Modelling biogeochemical controls on planetary habitability

The length of a planet's `habitable period' is an important controlling factor on the evolution of life and of intelligent observers. This can be defined as the amount of time the surface temperature on the planet remains within defined `habitable' limits. Complex states of habitability derived from complex interactions between multiple factors may arise over the course of the evolution of an individual terrestrial planet with implications for long-term habitability and biosignature detection. The duration of these habitable conditions are controlled by multiple factors, including the orbital distance of the planet, its mass, the evolution of the host star, and the operation of any (bio)geochemical cycles that may serve to regulate planetary climate. A stellar evolution model was developed to investigate the control of increasing main-sequence stellar luminosity on the boundaries of the radiative habitable zone, which was then coupled with a zero-D biogeochemical carbon cycle model to investigate the operation of the carbonate-silicate cycle under conditions of varying incident stellar flux and planet size. The Earth will remain within habitable temperature limits for 6.34 Gyr (1.8 Gyr from present), but photosynthetic primary producers will experience carbon-starvation due to greatly increased terrestrial weathering from 5.38 Gyr (0.84 Gyr from present) onwards, with significant implications for planetary habitability. Planet mass was discovered to have a significant control on the length of the habitable period of Earth-like planets, but more data on the bulk density and atmospheric composition of newly-discovered exoplanets is required before definitive estimates of their long-term habitability can be made. Exoplanet case studies reveal habitable periods significantly longer than that of the Earth, possibly up to 80 Gyr in the case of planets in the orbit of M-dwarfs. Contemporary measures of habitability that rely strongly on surface temperatures are becoming obsolete, and a move towards the inclusion of integrated biogeochemical cycle models and the development of multiparameter habitability indices will strengthen contemporary understanding of the distribution and evolution of potentially habitable terrestrial worlds

Habitable Zone Lifetime of Exoplanets around Main Sequence Stars

Funding: Dean's Scholarship at the University of East Anglia.The potential habitability of newly discovered exoplanets is initially assessed by determining whether their orbits fall within the circumstellar habitable zone of their star. However, the habitable zone (HZ) is not static in time or space, and its boundaries migrate outward at a rate proportional to the increase in luminosity of a star undergoing stellar evolution, possibly including or excluding planets over the course of the star’s main sequence lifetime. We describe the time that a planet spends within the HZ as its ‘‘habitable zone lifetime.’’ The HZ lifetime of a planet has strong astrobiological implications and is especially important when considering the evolution of complex life, which is likely to require a longer residence time within the HZ. Here, we present results from a simple model built to investigate the evolution of the ‘‘classic’’ HZ over time, while also providing estimates for the evolution of stellar luminosity over time in order to develop a ‘‘hybrid’’ HZ model. These models return estimates for the HZ lifetimes of Earth and 7 confirmed HZ exoplanets and 27 unconfirmed Kepler candidates. The HZ lifetime for Earth ranges between 6.29 and 7.79 · 109 years (Gyr). The 7 exoplanets fall in a range between ∼1 and 54.72 Gyr, while the 27 Kepler candidate planets’ HZ lifetimes range between 0.43 and 18.8 Gyr. Our results show that exoplanet HD 85512b is no longer within the HZ, assuming it has an Earth analog atmosphere. The HZ lifetime should be considered in future models of planetary habitability as setting an upper limit on the lifetime of any potential exoplanetary biosphere, and also for identifying planets of high astrobiological potential for continued observational or modeling campaigns.Publisher PDFPeer reviewe

The effect of land fraction and host star spectral energy distribution on the planetary albedo of terrestrial worlds

The energy balance and climate of planets can be affected by the reflective properties of their land, ocean, and frozen surfaces. Here we investigate the effect of host star spectral energy distribution (SED) on the albedo of these surfaces using a one-dimensional (1-D) energy balance model (EBM). Incorporating spectra of M-, K-, G- and F-dwarf stars, we determined the effect of varying fractional and latitudinal distribution of land and ocean surfaces as a function of host star SED on the overall planetary albedo, climate, and ice-albedo feedback response. While noting that the spatial distribution of land masses on a given planet will have an effect on the overall planetary energy balance, we find that terrestrial planets with higher average land/ocean fractions are relatively cooler and have higher albedo regardless of star type. For Earth-like planets orbiting M-dwarf stars the increased absorption of water ice in the near-infrared (NIR), where M-dwarf stars emit much of their energy, resulted in warmer global mean surface temperatures, ice lines at higher latitudes, and increased climate stability as the ice-albedo feedback became negative at high land fractions. Conversely, planets covered largely by ocean, and especially those orbiting bright stars, had a considerably different energy balance due to the contrast between the reflective land and the absorptive ocean surface, which in turn resulted in warmer average surface temperatures than land-covered planets and a stronger potential ice-albedo feedback. While dependent on the properties of individual planetary systems, our results place so constraints on a range of climate states of terrestrial exoplanets based on albedo and incident flux

The Effect of Land Albedo on the Climate of Land-Dominated Planets in the TRAPPIST-1 System

Variations in the reflective properties of the bulk material that comprises the surface of land-dominated planets will affect the planetary energy balance by interacting differently with incident radiation from the host star. Furthermore, low-mass cool stars, such as nearby M8V dwarf TRAPPIST-1, emit a significant fraction of their flux in longer wavelengths relative to the Sun in regions where terrestrial materials may exhibit additional variability in albedo. Using the Community Earth System Model (CESM) we investigate the effect of the composition of the land surface and its albedo on planetary climate in the context of spatially homogeneous, entirely land-covered planets with dry atmospheres at the orbital separation of TRAPPIST-1d, TRAPPIST-1e, and TRAPPIST-1f. We use empirically derived spectra of four terrestrial compositional endmembers (granite, calcite, aridisol, and dune sand) and a composite spectrum of TRAPPIST-1 for these simulations and compare these model output to an aquaplanet and several Sol-spectrum control cases. We report a difference of approximately 50 K in global mean surface temperature, variations in atmospheric rotational features, and a reduction in cross-equatorial heat transport between scenarios in which materials with higher albedo in the infrared (calcite and dune sand) were used and those with more absorptive crustal material, such as granite or dry soils. An aquaplanet TRAPPIST-1d scenario results in an unstable runaway greenhouse regime. Therefore, we demonstrate that determining the composition and albedo of continental landmasses is crucial for making accurate determinations of the climate of terrestrial exoplanets.Comment: 18 pages, 11 figures. Accepted for publication in the Astrophysical Journal (ApJ

A prospective stroke register in Sierra Leone: Demographics, stroke type, stroke care and hospital outcomes

Introduction Stroke is the second most common cause of adult death in Africa. This study reports the demographics, stroke types, stroke care and hospital outcomes for stroke in Freetown, Sierra Leone. Methods A prospective observational register recorded all patients 18 years and over with stroke between May 2019 and April 2020. Stroke was defined according to the WHO criteria. Pearson’s chi squared test was used to examine associations between categorical variables and unpaired t-tests for continuous variables. Multivariable logistic regression,to explain in-hospital death, was reported as odds ratios (OR) and 95% confidence intervals. Results 385 strokes were registered, 315 (81.8%) were first in a lifetime events. Mean age was 59.2 (SD 13.8) and 187 (48.6%) were male. 327 (84.9%) of strokes were confirmed by CT scan. 231 (60.0%) were ischaemic, 85 (22.1%) intracerebral haemorrhage, 11 (2.9%) subarachnoid haemorrhage and 58 (15.1%) undetermined stroke type. The median National Institute of Health Stroke Scale on presentation was 17 (IQR 9-25). Haemorrhagic strokes compared to ischaemic strokes were more severe, 20 (IQR 12-26) vs 13 (IQR 7-22) (p<0.001), and occurred in a younger population, mean age 52.3 (SD 12.0) vs 61.6 (SD 13.8) (p<0.001), with a lower level of educational attainment 28.2% vs 40.7% (p=0.04). The median time from stroke onset to arrival at the principal referral hospital was 25 hours (IQR 6-73). Half the patients (50.4%) sought care at another health provider prior to arrival. 151 patients died in hospital (39.5%). 43 deaths occurred within 48 hours of arriving at hospital with median time to death of 4 days (IQR 0-7 days). 49.6% of patients had ≥1 complication, 98 (25.5%) pneumonia, 33 (8.6%) urinary tract infection. Male gender (OR 3.33,1.65 - 6.75), pneumonia (OR 3.75, 1.82 – 7.76), subarachnoid haemorrhage (OR 43.1, 6.70-277.4) and undetermined stroke types (OR 6.35, 2.17– 18.60), were associated with higher risk of in-hospital death. Discussion We observed severe strokes occurring in a young population with high in hospital mortality. Further work to deliver evidence-based stroke care is essential to reduce stroke mortality in Sierra Leone

Exoplanet Science Priorities from the Perspective of Internal and Surface Processes for Silicate and Ice Dominated Worlds

The geophysics of extrasolar planets is a scientific topic often regarded as standing largely beyond the reach of near-term observations. This reality in no way diminishes the central role of geophysical phenomena in shaping planetary outcomes, from formation, to thermal and chemical evolution, to numerous issues of surface and near-surface habitability. We emphasize that for a balanced understanding of extrasolar planets, it is important to look beyond the natural biases of current observing tools, and actively seek unique pathways to understand exoplanet interiors as best as possible during the long interim prior to a time when internal components are more directly accessible. Such pathways include but are not limited to: (a) enhanced theoretical and numerical modeling, (b) laboratory research on critical material properties, (c) measurement of geophysical properties by indirect inference from imprints left on atmospheric and orbital properties, and (d) the purpose-driven use of Solar System object exploration expressly for its value in comparative planetology toward exoplanet-analogs. Breaking down barriers that envision local Solar System exploration, including the study of Earth's own deep interior, as separate from and in financial competition with extrasolar planet research, may greatly improve the rate of needed scientific progress for exoplanet geophysics. As the number of known rocky and icy exoplanets grows in the years ahead, we expect demand for expertise in 'exogeoscience' will expand at a commensurately intense pace. We highlight key topics, including: how water oceans below ice shells may dominate the total habitability of our galaxy by volume, how free-floating nomad planets may often attain habitable subsurface oceans supported by radionuclide decay, and how deep interiors may critically interact with atmospheric mass loss via dynamo-driven magnetic fields

Highly Volcanic Exoplanets, Lava Worlds, and Magma Ocean Worlds:An Emerging Class of Dynamic Exoplanets of Significant Scientific Priority

Highly volcanic exoplanets, which can be variously characterized as 'lava worlds', 'magma ocean worlds', or 'super-Ios' are high priority targets for investigation. The term 'lava world' may refer to any planet with extensive surface lava lakes, while the term 'magma ocean world' refers to planets with global or hemispherical magma oceans at their surface. 'Highly volcanic planets', including super-Ios, may simply have large, or large numbers of, active explosive or extrusive volcanoes of any form. They are plausibly highly diverse, with magmatic processes across a wide range of compositions, temperatures, activity rates, volcanic eruption styles, and background gravitational force magnitudes. Worlds in all these classes are likely to be the most characterizable rocky exoplanets in the near future due to observational advantages that stem from their preferential occurrence in short orbital periods and their bright day-side flux in the infrared. Transit techniques should enable a level of characterization of these worlds analogous to hot Jupiters. Understanding processes on highly volcanic worlds is critical to interpret imminent observations. The physical states of these worlds are likely to inform not just geodynamic processes, but also planet formation, and phenomena crucial to habitability. Volcanic and magmatic activity uniquely allows chemical investigation of otherwise spectroscopically inaccessible interior compositions. These worlds will be vital to assess the degree to which planetary interior element abundances compare to their stellar hosts, and may also offer pathways to study both the very young Earth, and the very early form of many silicate planets where magma oceans and surface lava lakes are expected to be more prevalent. We suggest that highly volcanic worlds may become second only to habitable worlds in terms of both scientific and public long-term interest.Comment: A white paper submitted in response to the National Academy of Sciences 2018 Exoplanet Science Strategy solicitation, from the NASA Sellers Exoplanet Environments Collaboration (SEEC) of the Goddard Space Flight Center. 6 pages, 0 figure

Stroke in Sierra Leone: Case fatality rate and functional outcome after stroke in Freetown

Background: There is limited information on long term outcomes after stroke in Sub-Saharan Africa (SSA). Current estimates of case fatality rate (CFR) in SSA are based on small sample sizes with varying study design and report high heterogeneity. Aims: We report CFR and functional outcomes from a large, prospective, longitudinal cohort of stroke patients in Sierra Leone and describe factors associated with mortality and functional outcome. Methods: A prospective longitudinal stroke register was established at both adult tertiary government hospitals in Freetown, Sierra Leone. It recruited all patients ≥18 years with stroke, using the World Health Organization definition, from May 2019 until October 2021. To reduce selection bias onto the register all investigations were paid by the funder and outreach conducted to raise awareness of the study. Sociodemographic data, National Institute of Health Stroke Scale (NIHSS) and Barthel Index (BI) was collected on all patients on admission, at seven days, 90 days, one year and two years post stroke. Cox proportional-hazards models were constructed to identify factors associated with all-cause mortality. A binomial logistic regression model reports odds ratio (OR) for functional independence at one year. Results: 986 patients with stroke were included, of which 847 (85.9%) received neuroimaging. Follow up rate was 81.5% at one year, missing item data was &lt;1% for most variables. Stroke cases were equally split by sex and mean age was 58.9 (SD: 14.0) years. 625 (63%) were ischaemic, 206 (21%) primary intracerebral haemorrhage, 25 (3%) subarachnoid haemorrhage and 130 (13%) were of undetermined stroke type. Median NIHSS was 16 (9-24). CFR at 30 days, 90 days, 1 year and 2 years was 37.1%, 44.4%, 49.7% and 53.2% respectively. Factors associated with increased fatality were male sex HR:1.28 (1.05-1.56), previous stroke HR:1.34 (1.04-1.71), atrial fibrillation HR:1.58(1.06-2.34), subarachnoid haemorrhage HR:2.31 (1.40-3.81), undetermined stroke type HR: 3.18(2.44-4.14) and in-hospital complications HR: 1.65 (1.36-1.98). 93% of patients were completely independent prior to their stroke, declining to 19% at one year after stroke. Functional improvement was most likely to occur between 7 and 90-days post stroke with 35% patients improving, and 13% improving between 90 days to one year. Increasing age OR: 0.97(0.95-0.99), previous stroke OR: 0.50 (0.26-0.98), NIHSS OR 0.89 (0.86-0.91), undetermined stroke type OR:0.18 (0.05-0.62) and ≥1 in hospital complication OR:0.52 (0.34-0.80) were associated with lower OR of functional independence at one year. Whilst hypertension OR:1.98 (1.14-3.44) and being the primary breadwinner of the household OR:1.59 (1.01-2.49) were associated with functional independence. Discussion: Stroke in Sierra Leone affected younger people, and resulted in high rates of fatality and functional impairment relative to global averages. Key clinical priorities for reducing fatality include preventing stroke-related complications through evidence-based stroke care; improved detection and management of atrial fibrillation, and increasing coverage of secondary prevention. Further research into care pathways and interventions to encourage care seeking for less severe strokes should be prioritized. Data availability: Requests for access to anonymized data for academic use should be made to the SISLE team https://www.kcl.ac.uk/research/stroke

Planned early delivery or expectant management for late preterm pre-eclampsia (PHOENIX): a randomised controlled trial

© 2019 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY 4.0 license Background: In women with late preterm pre-eclampsia, the optimal time to initiate delivery is unclear because limitation of maternal disease progression needs to be balanced against infant complications. The aim of this trial was to determine whether planned earlier initiation of delivery reduces maternal adverse outcomes without substantial worsening of neonatal or infant outcomes, compared with expectant management (usual care) in women with late preterm pre-eclampsia. Methods: In this parallel-group, non-masked, multicentre, randomised controlled trial done in 46 maternity units across England and Wales, we compared planned delivery versus expectant management (usual care) with individual randomisation in women with late preterm pre-eclampsia from 34 to less than 37 weeks' gestation and a singleton or dichorionic diamniotic twin pregnancy. The co-primary maternal outcome was a composite of maternal morbidity or recorded systolic blood pressure of at least 160 mm Hg with a superiority hypothesis. The co-primary perinatal outcome was a composite of perinatal deaths or neonatal unit admission up to infant hospital discharge with a non-inferiority hypothesis (non-inferiority margin of 10% difference in incidence). Analyses were by intention to treat, together with a per-protocol analysis for the perinatal outcome. The trial was prospectively registered with the ISRCTN registry, ISRCTN01879376. The trial is closed to recruitment but follow-up is ongoing. Findings: Between Sept 29, 2014, and Dec 10, 2018, 901 women were recruited. 450 women (448 women and 471 infants analysed) were allocated to planned delivery and 451 women (451 women and 475 infants analysed) to expectant management. The incidence of the co-primary maternal outcome was significantly lower in the planned delivery group (289 [65%] women) compared with the expectant management group (338 [75%] women; adjusted relative risk 0·86, 95% CI 0·79–0·94; p=0·0005). The incidence of the co-primary perinatal outcome by intention to treat was significantly higher in the planned delivery group (196 [42%] infants) compared with the expectant management group (159 [34%] infants; 1·26, 1·08–1·47; p=0·0034). The results from the per-protocol analysis were similar. There were nine serious adverse events in the planned delivery group and 12 in the expectant management group. Interpretation: There is strong evidence to suggest that planned delivery reduces maternal morbidity and severe hypertension compared with expectant management, with more neonatal unit admissions related to prematurity but no indicators of greater neonatal morbidity. This trade-off should be discussed with women with late preterm pre-eclampsia to allow shared decision making on timing of delivery. Funding: National Institute for Health Research Health Technology Assessment Programme