Aeolian features on Venus: Preliminary Magellan results

Magellan synthetic aperture radar data reveal numerous surface features that are attributed to aeolian, or wind processes. Wind streaks are the most common aeolian feature. They consist of radar backscatter patterns that are high, low, or mixed in relation to the surface on which they occur. A data base of more than 3400 wind streaks shows that low backscatter linear forms (long, narrow streaks) are the most common and that most streaks occur between 17°S to 30°S and 5°N to 53°N on smooth plains. Moreover, most streaks are associated with deposits from certain impact craters and some tectonically deformed terrains. We infer that both of these geological settings provide fine particulate material that can be entrained by the low-velocity winds on Venus. Turbulence and wind patterns generated by the topographic features with which many streaks are associated can account for differences in particle distributions and in the patterns of the wind streaks. Thus, some high backscatter streaks are considered to be zones that are swept free of sedimentary particles to expose rough bedrock; other high backscatter streaks may be lag deposits of dense materials from which low-density grains have been removed (dense materials such as ilmenite or pyrite have dielectric properties that would produce high backscatter patterns). Wind streaks generally occur on slopes < 2° and tend to be oriented toward the equator, consistent with the Hadley model of atmospheric circulation. In addition to wind streaks, other aeolian features on Venus include yardangs(?) and dune fields. The Aglaonice dune field, centered at 25°S, 340°E, covers ∼1290 km^2 and is located in an ejecta flow channel from the Aglaonice impact crater. The Meshkenet dune field, located at 67°N, 90°E, covers ∼17,120 km^2 in a valley between Ishtar Terra and Meshkenet Tessera. Wind streaks associated with both dune fields suggest that the dunes are of transverse forms in which the dune crests are perpendicular to the prevailing winds. Dunes on Venus signal the presence of sand-size (∼60 to 2,000 μm) grains. The possible yardangs are found at 9°N, 60.5°E, about 300 km southeast of the crater Mead. Although most aeolian features are concentrated in smooth plains near the equator, the occurrence of wind streaks is widespread, and some have been found at all latitudes and elevations. They demonstrate that aeolian processes operate widely on Venus. The intensity of wind erosion and deposits, however, varies with locality and is dependent on the wind regime and supply of particles

Climate dynamics and fluid mechanics: Natural variability and related uncertainties

The purpose of this review-and-research paper is twofold: (i) to review the role played in climate dynamics by fluid-dynamical models; and (ii) to contribute to the understanding and reduction of the uncertainties in future climate-change projections. To illustrate the first point, we focus on the large-scale, wind-driven flow of the mid-latitude oceans which contribute in a crucial way to Earth's climate, and to changes therein. We study the low-frequency variability (LFV) of the wind-driven, double-gyre circulation in mid-latitude ocean basins, via the bifurcation sequence that leads from steady states through periodic solutions and on to the chaotic, irregular flows documented in the observations. This sequence involves local, pitchfork and Hopf bifurcations, as well as global, homoclinic ones. The natural climate variability induced by the LFV of the ocean circulation is but one of the causes of uncertainties in climate projections. Another major cause of such uncertainties could reside in the structural instability in the topological sense, of the equations governing climate dynamics, including but not restricted to those of atmospheric and ocean dynamics. We propose a novel approach to understand, and possibly reduce, these uncertainties, based on the concepts and methods of random dynamical systems theory. As a very first step, we study the effect of noise on the topological classes of the Arnol'd family of circle maps, a paradigmatic model of frequency locking as occurring in the nonlinear interactions between the El Nino-Southern Oscillations (ENSO) and the seasonal cycle. It is shown that the maps' fine-grained resonant landscape is smoothed by the noise, thus permitting their coarse-grained classification. This result is consistent with stabilizing effects of stochastic parametrization obtained in modeling of ENSO phenomenon via some general circulation models.Comment: Invited survey paper for Special Issue on The Euler Equations: 250 Years On, in Physica D: Nonlinear phenomen

Etude d'ondes non linéaires générées par instabilités d'écoulements annulaires tournants cisaillés

Nous présentons des mesures globales par vidéo et des mesures locales par vélocimétrie laser Doppler permettant de caractériser des écoulements d'eau dans un canal annulaire creusé dans un plateau pouvant éventuellement tourner, cisaillé par un couvercle. À cisaillement élevé, l'écoulement de base axisymétrique devient instable et on obtient en général des ondes progressives, sauf dans certains cas où nous obtenons des structures localisées dans l'espace-temps. Ces résultats expérimentaux sont comparés aux résultats numériques d'un code pseudo-spectral

Backscatter model for the unusual radar properties of the Greenland Ice Sheet

Abstract. A number of planetary objects exhltilt unusual radar polarization properties and more recently a similar behavior has been observed over a vast portion of the Eart h‘s surface: the percolation facies of the Greenland Ice Sheet. Surface-based ranging radar data and snow stratigraphy studies demonstrated that the unusual radar properties of that portion of Greenland are caused by enhanced scattering from massive, large, solid-ice bod-ies buried in the top few meters of the dry, cold, clean snowy surface of the ice sheet and created by seasonal melting and refreezing events. Here, we model the icy inclusions as ran-domly distributed, horizontal and nearly vertical, discrete, dielectric cylinders embedded in a transparent snow medkm. An exact analytical solution is used to compute the scattered field from the cylinders, The model predictions are in good agreement with fully polari-met ric radar observations gathered by an airborne imaging system simultaneously at three radar wavelengths (5.6, 24 and 68 cm) with an incidence angle of the radar illumination varying bet ween 19 and 65 degrees. The diameter and number density of the cylinders that are inferred from the radar data using the backscatter model are consistent with in-situ observations of the icy inclusions, The large radar reflectivity and polarization ratios of the Greenland percolation facies are interpreted as arising from internal reflections of the radar signals in the icy inclusions that first-order scattering models fail to predict. The results compare favorably with predictions from the coherent backseat ter or weak localization the-ory and may provide a complementary framework for interpreting exotic radar echoes from other planet ary objects. Submitted to JGR Planets on September 26, 1994. 1