Ringed impact craters on Venus: An analysis from Magellan images

We have analyzed cycle 1 Magellan images covering approximately 90 percent of the venusian surface and have identified 55 unequivocal peak-ring craters and multiringed impact basins. This comprehensive study (52 peak-ring craters and at least 3 multiringed impact basins) complements our earlier independent analysis of Arecibo and Venera images and initial Magellan data and that of the Magellan team

Alien Registration- Mckinnon, William L. (Sanford, York County)

https://digitalmaine.com/alien_docs/3926/thumbnail.jp

Triton's surface age and impactor population revisited in light of Kuiper Belt fluxes: Evidence for small Kuiper Belt objects and recent geological activity

Neptune's largest satellite, Triton, is one of the most fascinating and enigmatic bodies in the solar system. Among its numerous interesting traits, Triton appears to have far fewer craters than would be expected if its surface was primordial. Here we combine the best available crater count data for Triton with improved estimates of impact rates by including the Kuiper Belt as a source of impactors. We find that the population of impactors creating the smallest observed craters on Triton must be sub-km in scale, and that this small-impactor population can be best fit by a differential power-law size index near -3. Such results provide interesting, indirect probes of the unseen small body population of the Kuiper Belt. Based on the modern, Kuiper Belt and Oort Cloud impactor flux estimates, we also recalculate estimated ages for several regions of Triton's surface imaged by Voyager 2, and find that Triton was probably active on a time scale no greater than 0.1-0.3 Gyr ago (indicating Triton was still active after some 90% to 98% of the age of the solar system), and perhaps even more recently. The time-averaged volumetric resurfacing rate on Triton implied by these results, 0.01 km$^3$ yr$^{-1}$ or more, is likely second only to Io and Europa in the outer solar system, and is within an order of magnitude of estimates for Venus and for the Earth's intraplate zones. This finding indicates that Triton likely remains a highly geologically active world at present, some 4.5 Gyr after its formation. We briefly speculate on how such a situation might obtain.Comment: 14 pages (TeX), plus 2 postscript figures Stern & McKinnon, 2000, AJ, in pres

Estimates of Comet Fragment Masses from Impact Crater Chains on Callisto and Ganymede

Chains of impact craters, or catenae, have been identified in Voyager images of Callisto and Ganymede. Although these resemble in some respects secondary crater chains, the source craters and basins for the catenae cannot be identified. The best explanation is a phenomenon similar to that displayed by former comet Shoemaker-Levy 9; tidal (or other) breakup close to Jupiter followed by gradual orbital separation of the fragments and collision with a Galilean satellite on the outbound leg of the trajectory. Because the trajectories must pass close to Jupiter, this constrains the impact geometry (velocity and impact angle) of the individual fragments. For the dominant classes of impactors, short period Jupiter-family comets and asteroids, velocities at Callisto and Ganymede are dominated by Jovian gravity and a satellite's orbital motion, and are insensitive to the pre-fragmentation heliocentric velocity; velocities are insensitive to satellite gravity for all impactor classes. Complex crater shapes on Callisto and Ganymede are determined from Voyager images and Schmidt-Holsapple scaling is used to back out individual fragment masses. We find that comet fragment radii are generally less than about 500 m (for ice densities) but can be larger. These estimates can be compared with those for the Shoemaker-Levy 9 impactors

Methods used in the Lives Saved Tool (LiST)

<p>Abstract</p> <p>Background</p> <p>Choosing an optimum set of child health interventions for maximum mortality impact is important within resource poor policy environments. The Lives Saved Tool (LiST) is a computer model that estimates the mortality and stillbirth impact of scaling up proven maternal and child health interventions. This paper will describe the methods used to estimate the impact of scaling up interventions on neonatal and child mortality.</p> <p>Model structure and assumptions</p> <p>LiST estimates mortality impact via five age bands 0 months, 1-5 months, 6-11 months, 12-23 months and 24 to 59 months. For each of these age bands reductions in cause specific mortality are estimated. Nutrition interventions can impact either nutritional statuses or directly impact mortality. In the former case, LiST acts as a cohort model where current nutritional statuses such as stunting impact the probability of stunting as the cohort ages. LiST links with a demographic projections model (DemProj) to estimate the deaths and deaths averted due to the reductions in mortality rates.</p> <p>Using LiST</p> <p>LiST can be downloaded at <url>http://www.jhsph.edu/dept/ih/IIP/list/</url> where simple instructions are available for installation. LiST includes default values for coverage and effectiveness for many less developed countries obtained from credible sources.</p> <p>Conclusions</p> <p>The development of LiST is a continuing process. Via technical inputs from the Child Health Epidemiological Group, effectiveness values are updated, interventions are adopted and new features added.</p

Obstructive sleep apnea as an independent predictor of postoperative delirium and pain: Protocol for an observational study of a surgical cohort [version 2; referees: 2 approved]

Introduction: Postoperative delirium and pain are common complications in adults, and are difficult both to prevent and treat. Obstructive sleep apnea (OSA) is prevalent in surgical patients, and has been suggested to be a risk factor for postoperative delirium and pain. OSA also might impact pain perception, and alter pain medication requirements. This protocol describes an observational study, with the primary aim of testing whether OSA is an independent predictor of postoperative complications, focusing on (i) postoperative incident delirium and (ii) acute postoperative pain severity. We secondarily hypothesize that compliance with prescribed treatment for OSA (typically continuous positive airway pressure or CPAP) might decrease the risk of delirium and the severity of pain. Methods and analysis: We will include data from patients who have been enrolled into three prospective studies: ENGAGES, PODCAST, and SATISFY-SOS. All participants underwent general anesthesia for a non-neurosurgical inpatient operation, and had a postoperative hospital stay of at least one day at Barnes Jewish Hospital in St. Louis, Missouri, from February 2013 to May 2018.  Patients included in this study have been assessed for postoperative delirium and pain severity as part of the parent studies. In the current study, determination of delirium diagnosis will be based on the Confusion Assessment Method, and the Visual Analogue Pain Scale will be used for pain severity. Data on OSA diagnosis, OSA risk and compliance with treatment will be obtained from the preoperative assessment record. Other variables that are candidate risk factors for delirium and pain will also be extracted from this record. We will use logistic regression to test whether OSA independently predicts postoperative delirium and linear regression to assess OSAs relationship to acute pain severity. We will conduct secondary analyses with subgroups to explore whether these relationships are modified by compliance with OSA treatment.</ns4:p

Large Impact Craters and Basins: Mechanics of Syngenetic and Postgenetic Modification

The impact crater is the ubiquitous landform of the solar system. Theoretical mechanical analyses are applied to the modification stage of crater formation, both syngenetic (immediate or short term) and postgenetic (long term). The mechanical stability of an impact crater is analyzed via a quasi-static, axisymmetric slip line theory of plasticity. The yield model incorporated is Mohr-Coulomb and a simplified rectangular profile is used for the transient cavity. The degree of stability (or instability) is described as a function of internal friction angle, depth/diameter ratio, and a dimensionless parameter ρgH/c (ρ = density, g = acceleration of gravity, H = depth, and c = cohesion strength). To match the observed slumping of large lunar craters the cohesion strength of the lunar surface material must be low (&lt;20 bars) and the angle of internal friction must be less than 2°. It is not implausible that these failure strength characteristics are realized by freshly shocked rock. A theoretical description of impact crater collapse is evolved which accounts for the development of wall scallops, slump terraces, and flat floors. A preliminary set of scale model experiments performed in a centrifuge corroborate the theory. The strength of terrestrial planet surfaces under impact is seen to vary by as much as a factor of two. Shortly after the excavation of a large impact crater the transient cavity collapses, driven by gravity. It is shown that at least one concentric fault scarp forms about the crater, if the strength of the target material decreases sufficiently rapidly with increasing depth. This is demonstrated by two classes of models: extrusion flow models which assume a weak layer underlying a strong layer, and plastic flow models which assume a continuous decrease of cohesion strength with depth. Both classes predict that the ratio of the radius of the scarp to the transient crater radius is between 1.2 and 2 for large craters. Large impact basins on Ganymede and Callisto are characterized by one or more concentric rings or scarps. The number, spacing, and morphology of the rings is a function of the thickness and strength of the lithosphere, and crater diameter. When the lithosphere is thin and weak, the collapse is regulated by flow induced in the asthenosphere. The lithosphere fragments in a multiply concentric pattern (e.g., Valhalla, Asgard, Galilee Regio, and a newly discovered ring system on Callisto). The thickness and viscosity of a planetary lithosphere increases with time as the mantle cools. A thicker lithosphere leads to the formation of one (or very few) irregular normal faults concentric to the crater (e.g., Gilgamesh). A gravity wave or tsunami induced by impact into a liquid mantle would result in both concentric and radial extension features. Since these are not observed , this process cannot be responsible for the generation of the rings around the basins on Ganymede and Callisto. The appearance of Galilee Regio and portions of Valhalla is best explained by ring graben, and though the Valhalla system is older, the lithosphere was 1.5-2.0 times as thick at the time of formation. The present lithosphere thickness is too great to permit development of any rings. It has been proposed that a mascon may be in the form of an annulus surrounding the Caloris basin on Mercury, associated with the smooth plains. The effects (stresses, deformation, surface tectonic style, gravity anomalies, etc.) of such a ring load on a floating elastic lithosphere of variable thickness are investigated. The main characteristics of the surface tectonic pattern are normal faulting within the basin and thrust faulting beneath the ring load both in agreement with observation Moreover, the dominant concentric trend of the basin normal faults is consistent with the ring load hypothesis provided the mercurian lithosphere was ≤125 km thick at the time of faulting. Simple updoming within the basin would produce normal faults of predominantly radial orientation.</p

Thin Ice Lithospheres and High Heat Flows on Europa From Large Impact Structure Ring-graben

Craters are probes of planetary surface and interior properties. Here we measure depths, widths, and spacing of circumferential ring-graben surrounding the two largest multiring impact structures on Europa, Tyre and Callanish. We estimate formation conditions including the ice shell structure. The radial extension necessary to form these graben is thought to be caused by asthenospheric drag of warmer, more ductile ice and/or water flowing toward the excavated center of the crater, under a brittle-elastic lithospheric lid. Measurements of graben depths from stereo-photoclinometric digital elevation models result in estimates of displacement, strain, and stress experienced by the ice shell. Graben widths are used to estimate the intersection depth of the bounding normal faults, a quantity related to the brittle-ductile transition depth that approximates elastic shell thickness during crater collapse. Heat flows at the time of crater formation as well as ice lithosphere and total shell thickness are thus also constrained. Average widths and depths tend to decrease with increasing distance from the structure center, while inter-graben spacing generally increases. Varied assumptions yield plausible total conductive ice shell thickness estimates between 4-8 and 2.5-5 km for Tyre and Callanish, respectively, and heat flows of ~70-115 (+/-30) mW m^-2 for realistic thermal conductivities, consistent with other geophysical estimates for Europa. Higher heat flows are consistent with thin (<10 km), conductive ice shells and impact breaching, or penetration of the stagnant lid for a convecting ice shell. Callanish, geologically younger, formed in a time or region of greater heat flow than Tyre.Comment: 50 pages, 18 figures, 2 tables, published in JGR-Planet