A total of 57 parabolic‐shaped and 9 approximately circular extended, impact crater related features have been found in Magellan synthetic aperture radar (SAR) and thermal emissivity data covering 92% of the surface of Venus. The parabolic features are, with seven exceptions, oriented E‐W with the apex to the east and the impact crater located just west of the apex. They were first identified in the surface emissivity data derived from Magellan radiometry measurements, but the great majority are only clearly visible in the SAR imagery. The overall sizes of both the parabolic and circular features range from several hundred to about two thousand kilometers and are loosely correlated with the diameters of the “parent” craters. The floors of almost all these craters have high specific radar backscatter cross sections (i.e., they are bright in the SAR imagery) relative to their surroundings and tend to have low emissivities. Approximately one‐third of the impact craters with diameters ≥15 km appear to have bright floors and about half of these have an associated parabolic feature which can be observed in the SAR or emissivity data. No features have been found which overlie the parabolic features, indicating that they are among the youngest features on the surface of the planet. This suggests that radar‐bright floors characterize the freshest impact craters and that modification processes subsequently darken their radar signature. A model for the formation of the parabolic features is developed based on the injection of small particles into the upper atmosphere at the time of impact and their transport to the west by the E‐W zonal winds. Fitting of a small perturbation scattering model to the measured average scattering law for the parabolic features placed an upper limit of about 0.6 cm on the wavelength scale (12.6 cm) surface roughness and, hence, of 1 to 2 cm on the largest particle sizes of interest. Fallout times from 50 km in the Venus atmosphere for particles of this size are about 2 hours, allowing westerly drifts of several hundred kilometers for zonal winds of 50 to 100 m s−1. Measurements of the change in backscatter cross section of features overlaid by these extended ejecta deposits, are consistent with deposit depths of a few centimeters to 1 or 2m.
Abstract.Early examinations of Galileo images of Europa revealed features that look like low topographic domes.These small (5-10 km in radius) domes have been interpreted as surface manifestations of diapirs. As a way to probe the subsurface structure of Europa, we investigate the possibility that thermally-driven ice diapirism created these surface features. We use a previously developed analytic model for rising diapirs to estimate the initial radii of the diapirs, their initial depth, the temperature of the medium through which they rise and their formation timescales. We assume that the diapirs originate at the boundary layer between solid ice and whatever underlies it, whether it be liquid water or solid rock. Our results show that the diapirs originate at a depth of no more than a few tens of kilometers.
Abstract. We develop a model of the combined thermal and mechanical evolution of diapirs in the mantle of Venus. The diapir is treated as an oblate spheroid rising through a viscous fluid and ultimately impinging on a rigid overlaying "lid". Drag forces imposed by the lid cause the diapir to spread and fiatten as it rises. We parameterize the heat loss from the rising diapir using a Nusselt number formulation and treat the resulting loss in buoyancy in our flow calculation. The model predicts the evolution timescale and degree of flattening of a diapir as it rises, as well as the stresses exerted on the underside of the lithosphere. In order to explore diapir behavior further, we check our analytical model against the predictions of the finite element code MANTLE.From the combined results of these models and observations of Venusian coronae, we are able to make a number of inferences about Venusian diapirism and mantle properties. We find that the diapirs responsible for formation of Venusian coronae have an initial size distribution extending from about 30 to 100 km in radius. Typical evolution timescales for these diapirs are 30-50 Myr. If Venus' global average resurfacing age is 300-500 Myr, then our results are consistent with a rate of diapirism that has been roughly constant over this period, and with an effective mantle viscosity of 1021 Pa s.
Between 1956 and 1981, 306 splenectomies for hematologic diseases were performed at the UCLA Medical Center. Of these operations, more than 75% were performed for therapeutic reasons to control anemia, thrombocytopenia, neutropenia, or painful symptoms of splenomegaly. Of the 65 patients who had idiopathic thrombocytopenic purpura, 77% showed an excellent response, and of the 39 patients who had hereditary spherocytosis, 90% responded. Other diseases with predictably good response rates were autoimmune hemolytic anemias, Felty's syndrome, and hairy cell leukemia. Forty patients with Hodgkin's disease had splenectomies for diagnostic purposes the last 10 years. The overall morbidity and mortality were 24% and 6%, respectively, the most common complications being pneumonia, wound infections, and local postoperative bleeding, and the most common cause of death being sepsis. The review supports the thesis that in carefully selected patients, therapeutic splenectomy can have desirable palliative effects and that diagnostic splenectomy has a sufficiently low risk to warrant its consideration in patients with Hodgkin's disease.
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