Context. Plate tectonics is considered a fundamental component for the habitability of the Earth. Yet whether it is a recurrent feature of terrestrial bodies orbiting other stars or unique to the Earth is unknown. The stagnant lid may rather be the most common tectonic expression on such bodies. Aims. To understand whether a stagnant-lid planet can be habitable, i.e. host liquid water at its surface, we model the thermal evolution of the mantle, volcanic outgassing of H 2 O and CO 2 , and resulting climate of an Earth-like planet lacking plate tectonics. Methods. We used a 1D model of parameterized convection to simulate the evolution of melt generation and the build-up of an atmosphere of H 2 O and CO 2 over 4.5 Gyr. We then employed a 1D radiative-convective atmosphere model to calculate the global mean atmospheric temperature and the boundaries of the habitable zone (HZ).Results. The evolution of the interior is characterized by the initial production of a large amount of partial melt accompanied by a rapid outgassing of H 2 O and CO 2 . The maximal partial pressure of H 2 O is limited to a few tens of bars by the high solubility of water in basaltic melts. The low solubility of CO 2 instead causes most of the carbon to be outgassed, with partial pressures that vary from 1 bar or less if reducing conditions are assumed for the mantle to 100-200 bar for oxidizing conditions. At 1 au, the obtained temperatures generally allow for liquid water on the surface nearly over the entire evolution. While the outer edge of the HZ is mostly influenced by the amount of outgassed CO 2 , the inner edge presents a more complex behaviour that is dependent on the partial pressures of both gases. Conclusions. At 1 au, the stagnant-lid planet considered would be regarded as habitable. The width of the HZ at the end of the evolution, albeit influenced by the amount of outgassed CO 2 , can vary in a non-monotonic way depending on the extent of the outgassed H 2 O reservoir. Our results suggest that stagnant-lid planets can be habitable over geological timescales and that joint modelling of interior evolution, volcanic outgassing, and accompanying climate is necessary to robustly characterize planetary habitability.

Understanding whether M dwarf stars may host habitable planets with Earth-like atmospheres and biospheres is a major goal in exoplanet research. If such planets exist, the question remains as to whether they could be identified via spectral signatures of biomarkers. Such planets may be exposed to extreme intensities of cosmic rays that could perturb their atmospheric photochemistry. Here, we consider stellar activity of M dwarfs ranging from quiet up to strong flaring conditions and investigate one particular effect upon biomarkers, namely, the ability of secondary electrons caused by stellar cosmic rays to break up atmospheric molecular nitrogen (N 2 ), which leads to production of nitrogen oxides (NO x ) in the planetary atmosphere, hence affecting biomarkers such as ozone (O 3 ). We apply a stationary model, that is, without a time dependence; hence we are calculating the limiting case where the atmospheric chemistry response time of the biomarkers is assumed to be slow and remains constant compared with rapid forcing by the impinging stellar flares. This point should be further explored in future work with time-dependent models. We estimate the NO x production using an air shower approach and evaluate the implications using a climate-chemical model of the planetary atmosphere. O 3 formation proceeds via the reaction O + O 2 + M/O 3 + M. At high NO x abundances, the O atoms arise mainly from NO 2 photolysis, whereas on Earth this occurs via the photolysis of molecular oxygen (O 2 ). For the flaring case, O 3 is mainly destroyed via direct titration, NO + O 3 /NO 2 + O 2 , and not via the familiar catalytic cycle photochemistry, which occurs on Earth. For scenarios with low O 3 , Rayleigh scattering by the main atmospheric gases (O 2 , N 2 , and CO 2 ) became more important for shielding the planetary surface from UV radiation. A major result of this work is that the biomarker O 3 survived all the stellar-activity scenarios considered except for the strong case, whereas the biomarker nitrous oxide (N 2 O) could survive in the planetary atmosphere under all conditions of stellar activity considered here, which clearly has important implications for missions that aim to detect spectroscopic biomarkers.

No abstract

Despite a fainter Sun, the surface of the early Earth was mostly ice-free. Proposed solutions to this so-called "faint young Sun problem" have usually involved higher amounts of greenhouse gases than present in the modern-day atmosphere.However, geological evidence seemed to indicate that the atmospheric CO 2 concentrations during the Archaean and Proterozoic were far too low to keep the surface from freezing. With a radiative-convective model including new, updated thermal absorption coefficients, we found that the amount of CO 2 necessary to obtain 273 K at the surface is reduced up to an order of magnitude compared to previous studies.For the late Archaean and early Proterozoic period of the Earth, we calculate that CO 2 partial pressures of only about 2.9 mb are required to keep its surface from freezing which is compatible with the amount inferred from sediment studies. This conclusion was not significantly changed when we varied model parameters such as relative humidity or surface albedo, obtaining CO 2 partial pressures for the late Archaean between 1.5 and 5.5 mb. Thus, the contradiction between sediment data and model results disappears for the late Archaean and early Proterozoic.

Aims. The effects of multi-layered clouds in the atmospheres of Earth-like planets orbiting different types of stars are studied. The radiative effects of cloud particles are directly correlated with their wavelength-dependent optical properties. Therefore the incident stellar spectra may play an important role for the climatic effect of clouds. We discuss the influence of clouds with mean properties measured in the Earth's atmosphere on the surface temperatures and Bond albedos of Earth-like planets orbiting different types of main sequence dwarf stars. The influence of clouds on the position of the habitable zone around these central star types is discussed. Methods. A parametric cloud model has been developed based on observations in the Earth's atmosphere. The corresponding optical properties of the cloud particles are calculated with the Mie theory accounting for shape effects of ice particles by the equivalent sphere method. The parametric cloud model is linked with a one-dimensional radiative-convective climate model to study the effect of clouds on the surface temperature and the Bond albedo of Earth-like planets in dependence of the type of central star.Results. The albedo effect of the low-level clouds depends only weakly on the incident stellar spectra because the optical properties remain almost constant in the wavelength range of the maximum of the incident stellar radiation. The greenhouse effect of the high-level clouds on the other hand depends on the temperature of the lower atmosphere, which is itself an indirect consequence of the different types of central stars. In general the planetary Bond albedo increases with the cloud cover of either cloud type. An anomaly was found for the K and M-type stars however, resulting in a decreasing Bond albedo with increasing cloud cover for certain atmospheric conditions. Depending on the cloud properties, the position of the habitable zone can be located either farther from or closer to the central star. As a rule, low-level water clouds lead to a decrease of distance because of their albedo effect, while the high-level ice clouds lead to an increase in distance. The maximum variations are about 15% decrease and 35% increase in distance compared to the clear sky case for the same mean Earth surface conditions in each case.

Planets orbiting in the habitable zone (HZ) of MDwarf stars are subject to high levels of galactic cosmic rays (GCRs) which produce nitrogen oxides in earthlike atmospheres. We investigate to what extent this NO x may modify biomarker compounds such as ozone (O 3 ) and nitrous oxide (N 2 O), as well as related compounds such as water (H 2 O) (essential for life) and methane (CH 4 ) (which has both abiotic and biotic sources) . Our model results suggest that such signals are robust, changing in the Mstar world atmospheric column by up to 20% due to the GCR NO x effects compared to an Mstar run without GCR effects and can therefore survive at least the effects of galactic cosmic rays. We have not however investigated stellar cosmic rays here. CH 4 levels are about 10 times higher than on the Earth related to a lowering in hydroxyl (OH) in response to changes in UV. The increase is less than reported in previous studies. This difference arose partly because we used different biogenic input. For example, we employed 23% lower CH 4 fluxes compared to those studies. Unlike on the Earth, relatively modest changes in these fluxes can lead to larger changes in the concentrations of biomarker and related species on the Mstar world.We calculate a CH 4 greenhouse heating effect of up to 4K. O 3 photochemistry in terms of the smog mechanism and the catalytic loss cycles on the Mstar world differs considerably compared with the Earth.

The potential habitability of a terrestrial planet is usually defined by the possible existence of liquid water on its surface, since life as we know it needs liquid water at least during a part of its life cycle. The potential presence of liquid water on a planetary surface depends on many factors such as, most importantly, surface temperatures. The properties of the planetary atmosphere and its interaction with the radiative energy provided by the planet's host star are thereby of decisive importance. In this study we investigate the influence of different main-sequence stars (F, G, and K-type stars) upon the climate of Earth-like extrasolar planets and their potential habitability by applying a state-of-the-art three-dimensional (3D) Earth climate model accounting for local and dynamical processes. The calculations have been performed for planets with Earth-like atmospheres at orbital distances (and corresponding orbital periods) where the total amount of energy received from the various host stars equals the solar constant. In contrast to previous 3D modeling studies, we include the effect of ozone radiative heating upon the vertical temperature structure of the atmospheres. The global orbital mean results obtained have been compared to those of a one-dimensional (1D) radiative convective climate model to investigate the approximation of global mean 3D results by those of 1D models. The different stellar spectral energy distributions lead to different surface temperatures and due to ozone heating to very different vertical temperature structures. As previous 1D studies we find higher surface temperatures for the Earth-like planet around the K-type star, and lower temperatures for the planet around the F-type star compared to an Earth-like planet around the Sun. However, this effect is more pronounced in the 3D model results than in the 1D model because the 3D model accounts for feedback processes such as the ice-albedo and the water vapor feedback. Whether the 1D model may approximate the global mean of the 3D model results strongly depends on the choice of the relative humidity profile in the 1D model, which is used to determine the water vapor profile. Hence, possible changes in the hydrological cycle need to be accounted for when estimating the potential habitability of an extrasolar planet.

Aims. The planetary system around the M star Gliese 581 contains at least three close-in potentially low-mass planets, Gl 581c, d, and e. In order to address the question of the habitability of Gl 581d, we performed detailed atmospheric modeling studies for several planetary scenarios. Methods. A 1D radiative-convective model was used to calculate temperature and pressure profiles of model atmospheres, which we assumed to be composed of molecular nitrogen, water, and carbon dioxide. The model allows for changing surface pressures caused by evaporation/condensation of water and carbon dioxide. Furthermore, the treatment of the energy transport has been improved in the model to account in particular for high CO 2 , high-pressure Super-Earth conditions. Results. For four high-pressure scenarios of our study, the resulting surface temperatures were above 273 K, indicating a potential habitability of the planet. These scenarios include three CO 2 -dominated atmospheres (95% CO 2 concentration with 5, 10, and 20 bar surface pressure) and a high-pressure CO 2 -enriched atmosphere (5% CO 2 concentration with 20 bar surface pressure). For all other considered scenarios, the calculated Gl 581d surface temperatures were below the freezing point of water, suggesting that Gl 581d would not be habitable then. The results for our CO 2 -dominated scenarios confirm very recent model results by Wordsworth et al. (2010). However, our model calculations imply that also atmospheres that are not CO 2 -dominated (i.e., 5% vmr instead of 95% vmr) could result in habitable conditions for Gl 581d.