Oxygenic photosynthesis appears to have evolved well before O2 levels increased in the atmosphere, at around 2.4Ga. This has led to numerous suggestions as to what may have kept O2 suppressed and ...then eventually allowed it to rise. These suggestions include changes in the recycling of carbon and sulfur relative to water (or hydrogen), a switch from dominantly submarine to dominantly subaerial volcanism, gradual oxidation of the continents and a concomitant decrease in reduced metamorphic gases, a decline in deposition of banded iron-formations, a decline in nickel availability, and various proposals to increase the efficiency of photosynthesis. Several of these different mechanisms could have contributed to the rise of O2, although not all of them are equally effective. To be considered successful, any proposed mechanism must make predictions that are consistent with the carbon isotope record in marine carbonates, which shows relatively little change with time, apart from transient (but occasionally spectacular) excursions. The reasons for this constancy are explored here, but are not fully resolved. In the process of making these comparisons, a self-consistent redox balance framework is developed which will hopefully prove useful to others who may work on this problem and to astronomers who may one day try to decipher spectral signatures of oxygen on Earth-like exoplanets.
•A self-consistent redox budget for the atmosphere–ocean system is presented.•Four different hypotheses for the rise of atmospheric O2 are analyzed.•The evolution of cyanobacteria was, of course, important.•Continued continental growth was also important.•Changes in crustal composition, from more to less mafic, were important as well.
The ongoing discoveries of extra-solar planets are unveiling a wide range of terrestrial mass (size) planets around their host stars. In this Letter, we present estimates of habitable zones (HZs) ...around stars with stellar effective temperatures in the range 2600 K-7200 K, for planetary masses between 0.1 M sub(+ in circle) and 5 M sub(+ in circle). Assuming H sub(2)O-(inner HZ) and CO sub(2)-(outer HZ) dominated atmospheres, and scaling the background N sub(2) atmospheric pressure with the radius of the planet, our results indicate that larger planets have wider HZs than do smaller ones. Specifically, with the assumption that smaller planets will have less dense atmospheres, the inner edge of the HZ (runaway greenhouse limit) moves outward (~10% lower than Earth flux) for low mass planets due to larger greenhouse effect arising from the increased H sub(2)O column depth. For larger planets, the H sub(2)O column depth is smaller, and higher temperatures are needed before water vapor completely dominates the outgoing longwave radiation. Hence the inner edge moves inward (~7% higher than Earth's flux). The outer HZ changes little due to the competing effects of the greenhouse effect and an increase in albedo. New, three-dimensional climate model results from other groups are also summarized, and we argue that further, independent studies are needed to verify their predictions. Combined with our previous work, the results presented here provide refined estimates of HZs around main-sequence stars and provide a step toward a more comprehensive analysis of HZs.
ABSTRACT Terrestrial planets at the inner edge of the habitable zone (HZ) of late-K and M-dwarf stars are expected to be in synchronous rotation, as a consequence of strong tidal interactions with ...their host stars. Previous global climate model (GCM) studies have shown that, for slowly rotating planets, strong convection at the substellar point can create optically thick water clouds, increasing the planetary albedo, and thus stabilizing the climate against a thermal runaway. However these studies did not use self-consistent orbital/rotational periods for synchronously rotating planets placed at different distances from the host star. Here we provide new estimates of the inner edge of the HZ for synchronously rotating terrestrial planets around late-K and M-dwarf stars using a 3D Earth-analog GCM with self-consistent relationships between stellar metallicity, stellar effective temperature, and the planetary orbital/rotational period. We find that both atmospheric dynamics and the efficacy of the substellar cloud deck are sensitive to the precise rotation rate of the planet. Around mid-to-late M-dwarf stars with low metallicity, planetary rotation rates at the inner edge of the HZ become faster, and the inner edge of the HZ is farther away from the host stars than in previous GCM studies. For an Earth-sized planet, the dynamical regime of the substellar clouds begins to transition as the rotation rate approaches ∼10 days. These faster rotation rates produce stronger zonal winds that encircle the planet and smear the substellar clouds around it, lowering the planetary albedo, and causing the onset of the water-vapor greenhouse climatic instability to occur at up to ∼25% lower incident stellar fluxes than found in previous GCM studies. For mid-to-late M-dwarf stars with high metallicity and for mid-K to early-M stars, we agree with previous studies.
Several lines of evidence indicate that the advent of oxygenic photosynthesis preceded the oxygenation of the atmosphere—perhaps by as much as 300million years. The fate of biogenic O2 prior to its ...appearance in the atmosphere remains speculative, but recent work suggests that O2 was locally available within the surface ocean to support aerobic microbial ecosystems. Simple mass balance predicts that locally oxygenated environments (oxygen oases) could exist in areas of high productivity if the local rate of O2 production by oxygenic photosynthesis exceeded the combined rate of O2 loss by a number of processes (e.g., exchange with the atmosphere, transport within the ocean, reaction with reduced aqueous species, biological consumption). The areal extent of these environments and the dissolved O2 concentrations that could have persisted in an otherwise anoxic ocean, however, are key uncertainties in our understanding of the spatiotemporal redox-evolution of the early earth system.
We use an earth system model of intermediate complexity that has been modified to simulate a theoretical Archean biosphere in order to explore redox heterogeneity in the late Archean surface ocean. We demonstrate that oxygen oases are an expected consequence of oxygenic photosynthesis beneath an essentially O2-devoid atmosphere—and that oxygenated surface waters need not be restricted to shallow coastal environments or microbial mats. Within oxygen oases, O2 concentrations locally approach ~1–10μM for a large range of plausible Archean conditions. Although O2 concentrations in the open ocean are exceedingly low, biologically relevant dissolved O2 concentrations are widespread in our hypothetical surface ocean.
•Oxygenic photosynthesis generates oxygen oases under late Archean conditions.•Oxygen oases are large-scale, steady state features of the pelagic surface ocean.•Up to ~1–10μM O2 is possible locally in an otherwise essentially anoxic world.•Aerobic metabolism could have been widespread in a broadly anoxic Archean ocean.
LIMIT CYCLES CAN REDUCE THE WIDTH OF THE HABITABLE ZONE Haqq-Misra, Jacob; Kopparapu, Ravi Kumar; Batalha, Natasha E. ...
Astrophysical journal/The Astrophysical journal,
08/2016, Letnik:
827, Številka:
2
Journal Article
Recenzirano
Odprti dostop
ABSTRACT The liquid water habitable zone (HZ) describes the orbital distance at which a terrestrial planet can maintain above-freezing conditions through regulation by the carbonate-silicate cycle. ...Recent calculations have suggested that planets in the outer regions of the HZ cannot maintain stable, warm climates, but rather should oscillate between long, globally glaciated states and shorter periods of climatic warmth. Such conditions, similar to "Snowball Earth" episodes experienced on Earth, would be inimical to the development of complex land life, including intelligent life. Here, we build on previous studies with an updated energy balance climate model to calculate this "limit cycle" region of the HZ where such cycling would occur. We argue that an abiotic Earth would have a greater CO2 partial pressure than today because plants and other biota help to enhance the storage of CO2 in soil. When we tune our abiotic model accordingly, we find that limit cycles can occur but that previous calculations have overestimated their importance. For G stars like the Sun, limit cycles occur only for planets with CO2 outgassing rates less than that on modern Earth. For K- and M-star planets, limit cycles should not occur; however, M-star planets may be inhospitable to life for other reasons. Planets orbiting late G-type and early K-type stars retain the greatest potential for maintaining warm, stable conditions. Our results suggest that host star type, planetary volcanic activity, and seafloor weathering are all important factors in determining whether planets will be prone to limit cycling.
The habitable zone (HZ) around a star is typically defined as the region where a rocky planet can maintain liquid water on its surface. That definition is appropriate, because this allows for the ...possibility that carbon-based, photosynthetic life exists on the planet in sufficient abundance to modify the planet’s atmosphere in a way that might be remotely detected. Exactly what conditions are needed, however, to maintain liquid water remains a topic for debate. In the past, modelers have restricted themselves to water-rich planets with CO ₂ and H ₂O as the only important greenhouse gases. More recently, some researchers have suggested broadening the definition to include arid, “Dune” planets on the inner edge and planets with captured H ₂ atmospheres on the outer edge, thereby greatly increasing the HZ width. Such planets could exist, but we demonstrate that an inner edge limit of 0.59 AU or less is physically unrealistic. We further argue that conservative HZ definitions should be used for designing future space-based telescopes, but that optimistic definitions may be useful in interpreting the data from such missions. In terms of effective solar flux, S ₑff, the recently recalculated HZ boundaries are: recent Venus—1.78; runaway greenhouse—1.04; moist greenhouse—1.01; maximum greenhouse—0.35; and early Mars—0.32. Based on a combination of different HZ definitions, the frequency of potentially Earth-like planets around late K and M stars observed by Kepler is in the range of 0.4–0.5.
Identifying terrestrial planets in the habitable zones (HZs) of other stars is one of the primary goals of ongoing radial velocity (RV) and transit exoplanet surveys and proposed future space ...missions. Most current estimates of the boundaries of the HZ are based on one-dimensional (1D), cloud-free, climate model calculations by Kasting et al. The inner edge of the HZ in the Kasting et al. model was determined by loss of water, and the outer edge was determined by the maximum greenhouse provided by a COsub 2 atmosphere. A conservative estimate for the width of the HZ from this model in our solar system is 0.95-1.67 AU. To assess the potential habitability of extrasolar terrestrial planets, we propose using stellar flux incident on a planet rather than equilibrium temperature. Our model does not include the radiative effects of clouds; thus, the actual HZ boundaries may extend further in both directions than the estimates just given.
ABSTRACT The NASA Kepler mission ha s discovered thousands of new planetary candidates, many of which have been confirmed through follow-up observations. A primary goal of the mission is to determine ...the occurrence rate of terrestrial-size planets within the Habitable Zone (HZ) of their host stars. Here we provide a list of HZ exoplanet candidates from the Kepler Q1-Q17 Data Release 24 data-vetting process. This work was undertaken as part of the Kepler HZ Working Group. We use a variety of criteria regarding HZ boundaries and planetary sizes to produce complete lists of HZ candidates, including a catalog of 104 candidates within the optimistic HZ and 20 candidates with radii less than two Earth radii within the conservative HZ. We cross-match our HZ candidates with the stellar properties and confirmed planet properties from Data Release 25 to provide robust stellar parameters and candidate dispositions. We also include false-positive probabilities recently calculated by Morton et al. for each of the candidates within our catalogs to aid in their validation. Finally, we performed dynamical analysis simulations for multi-planet systems that contain candidates with radii less than two Earth radii as a step toward validation of those systems.
The amazing science behind the search for Earth-like planets Ever since Carl Sagan first predicted that extraterrestrial civilizations must number in the millions, the search for life on other ...planets has gripped our imagination. Is Earth so rare that advanced life forms like us—or even the simplest biological organisms—are unique to the universe? How to Find a Habitable Planet describes how scientists are testing Sagan's prediction, and demonstrates why Earth may not be so rare after all.James Kasting has worked closely with NASA in its mission to detect habitable worlds outside our solar system, and in this book he introduces readers to the advanced methodologies being used in this extraordinary quest. He addresses the compelling questions that planetary scientists grapple with today: What exactly makes a planet habitable? What are the signatures of life astronomers should look for when they scan the heavens for habitable worlds? In providing answers, Kasting explains why Earth has remained habitable despite a substantial rise in solar luminosity over time, and why our neighbors, Venus and Mars, haven't. If other Earth-sized planets endowed with enough water and carbon are out there, he argues, chances are good that some of those planets sustain life. Kasting describes the efforts under way to find them, and predicts that future discoveries will profoundly alter our view of the universe and our place in it.This book is a must-read for anyone who has ever dreamed of finding other planets like ours—and perhaps even life like ours—in the cosmos. In a new afterword, Kasting presents some recent breakthroughs in the search for exoplanets and discusses the challenges facing space programs in the near future.
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