classes simplify an otherwise complex process and help explain some of the basic features of the observed internal wave spectrum.

Theory for passive microwave remote sensing of near‐surface soil moisture

Abstract: The theory of microwave thermal emission from a nonscattering half-space medium is developed for application to regions with nonuniform subsurface soil-moisture and temperature variations. A coherent stratified model is presented which is valid for nonuniform temperature profiles and rapidly varying moisture profiles, under which conditions the commonly used emissivity and radiative-transfer approaches become inaccurate. For naturally occurring profiles the stratified model gives more accurate results than the other approaches at frequencies below about 4 GHz. Experimental results from ground-based radiometric observations of a controlled target area compare systematically with brightness temperatures predicted from the theoretical model to within approximately 10 K. Results of dielectric-constant measurements of the sand are given at seven frequencies in the microwave range and for moisture contents in the range from 0% to 30% by volume. By using this model, the thermal microwave emission spectrum is computed for a number of representative moisture and temperature profiles in the frequency range from 0.25 to 25 GHz.

Meteorological results from the surface of Mars: Viking 1 and 2

Abstract: The meteorology equipment carried by the Viking landers was intended to measure atmospheric temperature, wind speed, wind direction, and pressure. During the summer months, the winds were a few meters per second, with a complex hodograph and the Lander-1 site, dominated by counterclockwise turning of the wind, and a simpler hodograph at the Lander-2 site, marked by clockwise turning of the wind. With advancing season, the repetitive wind pattern began to break down, and protracted northeasterly winds were recorded on several occasions (some of which are associated with lower than normal temperatures). Examples are given of wind and temperature traces over short periods, illustrating the effects of convection, static stability, and lander interference. A theoretical argument, based on the horizontal scale dictated by heating of slopes and on vertical mixing of momentum, is presented to explain the different sense of wind rotation at the two lander sites.

Substorm processes in the magnetotail - Comments on 'On hot tenuous plasmas, fireballs, and boundary layers in the earth's magnetotail' by L. A. Frank, K. L. Ackerson, and R. P. Lepping

Abstract: Various theories regarding the magnetotail are reviewed and discussed. These include the work of Dungey (1961) and Eastman et al., (1976) regarding the generation of the magnetotail, Frank et al., (1976) concerning the so-called magnetotail fireball and its characteristics, and Hones et al., (1976 and 1976a) on the formation of a neutral line across the near-earth plasma sheet near the substorm onset. A detailed discussion of a fireball encounter during 0900-1400 UT in April 1974 is presented, noting plasma and magnetic phenomena observed, and magnetic records from the earth. A critique is made by Hones of the interpretation of this fireball made by Frank et al. In an accompanying reply, Frank et al. comment on the observations made by Hones, with attention to the most evident discrepancy between the two theories, i.e., the generation of large closed magnetic loops in the plasma sheet during magnetic substorms.