The cratering record at Uranus: Implications for satellite evolution and the origin of impacting objects

The crater size/frequency distributions on the major Uranian satellites show two distinctly different crater populations of different ages. Any hypothesis on the origin of the objects responsible for the period of heavy bombardment must account for the occurrence of different crater populations (size/frequency distributions) in different parts of the solar system. A computerized simulation using short-period comet impact velocities and a modified Holsapple-Schmidt crater scaling law was used to recover the size distribution of cometary nuclei from the observed cratering record. The most likely explanation for the cratering record is that the period of heavy bombardment was caused by different families of accretional remnants indigenous to the system in which the different crater populations occurred

Glacial geology of the Hellas region on Mars

A glacial geologic interpretation was recently presented for Argyre, which is herein extended to Hellas. This glacial event is believed to constitute an important link in a global cryohydric epoch of Middle Amazonian age. At glacial maximum, ice apparently extended far beyond the regions of Argyre and Hellas, and formed what is termed as the Austral Ice Sheet, an agglomeration of several ice domes and lobes including the Hellas Lobe. It is concluded that Hellas was apparently heavily glaciated. Also glaciation was young by Martian standards (Middle Amazonian), and ancient by terrestrial standards. Glaciation appears to have occurred during the same period that other areas on Mars were experiencing glaciation and periglacial activity. Glaciation seems to have occurred as a geological brief epoch of intense geomorphic activity in an era characterized by long periods of relative inactivity

Bright crater outflows: Possible emplacement mechanisms

Lobate features with a strong backscatter are associated with 43 percent of the impact craters cataloged in Magellan's cycle 1. Their apparent thinness and great lengths are consistent with a low-viscosity material. The longest outflow yet identified is about 600 km in length and flows from the 90-km-diameter crater Addams. There is strong evidence that the outflows are largely composed of impact melt, although the mechanisms of their emplacement are not clearly understood. High temperatures and pressures of target rocks on Venus allow for more melt to be produced than on other terrestrial planets because lower shock pressures are required for melting. The percentage of impact craters with outflows increases with increasing crater diameter. The mean diameter of craters without outflows is 14.4 km, compared with 27.8 km for craters with outflows. No craters smaller than 3 km, 43 percent of craters in the 10- to 30-km-diameter range, and 90 percent in the 80- to 100-km-diameter range have associated bright outflows. More melt is produced in the more energetic impact events that produce larger craters. However, three of the four largest craters have no outflows. We present four possible mechanisms for the emplacement of bright outflows. We believe this 'shotgun' approach is justified because all four mechanisms may indeed have operated to some degree

Glacial and marine chronology of Mars

A hydrological model involving episodic oceans and ice sheets on Mars has been presented by Baker, et al. One of the main uncertainties concerning this model is the age and correlation of these events. Even more uncertain are their absolute ages. However, based on stratigraphic and cratering evidence, the most recent occurrence of these events was relatively late in Martian history. The cratering record on Mars can be divided into three general periods: (1) the period of late heavy bombardment; (2) a transition period at the end of late heavy bombardment; and (3) the post heavy bombardment era. The crater size/frequency distribution represented by the period of late heavy bombardment is characterized by a complex curve with a differential-2 slope (cumulative-1) at diameters less than about 50 km diameter, while the post heavy bombardment size distribution has a differential-3 slope (cumulative-2) over the same diameter range. An R plot is presented of the size/freqency distribution of ejecta blanket craters on the Argyre esker plains and similar craters in Hellas. The relative chronology is summarized of oceans, ice sheets, and other major events in Martian history

The origin of planetary impactors in the inner solar system

New insights into the history of the inner solar system are derived from the impact cratering record of the Moon, Mars, Venus and Mercury, and from the size distributions of asteroid populations. Old craters from a unique period of heavy bombardment that ended $\sim$3.8 billion years ago were made by asteroids that were dynamically ejected from the main asteroid belt, possibly due to the orbital migration of the giant planets. The impactors of the past $\sim$3.8 billion years have a size distribution quite different from the main belt asteroids, but very similar to the population of near-Earth asteroids.Comment: 12 pages (including 4 figures

Crater populations on Ganymede and Callisto

The discovery of heavily cratered surfaces on Ganymede and Callisto by Voyager 1 shows that like the inner Solar System, a period of heavy bombardment also occurred in the outer Solar System. Comparisons among the crater size/density curves of Ganymede, Callisto and the terrestrial planets show several striking features. The overall crater density of the most heavily cratered terrain on Ganymede is down by a factor of about 3 compared to Callisto, and when allowance is made for the difference in crater production rate due to the influence of Jupiter's gravity field it is down by a factor of nearly 6. This indicates that the oldest regions of Ganymede began recording the observed crater population at a later time than Callisto, and therefore Ganymede either experienced a large-scale (perhaps global) diameter-independent resurfacing event or simply developed a rigid crust capable of retaining craters later than Callisto. In either case, this process took place during the period of late heavy bombardment. Based on earlier studies of the terrestrial-planets' cratering record, neither Ganymede nor Callisto is saturated with craters. Compared to Callisto, a diameter-dependent loss of craters in the size range 10–40 km occurs on the grooved terrain of Ganymede and probably results from obliteration of small craters due to the formation of new ice. A similar but less severe loss also occurs on Ganymede's heavily cratered terrain and may be due to an earlier period of ice formation and/or the formation of arcuate troughs in this terrain. Seven different crater curves, in the diameter range of about 40–130 km, representing vastly different crater densities, different surface ages, different terrain types, and even different satellites all possess nearly the same distribution function. This together with other observational evidence strongly suggests that at least in this diameter range the curve basically represents its production function which is completely different from that on the terrestrial planets. This indicates that the population of bodies responsible for the period of late heavy bombardment in the inner Solar System was very different from that responsible for the late heavy bombardment in the outer Solar System. We can only speculate at this early stage that Ganymede and Callisto may principally record a population of bodies that never penetrated the inner Solar System in numbers great enough to leave a recognizable signature

Ancient oceans and Martian paleohydrology

The global model of ocean formation on Mars is discussed. The studies of impact crater densities on certain Martian landforms show that late in Martian history there could have been coincident formation of: (1) glacial features in the Southern Hemisphere; (2) ponded water and related ice features in the northern plains; (3) fluvial runoff on Martian uplands; and (4) active ice-related mass-movement. This model of transient ocean formation ties these diverse observations together in a long-term cyclic scheme of global planetary operation

Mitral paravalvular abscess with left ventriculo-atrial fistula in a patient on dialysis

Background: Infective endocarditis in hemodialysis patients is challenging but is becoming more common recently. Case report: A 64-year-old man with end-stage renal disease on hemodialysis presented with infective endocarditis of mitral valve and coronary artery disease after commencing training for home hemodialysis. During a course of antibiotic treatment the patient developed left ventriculo-atrial fistula due to mitral paravalvular abscess. Abscess debridement followed by reconstruction of the mitral annulus with fresh autologous pericardial patch and mitral valve replacement using a mechanical prosthesis with concomitant coronary artery bypass grafting was performed successfully. Conclusion: Timely diagnosis, proper antibiotic treatment and early surgical intervention including aggressive debridement should improve the outcome of this high-risk disease. © 2009 Kitamura et al; licensee BioMed Central Ltd.Tadashi Kitamura, James Edwards, Suchi Khurana and Robert G Stukli

Surface History of Mercury: Implications for Terrestrial Planets

A working hypothesis of Mercury's history is presented. We infer the surface of Mercury to record a sequence of events broadly similar to those recorded on the moon, implying similar histories of impact bombardment. The oldest terrain so n Mercury seem to be better preserved from modification by ejecta from subsequently formed impact basins because of higher surface gravity. The large lunarlike impact craters on Mercury can be interpreted as part of a distinct episode of bombardment which may have affected all the terrestrial planets about 4 b.y. ago. The light cratering accumulated on the surfaces of the Mercurian smooth plains is similar in diameter/frequency relationship to that of the lunar maria and of the oldest Martian plains units, consistent with recent interpretations of lunar and Martian flux histories by Wetherill(1974) and Soderblom et al. (1974). A straightforward interpretation of the Mercurian surface record thus supports recent order of magnitude increases in age estimates of many Martian features discovered by Mariner 9 but is not conclusive. The large core inferred for Mercury combined with the lack of recognizable evidence of past atmospheric activity is more easily understood in terms of radially heterogeneous accumulation than in terms of differentiation of a homogeneous planet. Early core cooling may be reflected by widespread evidence of crustal shortening. However, Mercury's surface seems little affected by any tectonic, atmospheric, or volcanic processes for the last 3 b.y. or so, raising questions concerning (1) the relationship of the origin of Mercury's magnetic field to that of earth's and (2) the primary cause of volcanic flooding, which may have begun, and ended, approximately synchronously on Mercury and the moon