Ridge belts: Are they compressional or extensional structures?

Today one of the main questions standing before geology of Venus is comprehension of tectonic style on surface of this planet. The observed ridge belts on the images of the radar surveyors of Venera-15 and -16 are related to one aspect of a solution of this problem. In general there are two views on the origin of the ridge belts on the planet. Some of the investigators consider them as a result of compression while others as extension. These discrepancies are connected with uncertainties of interpretations of the major structures of these belts, i.e., the ridges. This paper represents a review of evidence in favor of extension and compression observed within the ridge belts. Moreover it discusses the major speculative ideas and models of the ridge belts origin. The supporters of a ridge formation by extension suggest that these linear features are results of crust cracking and intruding of magma along these cracks so they propose that the ridge belts on the plains of Venus can represent analogies of the spreading zones on Earth. Other investigators assume that the ridge of belts represent structures of folding and or thrust faulting due to the tectonic compression environment so the ridge belts can be analogies of the orogenic belts and subduction zones on Earth. So the question of the ridge belts' origin remains controversial and for a solution, set one's hope on the Magellan mission.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

The Morphology of Small Craters and Knobs on the Surface of Jupiter's Satellite Callisto

Using the images of Callisto's surface acquired at 15-km resolution by the Galileo spacecraft during its C21 orbit, we studied the morphology of craters with diameters of less than 1–2 km and knobs. By analogy with other regions of Callisto that have been studied, these craters and knobs are thought to be formed by the sublimation degradation of the rims of larger craters that are also present in the region under study. The small craters closely resemble similar-sized lunar craters and, by analogy with the latter, are also divided into morphological classes. The depths of 42 craters of different morphological classes are estimated using shadow lengths visible in the craters. The fractions of the craters of different classes in the subpopulation are determined as a function of the crater diameter. Evidence has been obtained that larger craters degrade at a slower rate than smaller ones. The mean thickness of the mantle of dark material (40 m) is estimated from the sizes of the craters ejecting the blocks of the basement ice material. The shape of the knob shadows shows that the knobs are heights of mostly conical form with slopes whose steepness is close to the angle of repose. Analysis has shown that the observed landforms and material units of the region under investigation have been formed during two successive stages of the geologic history of Callisto. Large craters, knobs, and the mantle of dark material were formed mostly at the end of the period of heavy meteorite bombardment. The leading processes of this period are impact cratering, the sublimation of Callisto's crustal ice with the accumulation of residual non-icy material, and downslope mass movement. The next stage, which continues until the present time, involved the formation of the subpopulation of small (<1–2 km) craters. This formation was accompanied by the impact reworking of the upper portion of the dark mantle. The key processes occurring at this stage are impact cratering and downslope mass movement. The mean intensity of resurfacing at this stage is much lower than at the preceding stage.     

Natural experiment “Thermal Front in Lake Ladoga, 2010”

The subsatellite experiment ??Thermal Front in Lake Ladoga, 2010?? and its results are described. The experiment was performed at the initial stages of thermal bar existence at the end of May 2010. We estimated the horizontal gradients of water temperature and the front motion rate by two successive surveys carried out with a discreteness of four days. Cumulative heat flows coming to the lake surface were different on different front sides. The hydrochemical and hydrobiological parameters of the front zone were not characterized by any significant horizontal differences.     

Peculiarities of energy transport in the atmosphere by acoustic gravity waves

The dependence of energy transport by acoustic gravity waves (AGWs) on their spectral properties is studied. On the basis of the analysis of expressions for group velocities and energy fluxes of AGWs, it is shown that there exist separate frequencies and wavelengths at which the energy transport in space is most efficient. Comparison of the obtained results with the data of observations on board the Dynamic Explorer 2 satellite shows that, in the upper atmosphere of the Earth’s polar regions, AGWs with spectral parameters corresponding to the maximum of energy transport predominate.     

Dynamics and hierarchy of earthquakes

Temporal dependences of the Earth’s earthquake distribution over seismic moments are analyzed in the period from 1962 through 2004. They are shown to group in the vicinities of three hierarchical levels. The average seismic moments at neighboring levels differ by three times. The variations in the number of earthquakes at neighboring hierarchical levels are opposite in phase. The distribution of earthquakes over seismic moments at each level is described by the gamma function.     

Distribution of medium-scale acoustic gravity waves in polar regions according to satellite measurement data

The peculiarities of the distribution of medium-scale acoustic gravity waves (AGWs) in polar regions according to the data of measurements on board the Dynamics Explorer 2 satellite are studied. Over polar regions of both hemispheres at heights of 250–400 km, wave variations in neutral atmospheric parameters were systematically registered. These variations were identified as AGWs with horizontal wavelengths of 500–650 km. The relative amplitudes of polar AGWs in a neutral concentration reach 10%. Wave trains extend over the polar caps to thousands of kilometers and show a distinct spatial relationship with the auroral oval. A systematic direction is found in AGW propagation from the nighttime sector of the oval into the day-time sector, where wave activity is strictly limited. An assumption is formulated that this restriction is caused by dynamic interactions between AGWs and the zonal wind in the daytime sector of the auroral oval.     

Observed features of acoustic gravity waves in the heterosphere

According to measurements on the Dynamic Explorer 2 satellite, features of the propagation of acoustic gravity waves (AGWs) in the multicomponent upper atmosphere have been investigated. In the altitude range 250–400 km in wave concentration variations of some atmospheric gases, amplitude and phase differences have been observed. Using the approach proposed in this paper, in different gases, AGW variations have been divided into components associated with elastic compression, adiabatic expansion, and the vertical background distribution. The amplitude and phase differences observed in different gases are explained on the basis of analyzing these components. It is shown how to use this effect in order to determine the wave propagation, the vertical displacement of the volume element, the wave frequency, and the spatial distribution of the wave energy density.     

Properties of acoustic-gravity waves in the Earth’s polar thermosphere

The properties of acoustic-gravity waves in the polar regions of the Earth’s thermosphere have been studied. It has been shown that the change in AGW amplitudes occurs against the background of large-scale rotational movements of the medium in the polar thermosphere. The amplitudes of waves increase with AGW propagation against the motion of the medium and decrease when AGW propagate along rotation. An analytical expression for the gain coefficient of AGW perturbations is obtained; the wave’s amplification effect in the opposite wind given the characteristic parameters of the thermosphere is estimated. The results are consistent with the measurements of AGW parameters in the polar regions from the “Dynamic Explorer 2” satellite.)     

Latitude variability of acoustic-gravity waves in the upper atmosphere based on satellite data

Based on satellite measurements, we investigated the properties of acoustic-gravity waves in different geographical areas of the Earth’s upper atmosphere. To study wave activity at high latitudes, we used the concentration of neutral particles measured by the low-altitude polar satellite Dynamic Explorer 2 and measurements from the equatorial satellite Atmosphere Explorer-E for analysis of waves at low latitudes. In the range of heights 250–400 km, there are observed latitudinal variations of amplitudes, together with variations in the morphological and spectral properties of acoustic-gravity waves. In the polar regions of thermosphere, the wave amplitudes amount to 3–10% in terms of relative variations of density and do not exceed 3% at low and middle latitudes. At low latitudes, regular fluctuations induced by the solar terminator are clearly seen with a predominant wave mode moving synchronously with terminator. Moreover, at low and middle latitudes, there are observed sporadic local wave packets of small amplitudes (1–2%) that can have origins of various natures. We also investigated the relation between some of the observed wave trains and the earthquakes.     

Wind control of the propagation of acoustic gravity waves in the polar atmosphere

The relationship between the directions of polar acoustic gravity waves and a wind at 250–350 km altitudes has been studied based on an analysis of the Dynamics Explorer 2 satellite measurements. A method, which makes it possible to determine the direction of these waves relative to the satellite velocity vector based on one-point measurements of different neutral atmosphere parameters, is presented. It has been established that acoustic gravity waves observed over the polar caps systematically propagate upwind, which argues for their spatial wind filtering. In the polar regions, waves mainly propagate in two directions: toward magnetic noon and 15–16 MLT. Waves tend to move counterclockwise and clockwise over the northern and southern polar caps, respectively.