Showing papers in "Earth Moon and Planets in 1987"
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TL;DR: In this paper, the authors show that the radial spacing between concentric rings of impact basins that lack central peaks is statistically similar and nonrandom on the Moon, Mercury, and Mars, both inside and outside the main ring.
Abstract: Radial spacing between concentric rings of impact basins that lack central peaks is statistically similar and nonrandom on the Moon, Mercury, and Mars, both inside and outside the main ring. One spacing interval, (2.0 ± 0.3)0.5D, or an integer multiple of it, dominates most basin rings. Three analytical approaches yield similar results from 296 remapped or newly mapped rings of 67 multi-ringed basins: least-squares of rank-grouped rings, least-squares of rank and ring diameter for each basin, and averaged ratios of adjacent rings. Analysis of 106 rings of 53 two-ring basins by the first and third methods yields an integer multiple (2 ×) of 2.00.5D. There are two exceptions: (1) Rings adjacent to the main ring of multi-ring basins are consistently spaced at a slightly, but significantly, larger interval, (2.1 ± 0.3)0.5D; (2) The 88 rings of 44 protobasins (large peak-plus-inner-ring craters) are spaced at an entirely different interval (3.3 ± 0.6)0.5D. The statistically constant and target-invariant spacing of so many rings suggests that this characteristic may constrain formational models of impact basins on the terrestrial planets. The key elements of such a constraint include: (1) ring positions may not have been located by the same process(es) that formed ring topography; (2) ring location and emplacement of ring topography need not be coeval; (3) ring location, but not necessarily the mode of ring emplacement, reflects one process that operated at the time of impact; and (4) the process yields similarly-disposed topographic features that are spatially discrete at 20.5D intervals, or some multiple, rather than continuous. These four elements suggest that some type of wave mechanism dominates the location, but not necessarily the formation, of basin rings. The waves may be standing, rather than travelling. The ring topography itself may be emplaced at impact by this and/or other mechanisms and may reflect additional, including post-impact, influences.
92 citations
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TL;DR: In this paper, a 3-fold improvement over the previous mapping done in the late 1960's was achieved by using the Arecibo radar antenna beam for high-resolution radar observations of the Moon in 1981-1984.
Abstract: New radar observations of the Moon in 1981–1984 were made using the 430 MHz (70 cm wavelength) radar at the Arecibo observatory, Puerto Rico. The new observations have produced a high resolution lunar radar map with radar cell-sizes near 2–5 km. This new resolution is a three-fold improvement over the previous mapping done in the late 1960's. Since the Arecibo radar antenna beam is only ten arc-minutes (about one-third of the width of the lunar disk), this new map is a mosaic of some eighteen observations. A radarmetric control between the various pieces of the mosaic was obtained via a ‘beam-swing,’ limb-to-limb calibration. When the limb-to-limb calibration was combined with the mosaic, there were significant radar scattering differences across the maria. Eastern Mare Tranquillitatis and western Oceanus Procellarum have weaker echoes than other maria, while the central portion of Mare Serenitatis and northern Mare Imbrium have stronger echoes. There is a radar scattering difference across the southern terra as areas nearer Mare Orientale have stronger echoes than areas further from Mare Orientale.
61 citations
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TL;DR: In this article, the authors investigated the formation and migration of bodies from the feeding zones of the giant planets by using computer simulation of evolving disks, which initially consisted of hundreds of particles moving about the Sun and coagulating under collisions.
Abstract: Within the model of solid-body accumulation of planets (or their nuclei) the accumulation and migration of bodies from the feeding zones of the giant planets are investigated. The investigation is based on results of computer simulation of evolving disks which initially consisted of hundreds of particles moving about the Sun and coagulating under collisions. In some models the disks initially consisted of identical bodies. In other models they included also almost-formed planets. The computer simulation results as well as analytical investigations of the disk evolution depending on the number of particles in the disk allowed some estimates and conclusions on the accumulation process when the number of initial bodies was great (~ 106−1012). In this paper the characteristics of an initial protoplanetary circumsolar cloud, the body migration in the forming solar system, the planet orbit evolution, the formation of the beyond-Neptune belt and asteroid belts between the giant planet orbits are considered. The results obtained confirm many analytical estimates earlier made by V. S. Safronov and his colleagues.
28 citations
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TL;DR: In this paper, it is suggested that outbursts of cosmic ray electrons from the Galactic Center penetrate the Galaxy relatively undamped and are able to have a major impact on the Solar System through their ability to vaporize and inject cometary material into the interplanetary environment.
Abstract: It is proposed that outbursts of cosmic ray electrons from the Galactic Center penetrate the Galaxy relatively undamped and are able to have a major impact on the Solar System through their ability to vaporize and inject cometary material into the interplanetary environment. It is suggested that one such ‘superwave’, passing through the Solar System toward the end of the Last Ice Age, was responsible for producing major changes in the Earth's climate and for indirectly precipitating the terminal Pleistocene extinction episode. The high concentrations of 10Be, NO3
−, Ir and Ni observed in Late Wisconsin polar ice are consistent with this scenario. The intensities of the Galactic nonthermal radio background and diffuse X-ray emission ridge are shown to vary with Galactic longitude in the same manner as electron intensity along the proposed superwave ‘event horizon’. The high luminosities and unusual structural features which characterize the Crab Nebula and Cassiopeia A are shown to be attributable to the fact that these remnants happen to coincide with this event horizon and are being externally impacted by an intense volley of relativistic electrons travelling from the Galactic Center direction. The same cosmic ray volley is also shown to be able to account for the unusual structure of the extended radio source CTB 80.
25 citations
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TL;DR: In this article, the authors extended Hameen-Anttila's (1984) theory of self-gravitating collisional particle disks to include the effects of particle spin.
Abstract: Hameen-Anttila's (1984) theory of self-gravitating collisional particle disks is extended to include the effects of particle spin. Equations are derived for the coupled evolution of random velocities and spins, showing that friction and surface irregularity both reduce the local velocity dispersion and transfer significant amounts of random kinetic energy to rotational energy. Results for the equilibrium ratio of rotational energy to random kinetic energy are exact not only for identical nongravitating mass points, but also if finite size, self-gravitating forces, or size distribution are included. The model is applied to the dynamics of Saturn's rings, showing that the inclusion of rotation reduces the geometrical thickness of the layer of cm-sized particles to, at most, about one-half, with large particles being less affected.
24 citations
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TL;DR: In this article, a smoothed-particle method was used to calculate the three-dimensional gas flow in the solar nebula, which is subject to the gravity of the Sun and proto-Jupiter, numerically calculated by using a 3D hydrodynamic code.
Abstract: Three-dimensional gas flow in the solar nebula, which is subject to the gravity of the Sun and proto-Jupiter, is numerically calculated by using a three-dimensional hydrodynamic code - i.e., the socalled smoothed-particle method. The flow is circulating around the Sun as well as falling into a potential well of proto-Jupiter. The results for various masses of proto-Jupiter show that (1) the e-folding growth time of proto-Jupiter by accretion of the nebular gas is as short as about 300 years in stages where the mass of proto-Jupiter is 0.2 ~ 0.5 times the present Jovian mass, and that (2) proto-Jupiter begins to push away the nebular gas from the orbit of proto-Jupiter and form a gap around the orbit, when its mass is about 0.7 times the present Jovian mass. It is possible that this pushing-away process determined the present Jovian mass.
24 citations
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TL;DR: Bell Regio as discussed by the authors is a highland fragment south of Ishtar Terra, extending 1300 km in N-S direction and 900 km in E-W direction, with a semi-corona (other coronae on Venus are associated with volcanic-tectonic processes).
Abstract: Bell Regio is a highland fragment south of Ishtar Terra, extending 1300 km in N-S direction and 900 km in E-W direction. South of this region Eisila Regio is located with an E-W extension of 8000 km and a width of 2000 km. Bell Regio consists of two large massifs: a northern massif with maximum altitudes of 2.5 to 3.0 km above the 6051 km datum and with a semi-corona (other coronae on Venus are associated with volcanic-tectonic processes) and a southern massif with a maximum of 4 to 4.5 km above the datum. The possible shield volcano Tepev Mons of 250 km in diameter is superimposed on the southern massif. It shows a radar dark crater of 40 km diameter on its eastern flank, a crater-like feature of 15 km diameter on the top and a radar bright area extending from the dark crater across the summit. South of Tepev Mons are several volcanic structures with summit depressions. The crest of Bell Regio exhibits a N-S extending fossa system. The whole fresh appearing plain-like area has been classified as rather young compared to other units. Gravity data show a maximum of 33 mGal at Bell Regio and 35 mGal at eastern Eisila Regio. The basins north and south of the highland fragments are associated with gravity lows. Density models have been calculated along the gravity profile Rev. 163 of Pioneer Venus Orbiter across Bell and Eisila Regiones assuming Airy isostatic compensation of the topography and considering several boundary conditions (e.g. mean crustal thickness T 100 km. The highland of Beta Regio has, like Bell Regio, a N-S rifting system, volcanic structures, a fresh appearing plain-like surface and either deep-seating compensating masses or near surface surplus masses. Bell can be considered as little sister of Beta. The geological and geophysical results imply a volcanic-tectonic uplift over a hot spot. The conditions of Atla Regio in eastern Aphrodite Terra are similar. Thus the existence of volcanic-tectonic uplifts support the important role of hot spot volcanism on Venus.
22 citations
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TL;DR: The evidence for the existence of the magnetic field of the planet Mars and for the effectiveness of the dipolar part of the field as an obstacle to the solar wind at the most frequent parameters of the latter is presented in this paper.
Abstract: The evidence is presented for the existence of the magnetic field of the planet Mars and for the effectiveness of the dipolar part of the field as an obstacle to the solar wind at the most frequent parameters of the latter. The dipolar magnetic moment of Mars is (1.5–2.20 × 1022 G cm3. The dipole axis makes an angle i⩽15‡ with the rotation axis of the panel. The magnetic north pole of Mars is located in its southern hemisphere. In terms of the precession dynamo model, the magnetic fields of the Earth and Mars are similar. This indicates that the Martian magnetic field is associated with the present-day dynamo-process in the Martian liquid core.
20 citations
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TL;DR: In this paper, the influence of the initial stress on the propagation of Rayleigh waves in a homogeneous-isotropic generalized thermo-elastic body, subject to the boundary conditions that the outer surface is traction free, was studied.
Abstract: In this paper we studied the influence of the initial stress on the propagation of Rayleigh waves in a homogeneous-isotropic, generalized thermo-elastic body, subject to the boundary conditions that the outer surface is traction free. In addition it is subject to linear radiations, adiabatic isothermal transfer conditions. We found that the frequency equation of Rayleigh waves contains a term involving the initial stress and, therefore, the phase velocity of Rayleigh waves changes with respect of this initial stress, when the initial stress, vanishes, the derived frequency equation reduces to that one obtained in classical generalized thermo-elastic case which includes the relaxation time of heat conduction.
19 citations
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TL;DR: In this paper, a model for the calculation of the thermal history of a planet is constructed, based on the assumption that the core is not liquid, as given in models employing a dry mantle solidus, but solid to a large extent, which prevents the operation of a large-scale dynamo and explains in that way the lack of a magnetic field.
Abstract: Conventional evolutionary models for Mars adopt a dry mantle solidus. Taking into account the condensation conditions in the preplanetary nebula in the accretion zone of Mars, it can be concluded that large amounts of water or hydrated silicates have condensed in those regions. Therefore, water influences significantly the melting behaviour and the viscosity of the silicatic material. A model for the calculation of the thermal history of a planet is constructed. On this basis, and use of water — saturated solidus — it is possible to derive that the core is not liquid, as given in models employing a dry mantle solidus, but solid to a large extent, which prevents the operation of a large-scale dynamo and explains in that way the lack of a magnetic field. With these assumptions one can construct a possible evolutionary scheme that covers early crust differentiation, a hot thermal past and the missing magnetic field at present.
16 citations
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TL;DR: Based on regional maps of orbital geochemical variables, major geochemical heterogeneities in the Undarum/Sumans/Balmer region of the moon are found which are usually associated with distinct geological features, and which reveal a north/south dichotomy as discussed by the authors.
Abstract: Based on regional maps of orbital geochemical variables, major geochemical heterogeneities in the Undarum/Sumans/Balmer region of the moon are found which are usually associated with distinct geological features, and which reveal a north/south dichotomy. The mapped mare and plains units are often the site of heterogeneity, and the plains units in the Balmer basin are probably composed of volcanic material, possibly KREEP-enriched basalt. Data suggest that the northeastern part of the region may contain a minor spinel component. Major mare units of the region are found to have two distinct compositions, possibly indicating two different source regions for the basalts deposited there, but more likely indicating contamination with different amounts of highland debris.
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TL;DR: In this article, the auroral frequency of occurrences (A) for the 20th solar cycle and for the geomagnetic latitudes 54‡-63‡ N has been investigated in relation to sunspot numbers (Rz), number of flares (F), the solar wind streams derived from the coronal holes (H) and the Geomagnetic index (Ap).
Abstract: The auroral frequency of occurrences (A) for the 20th solar cycle and for the geomagnetic latitudes 54‡–63‡ N has been investigated in relation to sunspot numbers (Rz), number of flares (F), the solar wind streams derived from the coronal holes (H) and the geomagnetic index (Ap). The relationship between A and the other indices were found to be strongly latitude dependent. At around 57‡–58‡ N, a drastic change in this relationship occurs, and an attempt is made qualitatively to evaluate this latitudinal variation.
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TL;DR: In this article, the authors examined shuttle radar images (SIR-A) of volcanic and impact craters to assess their appearance on radar images and concluded that it will be difficult to separate small impact crater from small volcanic craters on Venus using radar images, and suggest that it is necessary to understand the geological setting of the areas containing in order to determine their origin.
Abstract: Shuttle radar images (SIR-A) of volcanic and impact craters were examined to assess their appearance on radar images. Radar characteristics were determined for: (a) 9 maarlike craters in the Pinacate volcanic field, Sonora, Mexico; (b) the caldera of Cerro Volcan Quemado, in the Bolivian Andes; (c) Talemzane impact crater, Algeria; and (d) Al Umchaimin, a possible impact structure in Iraq. SIR-A images were compared with conventional photographs and with results from field studies. Consideration was then given to radar images available for Venus, or anticipated from the Magellan mission. Of the criteria ordinarily used to identify impact craters, some can be assessed with radar images and others cannot be used; planimetric form — expressed as circularity — and ejecta-block distribution can be assessed on radar images, but rim and floor elevations relative to the surrounding plain and disposition of rim strata are difficult or impossible to determine. We conclude that it will be difficult to separate small impact craters from small volcanic craters on Venus using radar images, and suggest that it will be necessary to understand the geological setting of the areas containing in order to determine their origin.
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TL;DR: In this paper, high-resolution high-incidence-angle Arecibo images and Pioneer-Venus data of Southern Ishtar Terra and the flanking plains of Guinevere and Sena on Venus are analyzed.
Abstract: High-resolution high-incidence-angle Arecibo images and Pioneer-Venus data of Southern Ishtar Terra and the flanking plains of Guinevere and Sena on Venus are analyzed. The low predominantly volcanic plains of Guinevere and Sedna Planitae are the oldest of the mapped terrains. The complex tectonic deformation in the Southern Ishtar Transition Zone postdates much of the low plains, and delineates the steep-sloped flanks of Ishtar Terra. Lakshmi Planum is found to have a distinctive volcanic style, and to postdate the Southern Ishtar Transition Zone, at least in part. Data show relatively recent plains-style volcanism to have occurred locally in Sedna Planitae, and to embay the Southern Istar Transition Zone. Arecibo data show additional coronae in the lowlands, suggesting that corona formation may be a more widespread process than indicated by Venera 15/16 images.
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TL;DR: In this paper, an empirical relation for the cosmic-ray modulation has been found, which is well explained by a generalization of the Simpson solar wind model which has been proved by the spherically symmetric diffusion-convection theory.
Abstract: An analysis of monthly data from nine world-wide neutron monitoring stations over the period 1965–1975 is carried out for the study of the long-term cosmic-ray modulation. In an attempt to gain insight into the relationships which exist between solar activity, high-speed solar wind streams and various terrestrial phenomena an empirical relation for the cosmic-ray modulation has been found. Accordingly the modulated cosmic-ray intensity is equal to the galactic cosmic-ray intensity corrected by a few appropriate solar, interplanetary and terrestrial activity indices which causes the disturbances in interplanetary space, multiplying with the corresponding time-lag of cosmic-ray intensity from each of these indices. This relation is well explained by a generalization of the Simpson solar wind model which has been proved by the spherically symmetric diffusion-convection theory.
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TL;DR: In this article, economical and stable recurrence formulae for the Earth's zonal potential and its gradient for Burdet's regularized theory were established for any number N of the zonal harmonic coefficients.
Abstract: In this paper, economical and stable recurrence formulae for the Earth's zonal potential and its gradient for Burdet's regularized theory will be established for any number N of the zonal harmonic coefficients. A general recursive computational algorithm based on these formulae is also established for the initial value problem of Burdet oscillator for the prediction of artificial satellites in the Earth's gravitational field with axial symmetry. Applications of the algorithm for the problem of the final state prediction are illustrated by numerical examples of three test orbits each for two geopotential models corresponding to N = 2 and N = 36. A final state of any desired accuracy is obtained for each case study, a result which shows the flexibility of the algorithm.
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TL;DR: In this paper, the combined effect of global dust storms and the oblateness on the mean seasonal daily insolations at the Martian surface is investigated, and it is found that the percentage differences of the mean summer daily insolation are dependent upon the optical depths (Τ) considered.
Abstract: In this short paper, the combined effect of global dust storms and the oblateness on the mean seasonal daily insolations at the Martian surface is investigated. Due to the flattening, the mean summertime insolation is increased at equatorial and low latitudes, decreased at mid- and high latitudes. When comparing a spherical with an oblate planet Mars, it is found that the percentage differences of the mean summer daily insolations are dependent upon the optical depths (Τ) considered. For an atmosphere without aerosols, the maximum percentage differences are respectively equal to + 0.05 and − 0.2%; at Τ = 3.0 the corresponding values amount to about 0.1 and 2%. In winter, the mean daily insolations are decreased over the entire latitudinal interval, where the maximum values are found at polar region latitudes; at e.g. a latitude of 85 ‡ the loss of solar energy enhances from 2 (Τ = 0.0) to more than 30% (Τ = 3.0). The mean annual daily insolation is maximally reduced by about 0.5 and 2% for optical thicknesses of 0.0 and 3.0, respectively.
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TL;DR: An attempt is made here to outline the basic schemes envisaged in modern studies of the origin of the Earth and the ramification of consequences for its early evolution that currently appear allowable and supported by observation.
Abstract: Space exploration has changed our views not only on the properties of celestial bodies and the interplanetary medium but also our perspective on the formation of such bodies, including the Earth. However, on few points do the new hetegonic insights approach certainty. More appropriately, they can be said to raise physically meaningful questions and to decrease the degrees of freedom allowable in any given scenario, which necessarily extends into the early history of the planetary crust, ocean and atmosphere. An attempt is made here to outline the basic schemes envisaged in modern studies of the origin of the Earth and the ramification of consequences for its early evolution that currently appear allowable and supported by observation.
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TL;DR: In this paper, the authors show that distance from the Sun and the efficiency of retention of accretional energy control the early evolution of the terrestrial planets and explain why the third planet in our solar system is an "aqua" planet.
Abstract: On the basis of the model proposed by Matsui and Abe, we will show that two major factors — distance from the Sun and the efficiency of retention of accretional energy — control the early evolution of the terrestrial planets. A diagram of accretional energy versus the optical depth of a proto-atmosphere provides a means to follow the evolutionary track of surface temperature of the terrestrial planets and an explanation for why the third planet in our solar system is an ‘aqua’-planet.
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TL;DR: In this paper, the effect of interplanetary sector boundary crossing on grain distribution was studied for Halley's comet around the time of encounter by the GIOTTO and VEGA 1 and 2 spacecraft in March 1986.
Abstract: Recognizing that gains in the cometary dust tail are electrically charged, the effect of an interplanetary sector boundary crossing on grain distribution is studied. Specifically, Halley's comet around the time of encounter by the GIOTTO and VEGA 1 and 2 spacecraft in March 1986 is considered. The smallest dust particles are strongly effected, and the projection of their distributions in a plane containing the sun-comet axis and normal to the orbital plane shows a wavy appearance. Also, since reversals in the interplanetary magnetic field occur with a periodicity of 5 to 10 days, the spacecraft, which follow 3 to 4 days apart, are likely to encounter entirely different dust distributions at the lower end of the mass spectrum.
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TL;DR: In this paper, the axion flux from the giant planets is also estimated, based on the observed heat flow from planetary objects of the solar system, which can be used to put rather stringent limits on any background monopole flux.
Abstract: Recent improvements in the quantitative estimates of the observed heat flow from planetary objects of the solar system may be used to put rather stringent limits on any background monopole flux. The axion flux from the giant planets is also estimated.
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TL;DR: A general velocity-height relation for both antimatter and ordinary matter meteor is derived in this paper, which can be expressed as % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rN
Abstract: A general velocity-height relation for both antimatter and ordinary matter meteor is derived. This relation can be expressed as % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSaaaeaacq% aHfpqDdaWgaaWcbaGaamOEaaqabaaakeaacqaHfpqDdaWgaaWcbaGa% eyOhIukabeaaaaGccqGH9aqpcaqGLbGaaeiEaiaabchacaqGGaWaam% WaaeaacqGHsisldaWcaaqaaiaadkeaaeaacaWGHbaaaiaabwgacaqG% 4bGaaeiCaiaabIcacaqGTaGaamyyaiaadQhacaGGPaaacaGLBbGaay% zxaaGaeyOeI0YaaSaaaeaacaWGdbaabaGaamOqaiabew8a1naaBaaa% leaacqGHEisPaeqaaaaakmaacmaabaGaaGymaiabgkHiTiaabwgaca% qG4bGaaeiCamaadmaabaGaeyOeI0YaaSaaaeaacaWGcbaabaGaamyy% aaaacaqGLbGaaeiEaiaabchacaqGOaGaaeylaiaadggacaWG6bGaai% ykaaGaay5waiaaw2faaaGaay5Eaiaaw2haaiaacYcaaaa!64FD!\[\frac{{\upsilon _z }}{{\upsilon _\infty }} = {\text{exp }}\left[ { - \frac{B}{a}{\text{exp( - }}az)} \right] - \frac{C}{{B\upsilon _\infty }}\left\{ {1 - {\text{exp}}\left[ { - \frac{B}{a}{\text{exp( - }}az)} \right]} \right\},\]where υ z is the velocity of the meteoroid at height z, υ∞ its velocity before entrance into the Earth's atmosphere, α is the scale-height, and C parameter proportional to the atom-antiatom annihilation cross- section, which is experimentally unknown. The parameter B (B = DAϱ0/m) is the well known parameter for koinomatter (ordinary matter) meteors, D is the drag factor, ϱ0 is the air density at sea level, A is the cross sectional area of the meteoroid and m its mass.
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TL;DR: In this paper, it was proposed that the cometary analog of a terrestrial aurora was responsible for the enhanced fluxes of suprathermal (keV) electrons and associated plasma waves observed in the cometosheath of Comet Halley during its VEGA 2 encounter.
Abstract: It is proposed that the cometary analog of a terrestrial aurora was responsible for the enhanced fluxes of suprathermal (keV) electrons and associated plasma waves observed in the cometosheath of Comet Halley during its VEGA 2 encounter. The non-detection of such suprathermal electron fluxes during the GIOTTO encounter is ascribed to the much quieter solar wind conditions at that time.
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TL;DR: In this paper, it was shown that the radial compressional forces generated by the tendency for downslope movement of surface strata, vertical volcanic intrusions and traction of mantle spreading beneath Tharsis were transmitted through the lithosphere to form peripheral mare ridge zones.
Abstract: The tectonics of the Tharsis and adjoining areas is considered to be associated with the convection in the Martian mantle. Convection and mantle plume have been responsible for the primary uplift and volcanism of the Tharsis area. The radial compressional forces generated by the tendency for downslope movement of surface strata, vertical volcanic intrusions and traction of mantle spreading beneath Tharsis were transmitted through the lithosphere to form peripheral mare ridge zones. The locations of mare ridges were thus mainly controlled by the Tharsis-radial compression. The load-induced stresses then contributed on further ridge formation over an extended period of time by the isostatic readjustment which was reponsible for long-term stresses in the adjoining areas. Extrusions, changes in internal temperature and possible phase changes may also have caused changes in mantle volume giving rise to additional compressional forces and crustal deformations.
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TL;DR: In this paper, single and multiple scattering calculations were performed for a spherically symmetric cometary atmosphere irradiated by a plane parallel source, and it was suggested that the increased flux found for anisotropic phase functions is due to the effect of directional scattering in the forward sun-comet axis.
Abstract: Single and multiple scattering calculations were performed for a spherically symmetric cometary atmosphere irradiated by a plane parallel source. It is suggested that the increased flux found for anisotropic phase functions is due to the effect of directional scattering in the forward sun-comet axis. The isotropic multiply scattered flux at the surface is shown to be an increasing function of the opacity (tau) for tau of less than about 2.5. At large tau values, the maximum in the downward directed scattered flux still increases, but occurs at a height of several radii above the nucleus, resulting in a reduction at the surface. Results suggest that, except in the vicinity of the sun-comet axis, the plane parallel geometry tends to underestimate the degree of scattering.
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TL;DR: In this article, a thermal drive model was proposed based on an expression for externally derived gravitational influences between two bodies; and internal heat generation via radiogenic decay (expressed by surface area/volume ratio).
Abstract: The position of the satellite within the protonebula, the influence of the parent planet, particularly the relative effects of tidal (gravitational) as opposed to radiogenic (internal) heat generating processes, as well as the type of ice, exert a control on the evolutionary histories of the Jovian and Saturnian satellites. The landscapes of the moons are modified by surface deformational processes (tectonic activity derived from within the body) and externally derived cratering. The geological history of the Galilean satellites is deduced from surface stratigraphic successions of geological units. Io and Europa, with crater-free surfaces, are tectonically more advanced than crater-saturated Callisto. Two thermal-drive models are proposed based on: an expression for externally derived gravitational influences between two bodies; and internal heat generation via radiogenic decay (expressed by surface area/volume ratio). Both parameters, for the Galilean satellites, are plotted against an inferred product of tectonic processes — the age of the surface terrain. From these diagrams, the tectonic evolutionary state of the more distant Saturnian system are predicted. These moons are fitted into an evolutionary framework for the Solar System.
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TL;DR: In this article, the authors used topographic information, surface structures and construction of the Martian Tharsis bulge to estimate the previous stresses across the low-lying peripheral margins of the crustal blocks in terms of simple compensation models.
Abstract: Topographic information, surface structures and construction of the Martian Tharsis bulge are used to estimate the previous stresses across the low-lying peripheral margins of the crustal blocks in terms of simple compensation models. Hot mantle activity, crustal roots, isostasy, and late-stage extensive lithosphere thickening together with volcanic building have been in combined response to the high-elevated Tharsis bulge. The initial phases of the Tharsis building have been dominated by the mantle plume doming, followed by extrusional dome raising. The volcanism has been most important bulge building factor only after thickening of the crust. During the initial mantle-generated doming and igneous activity the thin-lithosphere block tectonics has been very important. There has been a compressional peripheral zone around the bulge giving rise to dorsa formation while the high bulge crests have been in tensional state. The situation may be favorable for comparative studies with other planets. We may have something to learn from this block tectonics on the one-plate planet Mars even in respect to the Earth's plate tectonic paradigm.
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TL;DR: In this paper, the results of photometric measurements (in UBV filters) of Comet P/Halley during four nights in January 1986 have been presented, showing that rapid recurring increases of P/halley's brightness were observed during these four nights.
Abstract: The results of photometric measurements (in UBV filters) of Comet P/Halley during four nights in January 1986 have been presented. Rapid recurring increases of P/Halley's brightness were observed during these four nights. This phenomenon seemed to occur at a time interval of about 23.7 ± 0.3 hr. The maximum variation of brightness during an individual night amounts to δU ~ 0.m70; δB ~ 0.m, 65 and δV ~ 0.m66 in U, B, and V filters, respectively. Rapid irregular short-lived fluctuations of brightness of the order of 0.m1 were also found present during an individual night. The present observations show the active nature of P/Halley.
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TL;DR: The supernova shell fragmentation model of solar system formation is used to calculate the mass distributions of other solar nebulae as mentioned in this paper, which is also successful in describing the mass distribution of our solar system.
Abstract: The supernova shell fragmentation model of solar system formation — previously shown to be successful in describing the mass distribution of our solar system — is used to calculate the mass distributions of other solar nebulae.
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TL;DR: In this paper, S-bursts and other short-lived events in Jupiter's decametric radio spectra were studied with an acousto-optical radio spectrograph and a charge-coupled device (CCD) readout.
Abstract: S-bursts and other short-lived events in Jupiter's decametric radio spectra are studied with an acousto-optical radio spectrograph (AORS) and a charge-coupled device (CCD) readout. The lifetimes of regular S-bursts are of the order of tens of milliseconds. Lifetimes of S-bursts that occur in quasi-periodic trains are mostly below 20 ms. Short-lived bursts that do not seem to drift in frequency are also observed. Such bursts (nondrift bursts) may occur in S-trains and at the vertex of tilted-V patterns. The vertex bursts have lifetimes comparable to those of the S-train bursts. Preliminary measurements of the risetimes of bursts indicate that a significant proportion of the vertex bursts exhibit risetimes of less than 2 ms.