273 research outputs found
Impactor flux and cratering on Ceres and Vesta: Implications for the early Solar System
We study the impactor flux and cratering on Ceres and Vesta caused by the
collisional and dynamical evolution of the asteroid Main Belt. We develop a
statistical code based on a well-tested model for the simultaneous evolution of
the Main Belt and NEA size distributions. This code includes catastrophic
collisions and noncollisional removal processes such as the Yarkovsky effect
and the orbital resonances. The model assumes that the dynamical depletion of
the early Main Belt was very strong, and owing to that, most Main Belt
comminution occurred when its dynamical structure was similar to the present
one. Our results indicate that the number of D > 1 km Main Belt asteroids
striking Ceres and Vesta over the Solar System history are approximately 4 600
and 1 100 respectively. The largest Main Belt asteroids expected to have
impacted Ceres and Vesta had diameters of 71.7 km and 21.1 km. The number of D
> 0.1 km craters on Ceres is \sim 3.4 \times 10^8 and 6.2 \times 10^7 on Vesta.
The number of craters with D > 100 km are 47 on Ceres and 8 on Vesta. Our study
indicates that the D = 460 km crater observed on Vesta had to be formed by the
impact of a D \sim 66.2 km projectile, which has a probability of occurr \sim
30% over the Solar System history. If significant discrepancies between our
results about the cratering on Ceres and Vesta and data obtained from the Dawn
Mission were found, they should be linked to a higher degree of collisional
evolution during the early Main Belt and/or the existence of the late heavy
bombardment. An increase in the collisional activity in the early phase may be
provided for an initial configuration of the giant planets consistent with, for
example, the Nice model. From this, the Dawn Mission would be able to give us
clues about the initial configuration of the early Solar System and its
subsequent dynamical evolution.Comment: Accepted for publication in Astronomy and Astrophysic
Probing the origin of the dark material on Iapetus
Among the icy satellites of Saturn, Iapetus shows a striking dichotomy
between its leading and trailing hemispheres, the former being significantly
darker than the latter. Thanks to the VIMS imaging spectrometer on-board
Cassini, it is now possible to investigate the spectral features of the
satellites in Saturn system within a wider spectral range and with an enhanced
accuracy than with previously available data. In this work, we present an
application of the G-mode method to the high resolution, visible and near
infrared data of Phoebe, Iapetus and Hyperion collected by Cassini/VIMS, to
search for compositional correlations. We also present the results of a
dynamical study on the efficiency of Iapetus in capturing dust grains
travelling inward in Saturn system to evaluate the viability of
Poynting-Robertson drag as the physical mechanism transferring the dark
material to the satellite. The results of spectroscopic classification are used
jointly with the ones of the dynamical study to describe a plausible physical
scenario for the origin of Iapetus' dichotomy. Our work shows that mass
transfer from the outer Saturnian system is an efficient mechanism,
particularly for the range of sizes hypothesised for the particles composing
the newly discovered outer ring around Saturn. Both spectral and dynamical data
indicate Phoebe as the main source of the dark material. However, we suggest a
multi-source scenario where now extinct prograde satellites and the disruptive
impacts that generated the putative collisional families played a significant
role in supplying the original amount of dark material.Comment: 20 pages, 4 tables, 11 figures, major revision (manuscript extended
and completed, figures added and corrected, new results added), minor
revision and finalization of author list, moderate revision (update of the
manuscript following reviewer's feedback and discovery of the new Saturnian
outer ring
Retrieving the distribution of comet 67P/Churyumov-Gerasimenko surface temperatures for individual Rosetta/VIRTIS-M spectra (pixel) by linear spectral unmixing - Method and first results -
Knowledge of surface temperature and its variations as function of illumination conditions is key for understanding the thermodynamical properties, the chemical properties and the physical structure of the regolith (porosity, roughness) of planets and small bodies in the solar system. The surface temperature can be retrieved from near-infrared spectra at wavelengths where thermal emission becomes non-negligible with respect to the reflected components. At 5 micron, the longest wavelength measured by VIRTIS-M on the Rosetta mission which observed comet 67P/Churyumov-Gerasimenko (67P/C-G), the minimum brightness temperature that can be measured is ~150K (instrumental noise equivalent temperature). The usual technique is to fit a Planck function to each spectrum, providing one temperature per pixel. However, the calculation of a distribution of temperatures per pixel is justified by the fact that the local topography changes at all scales resulting in variable illumination conditions (variable incidence angle and shadow casting) within the area covered by each pixel. This causes a distribution of temperatures which turns out in a distribution of thermal emission contributions. Furthermore, the combination of different thermal emission contributions (the linear combination of several Planck curves) is not a Planck function. Consequently, fitting one Planck function to a spectrum results in retrieving a value for the brightness temperature that is not representative of thermophysical properties of the regolith, because it is not the average of all temperatures in the area covered by that pixel
Nonspecific blockade of vascular free radical signals by methylated arginine analogues
The Geology of the Marcia Quadrangle of Asteroid Vesta: Assessing the Effects of Large, Young Craters
We used Dawn spacecraft data to identify and delineate geological units and landforms in the Marcia quadrangle of Vesta as a means to assess the role of the large, relatively young impact craters Marcia (approximately 63 kilometers diameter) and Calpurnia (approximately 53 kilometers diameter) and their surrounding ejecta field on the local geology. We also investigated a local topographic high with a dark-rayed crater named Aricia Tholus, and the impact crater Octavia that is surrounded by a distinctive diffuse mantle. Crater counts and stratigraphic relations suggest that Marcia is the youngest large crater on Vesta, in which a putative impact melt on the crater floor ranges in age between approximately 40 and 60 million years (depending upon choice of chronology system), and Marcia's ejecta blanket ranges in age between approximately 120 and 390 million years (depending upon choice of chronology system). We interpret the geologic units in and around Marcia crater to mark a major Vestan time-stratigraphic event, and that the Marcia Formation is one of the geologically youngest formations on Vesta. Marcia crater reveals pristine bright and dark material in its walls and smooth and pitted terrains on its floor. The smooth unit we interpret as evidence of flow of impact melts and (for the pitted terrain) release of volatiles during or after the impact process. The distinctive dark ejecta surrounding craters Marcia and Calpurnia is enriched in OH- or H-bearing phases and has a variable morphology, suggestive of a complex mixture of impact ejecta and impact melts including dark materials possibly derived from carbonaceous chondrite-rich material. Aricia Tholus, which was originally interpreted as a putative Vestan volcanic edifice based on lower resolution observations, appears to be a fragment of an ancient impact basin rim topped by a dark-rayed impact crater. Octavia crater has a cratering model formation age of approximately 280-990 million years based on counts of its ejecta field (depending upon choice of chronology system), and its ejecta field is the second oldest unit in this quadrangle. The relatively young craters and their related ejecta materials in this quadrangle are in stark contrast to the surrounding heavily cratered units that are related to the billion years old or older Rheasilvia and Veneneia impact basins and Vesta's ancient crust preserved on Vestalia Terra
Key Concepts for Estimating the Burden of Surgical Conditions and the Unmet Need for Surgical Care
Background: Surgical care is emerging as a crucial issue in global public health. Methodology is needed to assess the impact of surgical care from a public health perspective. Methods: A consensus opinion of a group of surgeons, anesthesiologists, and public health experts was established regarding the methodology for estimating the burden of surgical conditions and the unmet need for surgical care. Results: For purposes of analysis, we define surgical conditions as any disease state requiring the expertise of a surgically trained provider. Abnormalities resulting from a surgical condition or its treatment are termed surgical sequelae. Surgical care is defined as any measure that reduces the rates of physical disability or premature death associated with a surgical condition. To measure the burden of surgical conditions and unmet need for surgical care we propose using cumulative disability-adjusted life-year (DALY) curves generated from age-specific population-based data. This conceptual framework is based on the premise that surgically associated disability and death is determined by the incidence of surgical conditions and the quantity and quality of surgical care. The burden of surgical conditions is defined as the total disability and premature deaths that would occur in a population should there be no surgical care; the unmet need for surgical care is defined as the potentially treatable disability and premature deaths due to surgical conditions. Burden of surgical conditions should be expressed as DALYs and unmet need as potential DALYs avertable. Conclusions: Methodology is described for estimating the burden of surgical conditions and unmet need for surgical care. Using this approach it will be feasible to estimate the global burden of surgical conditions and help clarify where surgery fits among other global health priorities. These methods need to be validated using population-based data
The design and testing of a dual fiber textile matrix for accelerating surface hemostasis
The standard treatment for severe traumatic injury is frequently compression and application of gauze dressing to the site of hemorrhage. However, while able to rapidly absorb pools of shed blood, gauze fails to provide strong surface (topical) hemostasis. The result can be excess hemorrhage-related morbidity and mortality. We hypothesized that cost-effective materials (based on widespread availability of bulk fibers for other commercial uses) could be designed based on fundamental hemostatic principles to partially emulate the wicking properties of gauze while concurrently stimulating superior hemostasis. A panel of readily available textile fibers was screened for the ability to activate platelets and the intrinsic coagulation cascade in vitro. Type E continuous filament glass and a specialty rayon fiber were identified from the material panel as accelerators of hemostatic reactions and were custom woven to produce a dual fiber textile bandage. The glass component strongly activated platelets while the specialty rayon agglutinated red blood cells. In comparison with gauze in vitro, the dual fiber textile significantly enhanced the rate of thrombin generation, clot generation as measured by thromboelastography, adhesive protein adsorption and cellular attachment and activation. These results indicate that hemostatic textiles can be designed that mimic gauze in form but surpass gauze in ability to accelerate hemostatic reactions
TDP-43 Identified from a Genome Wide RNAi Screen for SOD1 Regulators
Amyotrophic Lateral Sclerosis (ALS) is a late-onset, progressive neurodegenerative disease affecting motor neurons in the brain stem and spinal cord leading to loss of voluntary muscular function and ultimately, death due to respiratory failure. A subset of ALS cases are familial and associated with mutations in superoxide dismutase 1 (SOD1) that destabilize the protein and predispose it to aggregation. In spite of the fact that sporadic and familial forms of ALS share many common patho-physiological features, the mechanistic relationship between SOD1-associated and sporadic forms of the disease if any, is not well understood. To better understand any molecular connections, a cell-based protein folding assay was employed to screen a whole genome RNAi library for genes that regulate levels of soluble SOD1. Statistically significant hits that modulate SOD1 levels, when analyzed by pathway analysis revealed a highly ranked network containing TAR DNA binging protein (TDP-43), a major component of aggregates characteristic of sporadic ALS. Biochemical experiments confirmed the action of TDP-43 on SOD1. These results highlight an unexpected relationship between TDP-43 and SOD1 which may have implications in disease pathogenesis
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