Asteroid flux and impact cratering rate on Venus

By the end of 1990, 65 Venus-crossing asteroids were recognized; these represent 59 percent of the known Earth-crossing asteroids. Further studies, chiefly numerical integrations of orbit evolution, may reveal one or two more Venus crossers among the set of discovered asteroids. A Venus crosser was defined as an asteroid whose orbit can intersect the orbit of Venus as a result of secular (long range) perturbations. Venus crossers revolving on orbits that currently overlap the orbit of Venus are called Venapol asteroids, and those on orbit that don't overlap are called Venamor asteroids; 42 Venapols and 23 Venamors were recognized. Collision probabilities with Venus for 60 of the known Venus crossers were determined

Gravity survey of the Mt. Toondina impact structure, South Australia

The Mt. Toondina impact structure is located in northern South Australia, about 45 km south of the town of Oodnadatta. Only the central uplift is exposed. The outcrops at Mt. Toondina reveal a remarkable structural anomaly surrounded by a broad expanse of nearly flat-lying beds of the Bulldog Shale of Early Cretaceous age. A gravity survey was undertaken in 1989 to determine the diameter of the impact structure, define the form of the central uplift, and understand the local crustal structure. Data were collected along two orthogonal lines across the structure. In addition to the profiles, a significant number of measurements were made on and around the central uplift. The 1989 gravity data combined with 1963 gravity data and the seismic reflection data provide an excellent data base to interpret the subsurface structure of the Mt. Toondina feature

Impact cratering and the surface age of Venus: The Pre-Magellan controversy

The average surface age of a planet is a major indicator of the level of its geologic activity and thus of the dynamics of its interior. Radar images obtained by Venera 15/16 from the northern quarter of the Venus (lat 30 to 90 degs) reveal about 150 features that resemble impact craters, and they were so interpreted by Soviet investigators B. A. Ivanov, A. T. Basilevsky, and their colleagues. These features range in diameter from about 10 to 145 km. Their areal density is remarkably similar to the density of impact structures found on the American and European continental shields. The basic difference between the Soviet and American estimates of the average surface age of Venus's northern quarter is due to which crater-production rate is used for the Venusian environment. Cratering rates based on the lunar and terrestrial cratering records, as well as statistical calculations based on observed and predicted Venus-crossing asteroids and comets, have been used in both the Soviet and American calculations. The single largest uncertainty in estimating the actual cratering rates near Venus involves the shielding effect of the atmosphere

Chemical fractionation of siderophile elements in impactites from Australian meteorite craters

The abundance pattern of siderophile elements in terrestrial and lunar impact melt rocks was used extensively to infer the nature of the impacting projectiles. An implicit assumption made is that the siderophile abundance ratios of the projectiles are approximately preserved during mixing of the projectile constituents with the impact melts. As this mixture occurs during flow of strongly shocked materials at high temperatures, however there are grounds for suspecting that the underlying assumption is not always valid. In particular, fractionation of the melted and partly vaporized material of the projectile might be expected because of differences in volatility, solubility in silicate melts, and other characteristics of the constituent elements. Impactites from craters with associated meteorites offer special opportunities to test the assumptions on which projectile identifications are based and to study chemical fractionation that occurred during the impact process

Mixed Layer Height Estimates – A Statistical Analysis of Algorithm Performance

The Air Force Technical Applications Center (AFTAC) conducts dispersion transport modeling as part of their mission support for the United States Atomic Energy Detection System. Part of that modeling effort requires knowledge of the height of the mixed layer in the lower atmosphere to determine the vertical extent through which particulates can be distributed. The mixed layer can be estimated by analyzing atmospheric profiles of parameters obtained from observations (e.g., upper air soundings) or atmospheric models. Six mixed layer algorithms were evaluated: Gradient Richardson Number (RICH), Potential Temperature (POTEMP), Potential Instability Mixing Depth (PIMIX), and three variations of the PIMIX algorithm that have never been statistically tested. The purpose of the research was to evaluate algorithm performance when observed and model-generated soundings were used to determine the height of the mixed layer. The research was divided into two sections: observed and forecast. In the observed section, observed soundings were hand-analyzed to obtain subjective mixed layer heights, which were compared to the algorithm heights. In the forecast section, soundings generated by the Regional Atmospheric Modeling System (RAMS) were subjectively analyzed, and the results were compared to the algorithms\u27 output. Additionally, the algorithms were evaluated to determine if their performance varied temporally (i.e., was algorithm performance dependent on observation time). Finally, the algorithms\u27 root mean square errors (RMSE) compared to the subjective heights were calculated

The crash of P/Shoemaker-Levy 9 into Jupiter and its implications for comet bombardment on Earth

Periodic Comet Shoemaker/Levy 9 will impact Jupiter in late July 1994. The comet, which broke into more than 20 telescopically detectable fragments when it passed with the Roche lobe of Jupiter on July 8, 1992, is captured in a highly eccentric orbit about Jupiter. The 21 recognized nuclei will be spread out in a train of the order 7 x 10(exp 6) km long at the time of impact, and the impacts will be spread in time over about 5 1/2 days centered on about July 21.2 UT. In addition to the train of recognized bright nuclei, the comet consists of 'wings' of unresolved bodies that are the source of a very broad composite dust tail. The linear extent of the wings is about an order of magnitude greater than that of the train of recognized discrete nuclei. Collision of the wings will be spread in time over several months. Thus the impact of P/S-L 9 with Jupiter will be an event of appreciable duration

REGULATION OF PANCREATIC β-CELL FUNCTION BY THE RENIN-ANGIOTENSIN SYSTEM IN TYPE 2 DIABETES

Diet-induced obesity promotes type 2 diabetes (T2D). Drugs that inhibit the renin-angiotensin system (RAS) have been demonstrated in clinical trials to decrease the onset of T2D. Previously, we demonstrated that mice made obese from chronic consumption of a high-fat (HF) diet have marked elevations in systemic concentrations of angiotensin II (AngII). Pancreatic islets have been reported to possess components of the renin-angiotensin system (RAS), including angiotensin type 1a receptors (AT1aR), the primary receptor for AngII, and angiotensin converting-enzyme 2 (ACE2), which negatively regulates the RAS by catabolizing AngII to angiotensin-(1-7) (Ang-(1-7)). These two opposing proteins have been implicated in the regulation of β-cell function. We hypothesized that the RAS contributes to the decline of β-cell function during the development of T2D with obesity. To test this hypothesis we first examined the effects of whole-body deficiency of ACE2 in mice on β-cell function in vivo and in vitro during the development of T2D. Whole-body deficiency of ACE2 resulted in impaired β-cell adaptation to insulin resistance with HF-feeding and a reduction of in vivo glucose-stimulated insulin secretion (GSIS) associated with reduced β- cell mass and proliferation. These results demonstrate that ACE2 plays a role in the adaptive response to hyperinsulinemia with obesity. In islets from HF-fed mice, AngII inhibited GSIS. In mice with pancreatic-specific deletion of AT1aR, AngII-induced inhibition of GSIS in vitro from islets of HF-fed mice was abolished. However, there was no effect of pancreatic AT1aR-deficiency on glucose homeostasis in vivo in HF-fed mice exhibiting pronounced hyperinsulinemia. Notably, pancreatic weight, insulin content and basal and glucose-stimulated insulin secretion from islets were decreased in mice with pancreatic AT1aR deficiency. These results suggest that AT1aR may contribute to pancreatic cell development, and also contribute to AngII-induced reductions in GSIS from islets of HF-fed mice. Overall, these studies suggest a role for the RAS in the regulation of β-cell function in T2D