We make the case that marine processes are an important component of the canonical silicate weathering feedback, and have played a much more important role in p CO2 regulation than traditionally imagined. Notably, geochemical evidence indicate that the weathering of marine sediments and off‐axis basalt alteration act as major carbon sinks. However, this sink was potentially dampened during Earth’s early history when oceans had higher levels of dissolved silicon (Si), iron (Fe), and magnesium (Mg), and instead likely fostered more extensive carbon recycling within the ocean‐atmosphere system via reverse weathering—that in turn acted to elevate ocean‐atmosphere CO2 levels.
Phosphate is central to the origin of life because it is a key component of nucleotides in genetic molecules, phospholipid cell membranes, and energy transfer molecules such as adenosine triphosphate. To incorporate phosphate into biomolecules, prebiotic experiments commonly use molar phosphate concentrations to overcome phosphates poor reactivity with organics in water. However, phosphate is generally limited to micromolar levels in the environment because it precipitates with calcium as low-solubility apatite minerals. This disparity between laboratory conditions and environmental constraints is an enigma known as the phosphate problem. Here we show that carbonate-rich lakes are a marked exception to phosphate-poor natural waters. In principle, modern carbonate-rich lakes could accumulate up to ?0.1 molal phosphate under steady-state conditions of evaporation and stream inflow because calcium is sequestered into carbonate minerals. This prevents the loss of dissolved phosphate to apatite precipitation. Even higher phosphate concentrations (>1 molal) can form during evaporation in the absence of inflows. On the prebiotic Earth, carbonate-rich lakes were likely abundant and phosphate-rich relative to the present day because of the lack of microbial phosphate sinks and enhanced chemical weathering of phosphate minerals under relatively CO2-rich atmospheres. Furthermore, the prevailing CO2 conditions would have buffered phosphate-rich brines to moderate pH (pH 6.5 to 9). The accumulation of phosphate and other prebiotic reagents at concentration and pH levels relevant to experimental prebiotic syntheses of key biomolecules is a compelling reason to consider carbonate-rich lakes as plausible settings for the origin of life.
Despite surface liquid water’s importance to habitability, observationally diagnosing its presence or absence on exoplanets is still an open problem. Inspired within the solar system by the differing sulfur cycles on Venus and Earth, we investigate thick sulfate (H2SO4–H2O) aerosol haze and high trace mixing ratios of SO2 gas as observable atmospheric features whose sustained existence is linked to the near absence of surface liquid water. We examine the fundamentals of the sulfur cycle on a rocky planet with an ocean and an atmosphere in which the dominant forms of sulfur are SO2 gas and H2SO4–H2O aerosols (as on Earth and Venus). We build a simple but robust model of the wet, oxidized sulfur cycle to determine the critical amounts of sulfur in the atmosphere–ocean system required for detectable levels of SO2 and a detectable haze layer. We demonstrate that for physically realistic ocean pH values (pH gsim 6) and conservative assumptions on volcanic outgassing, chemistry, and aerosol microphysics, surface liquid water reservoirs with greater than 10−3 Earth oceans are incompatible with a sustained observable H2SO4–H2O haze layer and sustained observable levels of SO2. Thus, we propose the observational detection of an H2SO4–H2O haze layer and of SO2 gas as two new remote indicators that a planet does not host significant surface liquid water.
Constraining the surface environment of the early Earth is essential for understanding the origin and evolution of life. The release of cations from silicate weathering depends on climatic temperature and urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0001, and such cations sequester urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0002 into carbonate minerals in or on the seafloor, providing a stabilizing feedback on climate. Previous studies have suggested that this carbonate?silicate cycle can keep the early Earth’s surface temperature moderate by increasing urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0003 to compensate for the faint young Sun. However, the Hadean Earth experienced a high meteorite impactor flux, which produced ejecta that is easily weathered by carbonic acid. In this study, we estimated the histories of surface temperature and ocean pH during the Hadean and early Archean using a new model that includes the weathering of impact ejecta, empirically justified seafloor weathering, and ocean carbonate chemistry. We find that relatively low urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0004 and surface temperatures are probable during the Hadean, for example, at 4.3 Ga, urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0005 (in bar) is urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0006 urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0007 and temperature is urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0008 urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0009 K. Such a low urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0010 would result in a circumneutral to basic pH of seawater, for example, urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0011 urn:x-wiley:ggge:media:ggge22102:ggge22102-math-0012 at 4.3 Ga. A probably cold and alkaline marine environment is associated with a high impact flux. Hence, if there was an interval of an enhanced impact flux, that is, Late Heavy Bombardment, similar conditions may have existed in the early Archean. Therefore, if the origin of life occurred in the Hadean, life likely emerged in a cold global environment and probably spread into an alkaline ocean.
The TRAPPIST-1 system, consisting of an ultracool host star having seven known Earth-sized planets, will be a prime target for atmospheric characterization with the James Webb Space Telescope (JWST). However, the detectability of atmospheric molecular species may be severely impacted by the presence of clouds and/or hazes. In this work, we perform 3D general circulation model (GCM) simulations with the LMD-G model supplemented by 1D photochemistry simulations at the terminator with the Atmos model to simulate several possible atmospheres for TRAPPIST-1e, 1f, and 1g: (1) modern Earth, (2) Archean Earth, and (3) CO2-rich atmospheres. The JWST synthetic transit spectra were computed using the GSFC Planetary Spectrum Generator. We find that the TRAPPIST-1e, 1f, and 1g atmospheres, with clouds and/or hazes, could be detected using JWST’s NIRSpec Prism from the CO2 absorption line at 4.3 ?m in less than 15 transits at 3? or less than 35 transits at 5?. However, our analysis suggests that other gases would require hundreds (or thousands) of transits to be detectable. We also find that H2O, mostly confined in the lower atmosphere, is very challenging to detect for these planets or similar systems if the planets’ atmospheres are not in a moist greenhouse state. This result demonstrates that the use of GCMs, self-consistently taking into account the effect of clouds and subsaturation, is crucial to evaluate the detectability of atmospheric molecules of interest, as well as for interpreting future detections in a more global (and thus robust and relevant) approach.
We examined standard biogeochemical proxies, including organic carbon and nitrogen isotopes, iron speciation, metal abundances and carbonate-associated sulfate. Much of the primary information has been lost because the rocks of the Callanna Group have experienced extensive metamorphism up to amphibolite facies and are altered by modern weathering. However, relics of these proxies, combined with sedimentological features, preserve evidence of redox stratification within this basin. Furthermore, our observations, in particular weakly fractionated nitrogen isotopes and abundant gypsum pseudomorphs, are incompatible with the interpretation of high alkalinity. The high salt content and occurrences of tidal indicators are most parsimoniously explained by frequent incursions of seawater. Thus, the Callanna Group cannot speak straightforwardly to environmental conditions in non-marine habitats at this time. Lastly, the absence of a large carbon isotope anomaly indicates that these rocks do not correlate with the Bitter Springs Formation.
The asteroid belt is characterized by an extreme low total mass of material on dynamically excited orbits. The Nice model explains many peculiar qualities of the Solar system, including the belt’s excited state, by invoking an orbital instability between the outer planets. However, previous studies of the Nice model’s effect on the belt’s structure struggle to reproduce the innermost asteroids’ orbital inclination distribution.
The Large UV/Optical/Infrared Surveyor (LUVOIR) mission is one of four Decadal Survey Mission Concepts studied by NASA in preparation for the US National Academies’ Astro2020 Decadal Survey. This observatory has the major goal of characterizing a wide range of exoplanets, including those that might be habitable — or even inhabited. It would simultaneously enable a great leap forward in a broad range of astrophysics — from the epoch of reionization, through galaxy formation and evolution, to star and planet formation. Powerful remote sensing observations of Solar System bodies will also be possible. This Final Report on the LUVOIR study presents the scientific motivations and goals of the mission concept, the engineering design, and technology development information. Please refer to the LUVOIR Final Report Appendices (separate document) for additional information.
? Centauri A is the closest solar-type star to the Sun and offers an excellent opportunity to detect the thermal emission of a mature planet heated by its host star. The MIRI coronagraph on the James Webb Space Telescope can search the 13 au (1”2”) region around ? Cen A which is predicted to be stable within the ? Cen AB system. We demonstrate that with reasonable performance of the telescope and instrument, a 20 hr program combining on-target and reference star observations at 15.5 ?m could detect thermal emission from planets as small as ~5 R?. Multiple visits every 36 months would increase the geometrical completeness, provide astrometric confirmation of detected sources, and push the radius limit down to ~3 R?. An exozodiacal cloud only a few times brighter than our own should also be detectable, although a sufficiently bright cloud might obscure any planet present in the system. While current precision radial velocity (PRV) observations set a limit of 50100 M? at 13 au for planets orbiting ? Cen A, there is a broad range of exoplanet radii up to 10 R? consistent with these mass limits. A carefully planned observing sequence along with state-of-the-art post-processing analysis could reject the light from ? Cen A at the level of ~10?5 at 1”2” and minimize the influence of ? Cen B located 7”8” away in the 20222023 timeframe. These space-based observations would complement on-going imaging experiments at shorter wavelengths as well as PRV and astrometric experiments to detect planets dynamically. Planetary demographics suggest that the likelihood of directly imaging a planet whose mass and orbit are consistent with present PRV limits is small, ~5%, and possibly lower if the presence of a binary companion further reduces occurrence rates. However, at a distance of just 1.34 pc, ? Cen A is our closest sibling star and certainly merits close scrutiny.
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 energy balance model. 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, 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 some constraints on a range of climate states of terrestrial exoplanets based on albedo and incident flux.
Transit spectroscopy of terrestrial planets around nearby M dwarfs will be a primary goal of space missions in coming decades. Three-dimensional climate modeling has shown that slow-synchronous rotating terrestrial planets may develop thick clouds at the substellar point, increasing the albedo. For M dwarfs with T eff > 3000 K, such planets at the inner habitable zone (IHZ) have been shown to retain moist greenhouse conditions, with enhanced stratospheric water vapor (fH 2O > 10?3) and low Earth-like surface temperatures. However, M dwarfs also possess strong UV activity, which may effectively photolyze stratospheric H2O. Prior modeling efforts have not included the impact of high stellar UV activity on the H2O. Here, we employ a 1D photochemical model with varied stellar UV, to assess whether H2O destruction driven by high stellar UV would affect its detectability in transmission spectroscopy. Temperature and water vapor profiles are taken from published 3D climate model simulations for an IHZ Earth-sized planet around a 3300 K M dwarf with an N2H2O atmosphere; they serve as self-consistent input profiles for the 1D model. We explore additional chemical complexity within the 1D model by introducing other species into the atmosphere. We find that as long as the atmosphere is well-mixed up to 1 mbar, UV activity appears to not impact detectability of H2O in the transmission spectrum. The strongest H2O features occur in the James Webb Space Telescope MIRI instrument wavelength range and are comparable to the estimated systematic noise floor of ~50 ppm.
Near-term studies of Venus-like atmospheres with James Webb Space Telescope (JWST) promise to advance our knowledge of terrestrial planet evolution. However, the remote study of Venus in the solar system and the ongoing efforts to characterize gaseous exoplanets both suggest that high altitude aerosols can limit observational studies of lower atmospheres, and potentially make it challenging to recognize exoplanets as “Venus-like.” To support practical approaches for exo-Venus characterization with JWST, we use Venus-like atmospheric models with self-consistent cloud formation of the seven TRAPPIST-1 exoplanets to investigate the atmospheric depth that can be probed using both transmission and emission spectroscopy. We find that JWST/Mid-IR Instrument Low Resolution Spectrometer secondary eclipse emission spectroscopy in the 6 ?m opacity window could probe at least an order of magnitude deeper pressures than transmission spectroscopy, potentially allowing access to the subcloud atmosphere for the two hot innermost TRAPPIST-1 planets. In addition, we identify two confounding effects of sulfuric acid aerosols that may carry strong implications for the characterization of terrestrial exoplanets with transmission spectroscopy: (1) there exists an ambiguity between cloud-top and solid surface in producing the observed spectral continuum; and (2) the cloud-forming region drops in altitude with semimajor axis, causing an increase in the observable cloud-top pressure with decreasing stellar insolation. Taken together, these effects could produce a trend of thicker atmospheres observed at lower stellar insolationa convincing false positive for atmospheric escape and an empirical “cosmic shoreline.” However, developing observational and theoretical techniques to identify Venus-like exoplanets and discriminate them from stellar windswept worlds will enable advances in the emerging field of terrestrial comparative planetology.
In this Letter we will consider the effect of tidal locking on limit cycling between snowball and warm climate states, which has been suggested could occur for rapidly rotating planets in the outer regions of the habitable zone with low CO2 outgassing rates. Here, we use a 3D Global Climate Model that calculates silicate-weathering to show that tidally locked planets with an active carbon cycle will not experience limit cycling between warm and snowball states. Instead, they smoothly settle into “Eyeball” states with a small unglaciated substellar region. The size of this unglaciated region depends on the stellar irradiation, the CO2 outgassing rate, and the continental configuration. Furthermore, we argue that a tidally locked habitable zone planet cannot stay in a snowball state for a geologically significant time. This may be beneficial to the survival of complex life on tidally locked planets orbiting the outer edge of their stars, but might also make it less likely for complex life to arise.
Molecular nitrogen is the most commonly assumed background gas that supports habitability on rocky planets. Despite its chemical inertness, nitrogen molecules are broken by lightning, hot volcanic vents, and bolide impacts, and can be converted into soluble nitrogen compounds and then sequestered in the ocean. The very stability of nitrogen, and that of nitrogen-based habitability, is thus called into question. Here we determine the lifetime of molecular nitrogen vis-à-vis aqueous sequestration, by developing a novel model that couples atmospheric photochemistry and oceanic chemistry. We find that HNO, the dominant nitrogen compound produced in anoxic atmospheres, is converted to N2O in the ocean, rather than oxidized to nitrites or nitrates as previously assumed. This N2O is then released back into the atmosphere and quickly converted to N2. We also find that the deposition rate of NO is severely limited by the kinetics of the aqueous-phase reaction that converts NO to nitrites in the ocean. Putting these insights together, we conclude that the atmosphere must produce nitrogen species at least as oxidized as NO2 and HNO2 to enable aqueous sequestration. The lifetime of molecular nitrogen in anoxic atmospheres is determined to be >1 billion years on temperate planets of both Sun-like and M dwarf stars. This result upholds the validity of molecular nitrogen as a universal background gas on rocky planets.
Planets that revolve around a binary pair of stars are known as circumbinary planets. The orbital motion of the stars around their center of mass causes a periodic variation in the total instellation incident upon a circumbinary planet. This study uses both an analytic and numerical energy balance model to calculate the extent to which this effect can drive changes in surface temperature on circumbinary terrestrial planets. We show that the amplitude of the temperature variation is largely constrained by the effective heat capacity, which corresponds to the ocean?to?land ratio on the planet. Planets with large ocean fractions should experience only modest warming and cooling of only a few degrees, which suggests that habitability cannot be precluded for such circumbinary planets. Planets with large land fractions that experience extreme periodic forcing could be prone to changes in temperature of tens of degrees or more, which could drive more extreme climate changes that inhibit continuously habitable conditions.
To investigate water vapor detection for exoplanets, we generated reflectance spectra over a grid of water vapor column masses (given by the integral of a species’ mass density over the atmospheric column, yielding the mass of that species per unit area). As the depth of a gas spectral feature is, to a large degree, controlled by the absorption optical depth across the feature, varying the column mass is akin to varying feature strength. Our column mass grid spans 10?5101?g?cm?2, where the total gas and water vapor column masses for Earth are 1.0 × 103 g?cm?2 and 3?g?cm?2, respectively. We adopt 1 bar of total atmospheric pressure, assume molecular nitrogen is the background atmospheric gas, use an Earth-like surface gravity of 10?m?s?2, and assume a gray surface albedo of 0.3. Our calculations generally apply to scenarios where the water vapor column is measured above some opaque “surface” (e.g., a solid surface or planet-wide cloud deck). Quadrature-phase spectra were modeled with the widely used Spectral Mapping Atmospheric Radiative Transfer (SMART) model (developed by D.?Crisp; Meadows & Crisp 1996), which is a line-by-line, multiple-scattering radiative transfer tool.
Planets residing in circumstellar habitable zones offer us the best opportunities to test hypotheses of life’s potential pervasiveness and complexity. Constraining the precise boundaries of habitability and its observational discriminants is critical to maximizing our chances at remote life detection with future instruments. Conventionally, calculations of the inner edge of the habitable zone (IHZ) have been performed using both 1D radiative-convective and 3D general circulation models. However, these models lack interactive 3D chemistry and do not resolve the mesosphere and lower thermosphere region of the upper atmosphere. Here, we employ a 3D high-top chemistry-climate model (CCM) to simulate the atmospheres of synchronously rotating planets orbiting at the inner edge of habitable zones of K- and M-dwarf stars (between T eff = 2600 and 4000 K). While our IHZ climate predictions are in good agreement with general circulation model studies, we find noteworthy departures in simulated ozone and HOx photochemistry. For instance, climates around inactive stars do not typically enter the classical moist greenhouse regime even with high (gsim10?3 mol mol?1) stratospheric water vapor mixing ratios, which suggests that planets around inactive M-stars may only experience minor water-loss over geologically significant timescales. In addition, we find much thinner ozone layers on potentially habitable moist greenhouse atmospheres, as ozone experiences rapid destruction via reaction with hydrogen oxide radicals. Using our CCM results as inputs, our simulated transmission spectra show that both water vapor and ozone features could be detectable by instruments NIRSpec and MIRI LRS on board the James Webb Space Telescope.
We find that in the Late Pennsylvanian units, an estuarine nutrient trap on the Midcontinent Shelf enabled vigorous selenium recycling, leading to very high concentrations in sediments and enrichment of heavy isotopes in the aqueous selenium reservoir. In contrast, we find that among the Late Devonian units, differences in local basinal hydrography led to a gradient in selenium abundance and isotopic fractionation, with the more restricted basins depleting their selenium reservoirs and causing enrichment of heavy isotopes in the residual aqueous reservoir. In both of these case studies, the additional context provided by complementary paleo-environmental proxies was critical for distinguishing between possible drivers of selenium isotopic variability. When extending such studies to other paleo-environmental settings, we suggest that the continued use of complementary datasets will enable the most robust use of the selenium paleo-redox proxy. Moreover, further development of techniques for high-precision and phase-specific selenium isotope measurements will greatly improve the ability to deduce subtle redox fluctuations with this proxy.
If the process of microbial N2 fixation records the δ15N value of atmospheric N2 in cycad foliage, the fossil record of cycads may provide an archive of atmospheric δ15N values. To explore this potential proxy, we conducted a survey of wild cycads growing in a range of modern environments to determine whether cycad foliage reliably records the isotopic composition of atmospheric N2. We find that neither biological nor environmental factors significantly influence the δ15N values of cycad foliage, suggesting that they provide a reasonably robust record of the δ15N of atmospheric N2. Application of this proxy to the record of carbonaceous cycad fossils may not only help to constrain changes in atmospheric nitrogen isotope ratios since the late Paleozoic, but also could shed light on the antiquity of the N2‐fixing symbiosis between cycads and cyanobacteria.
Habitability is a measure of an environment’s potential to support life, and a habitable exoplanet supports liquid water on its surface. However, a planet’s success in maintaining liquid water on its surface is the end result of a complex set of interactions between planetary, stellar, planetary system and even Galactic characteristics and processes, operating over the planet’s lifetime. In this chapter, we describe how we can now determine which exoplanets are most likely to be terrestrial, and the research needed to help define the habitable zone under different assumptions and planetary conditions. We then move beyond the habitable zone concept to explore a new framework that looks at far more characteristics and processes, and provide a comprehensive survey of their impacts on a planet’s ability to acquire and maintain habitability over time. We are now entering an exciting era of terrestiral exoplanet atmospheric characterization, where initial observations to characterize planetary composition and constrain atmospheres is already underway, with more powerful observing capabilities planned for the near and far future. Understanding the processes that affect the habitability of a planet will guide us in discovering habitable, and potentially inhabited, planets.
We use a one-dimensional (1D) cloud-free climate model to estimate habitable zone (HZ) boundaries for terrestrial planets of masses 0.1 ME and 5 ME around circumbinary stars of various spectral type combinations. Specifically, we consider binary systems with host spectral types F-F, F-G, F-K, F-M, G-G, G-K, G-M, K-K, K-M and M-M. Scaling the background N2 atmospheric pressure with the radius of the planet, we find that the inner edge of the HZ moves inwards toward the star for 5 ME compared to 0.1 ME planets for all spectral types. This is because the water-vapor column depth is smaller for larger planets and higher temperatures are needed before water vapor completely dominates the outgoing longwave radiation. The outer edge of the HZ changes little due to competing effects of the albedo and greenhouse effect. While these results are broadly consistent with the trend of single star HZ results for different mass planets, there are significant differences between single star and binary star systems for the inner edge of the HZ. Interesting combinations of stellar pairs from our 1D model results can be used to explore for in-depth climate studies with 3D climate models. We identify a common HZ stellar flux domain for all circumbinary spectral types.
Odd oxygen (O, O(1D), O3) abundance and its variability in the Martian atmosphere results from complex physical and chemical interactions among atmospheric species, which are driven mainly by solar radiation and atmospheric conditions. Although our knowledge of Mars ozone distribution and variability has been significantly improved with the arrival of several recent orbiters, the data acquired by such missions is not enough to properly characterize its diurnal variation. Thus, photochemical models are useful tools to assist in such a characterization. Here, both the Martian ozone vertical distribution and its diurnal variation for equatorial latitudes are studied, using the JPL/Caltech one-dimensional photochemical model and diurnally-variable atmospheric profiles. The chosen equatorial latitude-region is based on the recent and future plans of NASA and other agencies to study this region by different surface missions. A production and loss analysis is performed in order to characterize the chemical mechanisms that drive odd oxygen’s diurnal budget and variability on Mars making use of the comprehensive chemistry implemented in the model. The diurnal variation shows large differences in the abundance between daytime and nighttime; and variable behavior depending on the atmospheric layer. The photolysis-driven ozone diurnal profile is obtained at the surface, whilst a sharp decrease is obtained in the upper troposphere at daytime, which originates from the large differences in atomic oxygen abundances between atmospheric layers. Finally, no clear anticorrelation between ozone and water vapor is found in the diurnal cycle, contrary to the strong correlation observed by orbiters on a seasonal timescale.
The pathways through which incoming energy is distributed between the surface and atmosphere have been analyzed for the Earth. However, the effect of the spectral energy distribution of a host star on the energy budget of an orbiting planet may be significant given the wavelength-dependent absorption properties of atmospheric CO2, water vapor, surface ice, and snow. We have quantified the flow of energy on aqua planets orbiting M-, G-, and F-dwarf stars, using a 3D Global Climate Model with a static ocean. The atmosphere and surface of an M-dwarf planet receiving an instellation equal to 88% of the modern solar constant at the top of the atmosphere absorb 12% more incoming stellar radiation than those of a G-dwarf planet receiving 100% of the modern solar constant, and 17% more radiation than an F-dwarf planet receiving 108% of the modern solar constant, resulting in climates similar to that of modern-day Earth on all three planets, assuming a 24 hr rotation period and fixed CO2. At 100% instellation, a synchronously rotating M-dwarf planet exhibits smaller flux absorption in the atmosphere and on the surface of the dayside, and a dayside mean surface temperature that is 37 K colder than its rapidly rotating counterpart. Energy budget diagrams are included to illustrate the variations in global energy budgets as a function of host star spectral class, and can contribute to habitability assessments of planets as they are discovered.
The structure and function of microbial communities inhabiting the subseafloor near hydrothermal systems are influenced by fluid geochemistry, geologic setting and fluid flux between vent sites, as well as biological interactions. Here, we used genome?resolved metagenomics and metatranscriptomics to examine patterns of gene abundance and expression and assess potential niche differentiation in microbial communities in venting fluids from hydrothermal vent sites at the Mid?Cayman Rise. We observed similar patterns in gene and transcript abundance between two geochemically distinct vent fields at the community level but found that each vent site harbours a distinct microbial community with differing transcript abundances for individual microbial populations. Through an analysis of metabolic pathways in 64 metagenome?assembled genomes (MAGs), we show that MAG transcript abundance can be tied to differences in metabolic pathways and to potential metabolic interactions between microbial populations, allowing for niche?partitioning and divergence in both population distribution and activity. Our results illustrate that most microbial populations have a restricted distribution within the seafloor, and that the activity of those microbial populations is tied to both genome content and abiotic factors.
To date a dozen transiting Tatooines or circumbinary planets (CBPs) have been discovered, by eye, in the data from the Kepler mission; by contrast, thousands of confirmed circumstellar planets orbiting around single stars have been detected using automated algorithms. Automated detection of CBPs is challenging because their transits are strongly aperiodic with irregular profiles. Here, we describe an efficient and automated technique for detecting circumbinary planets that transit their binary hosts in Kepler light curves. Our method accounts for large transit timing variations (TTVs) and transit duration variations (TDVs), induced by binary reflex motion, in two ways: (1) We directly correct for large-scale TTVs and TDVs in the light curves by using Keplerian models to approximate binary and CBP orbits; and (2) We allow additional aperiodicities on the corrected light curves by employing the Quasi-periodic Automated Transit Search algorithm. We demonstrate that our method dramatically improves detection significance using simulated data and two previously identified CBP systems, Kepler-35 and Kepler-64.