Cascading parallel fractures on Enceladus

Active eruptions from the south polar region of Saturn's small (~500 km diameter) moon Enceladus are concentrated along a series of lineaments known as the `tiger stripes', thought to be partially open fissures that connect to the liquid water ocean beneath the ice shell. Whereas aspects of the tiger stripes have been addressed in previous work, no study to date simultaneously explains why they should be located only at the south pole, why there are multiple approximately parallel and regularly spaced fractures, and what accounts for their spacing of ~35 km. Here we propose that secular cooling and the resulting ice shell thickening and global tensile stresses cause the first fracture to form at one of the poles, where the ice shell is thinnest due to tidal heating. The tensile stresses are thereby partially relieved, preventing a similar failure at the opposite pole. We propose that subsequent activity then concentrates in the vicinity of the first fracture as the steadily erupted water ice loads the flanks of the open fissure, causing bending in the surrounding elastic plate and further tensile failure in bands parallel to the first fracture, leading to a cascading sequence of parallel fissures until the conditions no longer permit through-going fractures.Comment: 18 pages, 9 figure

Isostatic equilibrium in spherical coordinates and implications for crustal thickness on the Moon, Mars, Enceladus, and elsewhere

Isostatic equilibrium is commonly defined as the state achieved when there are no lateral gradients in hydrostatic pressure, and thus no lateral flow, at depth within the lower viscosity mantle that underlies a planetary body's outer crust. In a constant-gravity Cartesian framework, this definition is equivalent to the requirement that columns of equal width contain equal masses. Here we show, however, that this equivalence breaks down when the spherical geometry of the problem is taken into account. Imposing the "equal masses" requirement in a spherical geometry, as is commonly done in the literature, leads to significant lateral pressure gradients along internal equipotential surfaces, and thus corresponds to a state of disequilibrium. Compared with the "equal pressures" model we present here, the "equal masses" model always overestimates the compensation depth--by ~27% in the case of the lunar highlands and by nearly a factor of two in the case of Enceladus.Comment: 23 pages of text; 3 figures; accepted for publication in GR

Global drainage patterns and the origins of topographic relief on Earth, Mars, and Titan

Rivers have eroded the topography of Mars, Titan, and Earth, creating diverse landscapes. However, the dominant processes that generated topography on Titan (and to some extent on early Mars) are not well known. We analyzed drainage patterns on all three bodies and found that large drainages, which record interactions between deformation and erosional modification, conform much better to long-wavelength topography on Titan and Mars than on Earth. We use a numerical landscape evolution model to demonstrate that short-wavelength deformation causes drainage directions to diverge from long-wavelength topography, as observed on Earth. We attribute the observed differences to ancient long-wavelength topography on Mars, recent or ongoing generation of long-wavelength relief on Titan, and the creation of short-wavelength relief by plate tectonics on Earth

Stressful life-events exposure is associated with 17-year mortality, but it is health-related events that prove predictive

Objectives Despite the widely-held view that psychological stress is a major cause of poor health, few studies have examined the relationship between stressful life-events exposure and death. The present analyses examined the association between overall life-events stress load, health-related and health-unrelated stress, and subsequent all-cause mortality.\ud \ud Design This study employed a prospective longitudinal design incorporating time-varying covariates.\ud \ud Methods Participants were 968 Scottish men and women who were 56 years old. Stressful life-events experience for the preceding 2 years was assessed at baseline, 8–9 years and 12–13 years later. Mortality was tracked for the subsequent 17 years during which time 266 participants had died. Cox's regression models with time-varying covariates were applied. We adjusted for sex, occupational status, smoking, BMI, and systolic blood pressure.\ud \ud Results Overall life-events numbers and their impact scores at the time of exposure and the time of assessment were associated with 17-year mortality. Health-related event numbers and impact scores were strongly predictive of mortality. This was not the case for health-unrelated events.\ud \ud Conclusions The frequency of life-events and the stress load they imposed were associated with all-cause mortality. However, it was the experience and impact of health-related, not health-unrelated, events that proved predictive. This reinforces the need to disaggregate these two classes of exposures in studies of stress and health outcomes.\u

Antipsychotic drugs and risks of myocardial infarction: a self-controlled case series study.

AIM: Antipsychotics increase the risk of stroke. Their effect on myocardial infarction remains uncertain because people prescribed and not prescribed antipsychotic drugs differ in their underlying vascular risk making between-person comparisons difficult to interpret. The aim of our study was to investigate this association using the self-controlled case series design that eliminates between-person confounding effects. METHODS AND RESULTS: All the patients with a first recorded myocardial infarction and prescription for an antipsychotic identified in the Clinical Practice Research Datalink linked to the Myocardial Ischaemia National Audit Project were selected for the self-controlled case series. The incidence ratio of myocardial infarction during risk periods following the initiation of antipsychotic use relative to unexposed periods was estimated within individuals. A classical case-control study was undertaken for comparative purposes comparing antipsychotic exposure among cases and matched controls. We identified 1546 exposed cases for the self-controlled case series and found evidence of an association during the first 30 days after the first prescription of an antipsychotic, for first-generation agents [incidence rate ratio (IRR) 2.82, 95% confidence interval (CI) 2.0-3.99] and second-generation agents (IRR: 2.5, 95% CI: 1.18-5.32). Similar results were found for the case-control study for new users of first- (OR: 3.19, 95% CI: 1.9-5.37) and second-generation agents (OR: 2.55, 95% CI: 0.93-7.01) within 30 days of their myocardial infarction. CONCLUSION: We found an increased risk of myocardial infarction in the period following the initiation of antipsychotics that was not attributable to differences between people prescribed and not prescribed antipsychotics

Prognosis research strategy (PROGRESS) 1: a framework for researching clinical outcomes.

The PROGRESS series (www.progress-partnership.org) sets out a framework of four interlinked prognosis research themes and provides examples from several disease fields to show why evidence from prognosis research is crucial to inform all points in the translation of biomedical and health related research into better patient outcomes. Recommendations are made in each of the four papers to improve current research standards What is prognosis research? Prognosis research seeks to understand and improve future outcomes in people with a given disease or health condition. However, there is increasing evidence that prognosis research standards need to be improved Why is prognosis research important? More people now live with disease and conditions that impair health than at any other time in history; prognosis research provides crucial evidence for translating findings from the laboratory to humans, and from clinical research to clinical practice This first article introduces the framework of four interlinked prognosis research themes and then focuses on the first of the themes - fundamental prognosis research, studies that aim to describe and explain future outcomes in relation to current diagnostic and treatment practices, often in relation to quality of care Fundamental prognosis research provides evidence informing healthcare and public health policy, the design and interpretation of randomised trials, and the impact of diagnostic tests on future outcome. It can inform new definitions of disease, may identify unanticipated benefits or harms of interventions, and clarify where new interventions are required to improve prognosis

Symptoms of depression and cardiovascular reactions to acute psychological stress: Evidence from a population study

Depression and exaggerated cardiovascular reactivity are considered risk factors for cardiovascular disease, possibly as a result of common antecedents, such as altered autonomic nervous system function. We examined the association between depressive symptomatology and cardiovascular reactions to psychological stress in 1608 adults (875 women) comprising three distinct age cohorts: 24-, 44-, and 63-year olds. Depression was assessed using the Hospital Anxiety and Depression Scale. Blood pressure and heart rate were measured at baseline and during the paced auditory serial arithmetic test. Depression scores were negatively associated with systolic blood pressure and heart rate reactions, after adjustment for likely confounders such as sex, cohort, occupational status, body mass index, stress task performance score, baseline cardiovascular activity, antidepressant and antihypertensive medication. The direction of association was opposite to that which would be expected if excessive reactivity were to mediate the association between depression and cardiovascular disease outcomes or if they shared common antecedents

The science of clinical practice: disease diagnosis or patient prognosis? Evidence about "what is likely to happen" should shape clinical practice.

BACKGROUND: Diagnosis is the traditional basis for decision-making in clinical practice. Evidence is often lacking about future benefits and harms of these decisions for patients diagnosed with and without disease. We propose that a model of clinical practice focused on patient prognosis and predicting the likelihood of future outcomes may be more useful. DISCUSSION: Disease diagnosis can provide crucial information for clinical decisions that influence outcome in serious acute illness. However, the central role of diagnosis in clinical practice is challenged by evidence that it does not always benefit patients and that factors other than disease are important in determining patient outcome. The concept of disease as a dichotomous 'yes' or 'no' is challenged by the frequent use of diagnostic indicators with continuous distributions, such as blood sugar, which are better understood as contributing information about the probability of a patient's future outcome. Moreover, many illnesses, such as chronic fatigue, cannot usefully be labelled from a disease-diagnosis perspective. In such cases, a prognostic model provides an alternative framework for clinical practice that extends beyond disease and diagnosis and incorporates a wide range of information to predict future patient outcomes and to guide decisions to improve them. Such information embraces non-disease factors and genetic and other biomarkers which influence outcome. SUMMARY: Patient prognosis can provide the framework for modern clinical practice to integrate information from the expanding biological, social, and clinical database for more effective and efficient care