Showing papers in "Earth Moon and Planets in 1992"

Martian lake basins and lacustrine plains

Abstract: Outflow channels and valley systems are evidence of water flow on the surface of Mars. Whenever there is a consequent flow of water on an irregular surface, temporary impoundment in surface depressions will form lakes. A classification of martian lake basins based on the location of the basin in respect to water sources is proposed. The classes are Type 1: Valley-head basins, Type 2: Intravalley basins, Type 3: Valley-terminal basins, and Type 4: Isolated basins. Martian lakes are ephemeral features. Many craters and irregular depressions impounded water only until the basins filled and overflowed. Water escaping by spillover rapidly cut crevasses in the downstream side of basins and drained the ponds. Clastic lacustrine sediments collected in the lakes as flowing water lost velocity and turbulence. Evaporitic deposits may be significant in those basins that were not rapidly drained. Sediments deposited in lake basins form smooth, featureless plains. Lacustrine plains are potentially candidate sites for Mars landings and for the search for evidence of ancient life.

Dynamics of dust in a plasma sheath and injection of dust into the plasma sheath above Moon and asteroidal surfaces

Abstract: We have examined single dust particle dynamics in a plasma sheath near the surface of solid bodies in space, considering conditions which resemble those of planetary system bodies, when photoelectric effect can be neglected. The forces on the dust particles are assumed to be from the electric field in the sheath and from gravitation only. As the dust particles will charge negatively in the sheath, these forces will act in opposite directions and may balance. The charge delay of a moving dust particle is responsible for many of the interesting dynamical properties, and we show that for a stationary plasma, dust motion is unstable to about one Debye length out from the surface of the solid body. This part of the sheath will therefore be devoid of dust particles as they will either fall down, escape completely from the solid body or collect and make damped oscillations at stable positions in the outer part of the sheath. With increasing plasma bulk speed towards the surface, the inner unstable part of the sheath will decrease in thickness. The sources for the dust in the sheath are assumed to be mainly ejecta from meteorites and micrometeorites, but may also, for the smallest solid bodies, be from electrostatic levitation of very small dust particles. We have for different sizes of solid bodies calculated the sizes of ejecta that can be ‘floated’ in the sheath. For the solar wind plasma, the suspended dust particles range from less than 1 Μm for the Moon to about 80 Μm for an asteroid with radius 1 km. These particles create a ‘dust atmosphere’. The results in this paper hold when the dust particle density is so low that the charges on the dust particles do not contribute significantly to the total space charge; a higher density will lead to a modification of the sheath. Our calculations show that ejecta below a certain size will be accelerated in the sheath and totally escape from the body even if they have near zero initial vertical velocity, while ejecta above this size will need a much larger velocity to escape. This is especially significant for the small solid bodies (radius of order km and less) which will therefore act as important sources of micronsized dust. This could be of significance for the dust production and the size distribution of dust in planetary ring systems.

Poynting-Robertson effect I. Equation of motion

Abstract: Derivations of the Poynting-Robertson effect are presented. They are based on the corpuscular nature of light (unlike Robertson's 1937 derivation). It is justified why currently presented derivations are incorrect and why classical (nonrelativistic) physics is not able to understand this effect. Relativistically covariant derivations not only for perfectly absorbing (spherical) dust particles are presented. Fundamental feature of the interaction between the dust particle and the electromagnetic radiation is the conservation of the (proper) mass of the particle.

Polarization Characteristics of the Coherent Backscatter Opposition Effect

Abstract: It has been suggested recently that coherent backscattering of photons from discrete disordered media, which has been recently discovered in laboratory-controlled experiments, may play a role in peculiar radar and light scattering from some atmosphereless solar system bodies. In this paper, a rigorous vector theory recently developed by Mishchenko (1991b, 1992a) is used to study polarization characteristics of the coherent backscatter opposition effect. Backscattering enhancement in different polarization components is studied and results of computer calculations for a representative selection of scattering models are presented. It is pointed out that these calculations support recent Hapke's (1990) explanation of unusual radar characteristics of icy outer planet satellites.

Poynting-Robertson effect II. Perturbation equations

Abstract: The problem of the complete set of perturbation equations of celestial mechanics in application to the Poynting-Robertson effect is discussed. Differential equations and initial conditions for them are justified. Sudden beginning of operation of the Poynting-Robertson effect (e.g., sudden release of dust particle from comet) is taken into account.

Multivariate classification methods in planetary sciences

Abstract: In this paper the problem of the classification of natural samples is discussed. An updated version of the G-mode multivariate statistical method for the classification of natural samples, applicable to a wide range of research fields, is discussed in this paper. This method allows an automatic classification in terms of homogeneous taxonomic units, without any a priori knowledge of the taxonomic structure of the natural observations; it provides informations on the different levels of classification present in the data set under study (classes and subclasses), on the level of information residing in each variable, on the level of similarity and/or difference among homogeneous classes.

Love numbers of the moon and of the terrestrial planets

Abstract: In the IERS Standards (1989), for the Moon the adopted value of the tide Love number, k 2, is equal to 0.0222. In this paper using the latest geodetic parameters of the Moon a group of internal structure models are constructed for this celestial body (see Table V), then the dependence of the Moon's core size on calculated value of k 2 is explored. The obtained results indicate that the second degree Love number, k 2 = 0.02664, of the lunar model 91–04 is near its observed value (0.027 ± 0.006). This implies that the Moon may possess an outer core of 660 km radius and of 300 kbar mean rigidity. With the same method the static Love numbers from degree 2 to 30 are computed for the terrestrial planets — Mercury, Venus, and Mars (see Table VII), and the influence of some parameters (such as the rigidity) of the outer core on low degree Love numbers is discussed. Finally, the likely range of the second degree Love numbers is determined for the terrestrial planets (see Table XI). It seems that if low degree Love numbers of a terrestrial planet can be detected in the future space explorations, there is some possibility to improve the planetary internal structure model. For example, as soon as space techniques yield an observed value of k 2 > 0.10 for Mercury, there will be reason to anticipate that a partly melted iron core exists in this planet.

Secular ring instability in the protoplanetary accretion disk

Abstract: The aim of the present paper is to show that an accretion disk, the material of which satisfies the Navier-Stokes equations of a compressible fluid, is secularly unstable to axisymmetric density disturbances even if Toomre's parameter Q is greater than one. This instability process, which can also be interpreted as negative diffusion, leads immediately to the formation of rings in the accretion disk as an intermediate step in the formation of planets, at least for the outer gaseous ones. We believe that the same process is also responsible for the ringlet-structure of planetary rings. Only a dynamical instability of axisymmetric density wave perturbations, represented by Toomre's condition Q < 1, does not exist in a viscous self-gravitating disk. On the base of a local theory we calculate the characteristic wave number and the corresponding time scale of the mode of maximum secular instability. In the global theory, we formulatea complex linear integral equation describing the diffusion instability in the accretion disk. The radial distances of the rings to the protosun obey in the outer range of the disk an exponential law provided that the quantity vΣ (the vertically integrated viscosity) is nearly constant throughout the disk. Our treatment gives arguments in favour for the result, that the spacing ratio q = r n+1 /r n between two consecutive protoplanetary orbits is approximately given by the formula q ≈ exp(2πΜ1/3), where Μ is close to the ratio of the accreted ring-masses in the disk and the mass of the central body. We believe that the formation process is shorter for the outer gaseous than for the inner rocky planets, because the global secular ring instability is evanescent in the inner regions of the disk.

Poynting-Robertson effect: `Circular' orbit

Abstract: Time evolution of the interplanetary dust particle under the action of the solar electromagnetic radiation (Poynting-Robertson effect) is investigated. Evolution of the initially circular orbit in terms of the orbital elements present in the standard equations for their secular changes is considered. It is pointed out that the osculating eccentricity is practically constant during the motion in spite of generally accepted opinion that the standard equations for the secular changes of orbital elements represent time evolution of the osculating elements.

Light scattering by nonspherical ice grains: An application to noctilucent cloud particles

Abstract: The light scattered by noctilucent cloud particles is nearly fully polarized at scattering angles in the vicinity of 90 ‡. This was one of the reasons to conclude that the upper limit of their sizes is not larger than about 0.12 Μm. Nevertheless, this estimate was made on the basis of the Mie scattering theory for spherical particles, whereas many investigators noted usefulness of highly aspherical shapes of noctilucent cloud particles. In this paper, we used rigorous light scattering theory for randomly oriented nonspherical particles to calculate the degree of linear polarization of the scattered light for ice grains of different shape. By comparing these calculations with rocket polarization measurements of noctilucent clouds, we show that, as for spherical particles, the upper limit of particle equal-volume radii for slightly flattened and elongated grains is of about 0.12 Μm, while for highly aspherical plate-like and needle-like particles this upper limit is substantially larger and is of about 0.18–0.20 Μm. We also report calculations of the volumetric scattering cross-section for particles of different shape and show that randomly oriented spheroids have (slightly) smaller scattering cross section per unit particle mass than equal-volume spherical grains. Nevertheless, if in noctilucent clouds plate-like and needle-like grains grow to much larger sizes than spherical particles, their scattering efficiency may be much greater.

Possible application of circular polarization for remote sensing of cosmic bodies

Abstract: Phase dependences of circular polarization were obtained with a precision Stokes polarimeter designed and constructed at the Main Astronomical Observatory of AS Ukraine. A study was made of dielectric and metallic powders with grains of diameter 10–100 Μm. Metallic powders were found to produce an essential circular polarization - up to 3%, just as dielectric powders did not show circular polarization values more than 0.05% Change of circular polarization with phase angle V is greatly depended on surface structure. Loose powders give phase curves with the same sign of circular polarization everywhere and with maximum at large phase angles V > 120 ‡. Measurements of compacted powders show curves which change the sign repeatedly and have additional maxima, including a maximum at small phase angles V < 40 ‡. A theory was created which considers a circular polarization as a result of multiple reflections of light from particulate surface. The theory provides reasonable good fit to the experimental data. It was concluded that measurements of circular polarization can be used to find metals in surface material of cosmic bodies (especially asteroids) and to determine characteristics of surface structure, in particular, to establish presence of regolith on metal-rich bodies.

Periodicity and Decay of Craters Over the Past 600-MYR

Abstract: The data set of Grieve which provides diameters and ages of craters is analyzed to obtain periodicity of the formation rate and decay constant of craters. It is confirmed that large craters (D ⩾ 10 km) do not exhibit any periodicity while small ones appear to satisfy the Broadbent criterion for quantum (periodicity) hypothesis at P ≃ 29.5 myr. The result is consistent with a recent study of Bailey who showed that large craters are largely due to asteroids. Allowing for the decay of craters, an excess of 4 small craters within the nearest past is detected. In this sense, one may argue that the solar system is now in a moderate comet shower.

A model and map of Amalthea

Abstract: A topographic model of Amalthea (JV) was derived from the shapes of limbs and terminators in Voyager images, modified locally to accommodate large craters and ridges. The model is presented in tabular and graphic form, including the first detailed shaded relief maps of the satellite. The shape is very irregular, with radii varying between about 53 and 151 ± 5 km. The minimum value occurs in a deep crater at the south pole. The volume is estimated to be 2.5 ± 0.5 × 106km3. A prominent groove or valley extends some 150 km across the trailing side. High albedo, spectrally distinct markings are mapped and found to have a less obvious relationship with relief than previously suggested.

Perturbation equations of celestial mechanics

Abstract: Perturbation equations of celestial mechanics in terms of orbital elements are completely derived in application to the motion of interplanetary dust particle in the gravational field of the Sun and under the action of disturbing forces. Consideration of change of mass of interplanetary dust particle is the most important feature of this derivation. The results obtained are completely general in the case of constant masses.

Secular Love numbers of the major planets

Abstract: Secular Love numbers of the major planets have been determined by solving the first (Dirichlet's) boundary — value problem for the level ellipsoid as the boundary surface It has been demonstrated that the rotational distortions are responsible for the actual figure of the bodies above The 6th degree Stokes zonal parameter for Uranus and for Neptune have been predicted

Yardang identification in Magellan imagery of Venus

Abstract: Yardang complexes should appear as dark lineation groups in Magellan imagery of Venus. Such structures are not found in preliminary analysis of Magellan imagery.

Mass distribution in the asteroid belt

Abstract: The dependence of the cumulative number of numbered asteroids (up to 3720) on their absolute magnitude is investigated. The differential mass index k is derived from these relations for fainter asteroids. A steeper slope (2.2 < k < 2.4) is found in the four most populous asteroid familes (Flora, Koronis, Eos and Themis) and a flatter slope (1.3 < k < 1.6) for non-family asteroids. This indicates that there are two different asteroid populations in the asteorid belt. Total masses of the asteroid families may be greater than it is commonly accepted.

Periodic variation in natural radioactivity of lake Bouchet sediments

Abstract: Low level counting of alpha and beta particles has been carried out on a 5 long sediment core from the lake La Bouchet in France, spanning the past around 24 000 years. A total of around 60 data points were spectrally analysed with the successive approximations method. Several whole or quasi-periodicities were located for the two radioactivity data sets, in different time intervals; for the alpha counting, periods of 1000–1200, 2000, 6000 and 24 000 years and for the beta counting periods of 1000, 3000, 4000, 8000 and 24 000 years were obtained. The non-stationarity nature of the obtained periods are discussed in relation to (a) palaeoclimatic fluctuations impact on mineral carrying natural radiosotopes partitioning of the sediment deposition of the last glacial and (b) other solar-terrestrial agents.

Shear-induced folding in Arsia Mons aureole: Evidence for low-latitude martian glaciations

Abstract: Folds up to 50 km across have been identified on Arsia Mons aureole. Tharsis Province, Mars. The structures, located on Mars for the first time, are close to Aganippe Fossa and other huge faults which have behaved as left-lateral shear zones and then as extensional features. A tectonic scheme is proposed to explain the folds as shear-induced structures. Folding reveals a layered sequence in the aureole, and that is taken as a definitive evidence for its deposition by ice.

Impacts and tectonism in Earth and Moon history of the past 3800 million years

Abstract: The moon's surface, unlike the Earth's, displays a comparatively clear record of its past bombardment history for the last 3800 Myr, the time since active lunar tectonism under the massive premare bombardment ended. From Baldwin's (1987) tabulation of estimated ages for a representative sample of large lunar craters younger than 3800 Ma, six major cratering episodes can be discerned. These six bombardment episodes, which must have affected the Earth too, appear to match in time the six major episodes of orogenic tectonism on Earth, despite typical resolution errors of +/- 100 Myr and the great uncertainties of the two chronologies. Since more highly resolved events during the Cenozoic and Mesozoic Eras suggest the same correlation, it is possible that large impacts have influenced plate tectonics and other aspects of geologic history, perhaps by triggering flood basalt eruptions.

Heat loss and tectonic style of Venus

Abstract: The tectonic style of a terrestrial planet depends strongly on the mechanisms of heat release from the mantle through the lithosphere to the surface. Three types of lithospheric heat transfer have been proposed. (1) Lithospheric conduction, (2) (hot spot) volcanism, (3) plate recycling (mainly at spreading plate margins). In the case of the Earth the total heat flow is determined by plate recycling 65%, heat conduction through the lithosphere 20%, decay of radioactive elements in the crust 15%, hot spot volcanism <1%. Scaling the mean surface heat flow density of the Earth to venusian conditions leads to 66 mW/m2. In the case of Venus plate tectonics play only a minor role. Thus, two processes remain for heat release: (hot spot) volcanism and conduction. The term “hot spot” is written in brackets because volcanism on Venus occurs globally, not necessarily associated with hot spots. The volcanic lava production has been estimated from Venera 15/16 scenes. Arecibo and Magellan images revealed that the surface character south of 30° N is very similar to the area covered by Venera. The main results of the estimation are: (i) The maximum thickness of the plain lavas is 3 km. (ii) With plain lava thicknesses larger than 200 m the lava production from central volcanoes is negligible, (iii) Two age models have been used for the mean age of the area obseved: Δt 1 = 109 a, Δt 2 = 400 x 106 a. Δt 1 leads to the maximum lava production rate of 3 km3/a compared to 20 to 25 km3/a of the Earth; this gives a maximum contribution of 0.75mW/m2 to the heat flow density of Venus, i.e. about 1%. This implies that either heat conduction is the only dominating process for heat release or there is a hidden reservoir of the “missing basalt” somewhere or there is another unknown tectonic process. Assuming pure conduction and correcting the surface heat flow density for radioactive elements in the crust leads to a thickness of the thermal lithosphere of 45km. A reservoir for the “missing basalt” could be basaltic underplating to a depth of 100 km. This gives a contribution of about 20 mW/m2 with the age model δt 2 to the heat flow density from first order calculations. While the tectonic style of the Earth can be described to be linear formed at the plate margins, the surface of Venus is characterized by global spotty volcanism. The surface is more dominated by volcanic landforms than in the case of the Earth despite the relatively low lava production rate with a maximum of 3 km3/a. As plate tectonics is a minor process on Venus, conduction through a rather thin lithosphere should play an important role for heat release.

Ridge systems related to Martian impact craters

Abstract: Wrinkle ridge systems within and around Martian highland craters were studied in order to find their basin-induced and regional aspects. Most prominent ridge directions indicate regional tectonic patterns. Radial ridges near large craters are often slightly deflected along regional or global ridge systems. Crater floor ridges have simpler local distributions. Smaller or older craters are less resistant against the effects of global or regional stress systems. In craters concentric ridge rings locate at 0.8 crater radius with additional minor rings at 0.66, 0.44 and 0.94 crater radius. This pattern illustrates compression of lava fill over buried topography.

Phase transitions in the Mars mantle

Abstract: A number of the models of Mars was constructed on the basis of different mass concentration of chemical elements and equation of station, proposed by different authors. It was shown that, in the majority of cases the pressure, necessary for the second phase transition, is reached in the mantle.

Observations of the Giotto Target Comet p/ Grigg / Skjellerup at the Calar-Alto Observatory

Abstract: CCD images of comet P/Grigg-Skjellerup, obtained for astrometric purposes with the 3.5 m telescope at the Calar Alto Observatory/Spain, were used for an analysis of the activity status of the nucleus and for a search of faint coma structures. The nucleus was found essentially inactive beyond 2.7 AU solar distance both inbound and outbound (observations on 12–13 August, 1986, 21–23 October, 1986, 22 August, 1988, 18 October, 1988, 9 and 12 September, 1991 and 3 December, 1991). The coma of the comet was well developed in May and July 1987 with a diameter of at least 190 000 km on 24 May, 1987 and of at least 80 000 km on 24 July 1987. The coma showed a cone of diffuse brightness enhancement in the sunward hemisphere. The orientation of the cone axis changed from the Sun direction in May 1987 towards about North in July 1987, i.e., it was almost perpendicular to the projected Sun-nucleus line on the sky. The cone opening angle became smaller from about 100‡ in May to about 50 ‡ in July 1987. A weak and narrow plasma tail was found in the images of May 1987.

Evaluation of gas production rates in some comets

Abstract: Emission fluxes of CN, C2 and C3 carbon-bearing molecular species observed in the coma of comets Bennett (1969i ≡ 1970II), West (1975n ≡ 1976VI), P Halley (1982i), Hartley-Good (19851) and Bradfield (1987s) are analysed in the framework of Haser model. CN, C2 and C3 production rates are determined using recently derived fluorescence efficiencies. The dependence of CN, C2 and C3 production rates on the heliocentric distance and the possible correlations among these radicals is studied and briefly discussed.

Application of ion irradiation experiments to planetary surfaces in the Outer Solar System

Abstract: Experimental results on the interaction between fast bombarding ions and solid targets simulating satellite surfaces in the Outer Solar System are reviewed. Applications to Jovian, Saturnian, Uranian, Neptunian, and Plutonian systems suggest the important role played by cosmic and magnetospheric ions in eroding material, in redistributing it on the surfaces of some objects, and in producing either thin or thick mantles of dark organics.

Physical significance and the role of Lagrangian points and the Oort clouds of planets in the Solar System

Abstract: Here, the role played by Lagrangian points and the Oort clouds of planets in the evolution and structure of the Solar System has been discussed. It is revealed that the Lagrangian points are not mere isolated points in space associated with the orbit of a planet at which the resultant gravitational force of the system of three bodies is zero as thought previously to be, but their existence has much deeper physical significance as regards the origin of the Solar System and those of satellite systems of planets.

A note on the Hori-Lie perturbation technique

Abstract: In this paper we describe how to avoid the introduction of Hori's pseudo time in general planetary theory in the case of two planets.

Propagation of vortices within a rotating fluid shell

Abstract: We consider a spherical, solid planet surrounded by a thin layer of an incompressible, inviscid fluid. The planet rotates with constant angular velocity

A preliminary study of the relationship between large earthquakes and precipitation for the region of Athens, Greece

Abstract: The annual rainfall data of Athens rain gauge stations, for the last 119 years, is compared with the occurrence of large (M ⩾ 6) earthquakes along the fault and thrust systems in the vicinity of Athens and interesting correlations have been observed.