Possible climates on terrestrial exoplanets Forget, F.; Leconte, J.
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
04/2014, Letnik:
372, Številka:
2014
Journal Article
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What kind of environment may exist on terrestrial planets around other stars? In spite of the lack of direct observations, it may not be premature to speculate on exoplanetary climates, for instance, ...to optimize future telescopic observations or to assess the probability of habitable worlds. To begin with, climate primarily depends on (i) the atmospheric composition and the volatile inventory; (ii) the incident stellar flux; and (iii) the tidal evolution of the planetary spin, which can notably lock a planet with a permanent night side. The atmospheric composition and mass depends on complex processes, which are difficult to model: origins of volatiles, atmospheric escape, geochemistry, photochemistry, etc. We discuss physical constraints, which can help us to speculate on the possible type of atmosphere, depending on the planet size, its final distance for its star and the star type. Assuming that the atmosphere is known, the possible climates can be explored using global climate models analogous to the ones developed to simulate the Earth as well as the other telluric atmospheres in the solar system. Our experience with Mars, Titan and Venus suggests that realistic climate simulators can be developed by combining components, such as a dynamical core, a radiative transfer solver, a parametrization of subgrid-scale turbulence and convection, a thermal ground model and a volatile phase change code. On this basis, we can aspire to build reliable climate predictors for exoplanets. However, whatever the accuracy of the models, predicting the actual climate regime on a specific planet will remain challenging because climate systems are affected by strong positive feedbacks. They can drive planets with very similar forcing and volatile inventory to completely different states. For instance, the coupling among temperature, volatile phase changes and radiative properties results in instabilities, such as runaway glaciations and runaway greenhouse effect.
Collision-induced absorption is of great importance to the overall radiative budget in dense CO2-rich atmospheres, but its representation in climate models remains uncertain, mainly due to a lack of ...accurate experimental and theoretical data. Here we compare several parameterisations of the effect, including a new one that makes use of previously unused measurements in the 1200-1800cma1 spectral range. We find that a widely used parameterisation strongly overestimates absorption in pure CO2 atmospheres compared to later results, and propose a new approach that we believe is the most accurate possible given currently available data.
The long-standing debate on the existence of ancient oceans on Mars has been recently revived by evidence for tsunami resurfacing events that date from the Late Hesperian geological era. It has been ...argued that these tsunami events originated from the impact of large meteorites on a deglaciated or nearly deglaciated ocean present in the northern hemisphere of Mars. Here we show that the presence of such a late ocean faces a paradox. If cold, the ocean should have been entirely frozen shortly after its formation, thus preventing the formation of tsunami events. If warm, the ice-free ocean should have produced fluvial erosion of Hesperian Mars terrains much more extensively than previously reported. To solve this apparent paradox, we suggest a list of possible tests and scenarios that could help to reconcile constraints from climate models with tsunami hypothesis. These scenarios could be tested in future dedicated studies.
The inner edge of the classical habitable zone is often defined by the critical flux needed to trigger the runaway greenhouse instability. This 1D notion of a critical flux, however, may not be all ...that relevant for inhomogeneously irradiated planets, or when the water content is limited (land planets). Based on results from our 3D global climate model, we present general features of the climate and large-scale circulation on close-in terrestrial planets. We find that the circulation pattern can shift from super-rotation to stellar/anti stellar circulation when the equatorial Rossby deformation radius significantly exceeds the planetary radius, changing the redistribution properties of the atmosphere. Using analytical and numerical arguments, we also demonstrate the presence of systematic biases among mean surface temperatures and among temperature profiles predicted from either 1D or 3D simulations. After including a complete modeling of the water cycle, we further demonstrate that two stable climate regimes can exist for land planets closer than the inner edge of the classical habitable zone. One is the classical runaway state where all the water is vaporized, and the other is a collapsed state where water is captured in permanent cold traps. We identify this “moist” bistability as the result of a competition between the greenhouse effect of water vapor and its condensation on the night side or near the poles, highlighting the dynamical nature of the runaway greenhouse effect. We also present synthetic spectra showing the observable signature of these two states. Taking the example of two prototype planets in this regime, namely Gl 581 c and HD 85512 b, we argue that depending on the rate of water delivery and atmospheric escape during the life of these planets, they could accumulate a significant amount of water ice at their surface. If such a thick ice cap is present, various physical mechanisms observed on Earth (e.g., gravity driven ice flows, geothermal flux) should come into play to produce long-lived liquid water at the edge and/or bottom of the ice cap. Consequently, the habitability of planets at smaller orbital distance than the inner edge of the classical habitable zone cannot be ruled out. Transiting planets in this regime represent promising targets for upcoming exoplanet characterization observatories, such as EChO and JWST.
The vertical distribution of water vapor is key to the study of Mars' hydrological cycle. To date, it has been explored mainly through global climate models because of a lack of direct measurements. ...However, these models assume the absence of supersaturation in the atmosphere of Mars. Here, we report observations made using the SPICAM (Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars) instrument onboard Mars Express that provide evidence of the frequent presence of water vapor in excess of saturation, by an amount far surpassing that encountered in Earth's atmosphere. This result contradicts the widespread assumption that atmospheric water on Mars cannot exist in a supersaturated state, directly affecting our long-term representation of water transport, accumulation, escape, and chemistry on a global scale.
Water ice clouds play a key role in the radiative transfer of the Martian atmosphere, impacting its thermal structure, its circulation, and, in turn, the water cycle. Recent studies including the ...radiative effects of clouds in global climate models (GCMs) have found that the corresponding feedbacks amplify the model defaults. In particular, it prevents models with simple microphysics from reproducing even the basic characteristics of the water cycle. Within that context, we propose a new implementation of the water cycle in GCMs, including a detailed cloud microphysics taking into account nucleation on dust particles, ice particle growth, and scavenging of dust particles due to the condensation of ice. We implement these new methods in the Laboratoire de Météorologie Dynamique GCM and find satisfying agreement with the Thermal Emission Spectrometer observations of both water vapor and cloud opacities, with a significant improvement when compared to GCMs taking into account radiative effects of water ice clouds without this implementation. However, a lack of water vapor in the tropics after Ls = 180° is persistent in simulations compared to observations, as a consequence of aphelion cloud radiative effects strengthening the Hadley cell. Our improvements also allow us to explore questions raised by recent observations of the Martian atmosphere. Supersaturation above the hygropause is predicted in line with Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars observations. The model also suggests for the first time that the scavenging of dust by water ice clouds alone fails to fully account for the detached dust layers observed by the Mars Climate Sounder.
Key Points
Radiatively active clouds impact atmospheric water vapor and ice in a GCMCloud microphysics with dynamic nuclei is needed in GCMs
Radiatively active water ice clouds (RAC) play a key role in shaping the thermal structure of the Martian atmosphere. In this paper, RAC are implemented in the LMD Mars Global Climate Model (GCM) and ...the simulated temperatures are compared to Thermal Emission Spectrometer observations over a full year. RAC change the temperature gradients and global dynamics of the atmosphere and this change in dynamics in turn implies large‐scale adiabatic temperature changes. Therefore, clouds have both a direct and indirect effect on atmospheric temperatures. RAC successfully reduce major GCM temperature biases, especially in the regions of formation of the aphelion cloud belt where a cold bias of more than 10 K is corrected. Departures from the observations are however seen in the polar regions, and highlight the need for better modeling of cloud formation and evolution.
Key Points
Radiatively active clouds (RAC) are implemented in the LMD global climate model
Whatever the season, including RAC is required to fit the observed temperatures
Renewed attention on the polar regions, where cold biases remain, is needed
Most data dealing with the biopreservative activity of lactic acid bacteria (LAB) are focused on their antibacterial effects. Food spoilage by mould and the occurrence of their mycotoxins constitute ...a potential health hazard. Development of biological control should help improve the safety of products by controlling mycotoxin contamination. Data have actually shown that many LAB can inhibit mould growth and that some of them have the potential to interact with mycotoxins.
This review summarizes these findings and demonstrates that LAB are promising biological agents for food safety.
Hydrogen chloride was discovered in the atmosphere of Mars for the first time during the global dust storm in Mars year (MY) 34 (July 2018) using the Atmospheric Chemistry Suite mid-infrared channel ...(ACS MIR) on the ExoMars Trace Gas Orbiter. The simultaneity of variations in dust and HCl, and a correlation between water vapour and HCl, led to the proposal of a novel surface-atmosphere coupling analogous to terrestrial HCl production in the troposphere from salt aerosols. After seasonal dust activity restarted in MY 35 (August 2020), we have been monitoring HCl activity to determine whether such a coupling was validated. Here we present a new technique for analysing the absorption features of trace gases close to the ACS MIR noise level and report that HCl mixing ratios are observed to rapidly increase in both hemispheres coincidentally with the onset of the MY 35 perihelion dust season. We present the temporal evolution of the vertical distribution of HCl (0.1–6 ppbv) and of dust activity in both hemispheres. We also report two observations of >2 ppbv HCl below 10 km in the northern hemisphere during the aphelion period.
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