FastChem Cond: Equilibrium chemistry with condensation and rainout for cool planetary and stellar environments

Cool astrophysical objects, such as (exo)planets, brown dwarfs, or asymptotic giant branch stars, can be strongly affected by condensation. Condensation does not only directly affect the chemical composition of the gas phase by removing elements but the condensed material also influences other chemical and physical processes in these object. This includes, for example, the formation of clouds in planetary atmospheres and brown dwarfs or the dust-driven winds of evolved stars. In this study we introduce FastChem Cond, a new version of the FastChem equilibrium chemistry code that adds a treatment of equilibrium condensation. Determining the equilibrium composition under the impact of condensation is complicated by the fact that the number of condensates that can exist in equilibrium with the gas phase is limited by a phase rule. However, this phase rule does not directly provide information on which condensates are stable. As a major advantage of FastChem Cond is able to automatically select the set stable condensates satisfying the phase rule. Besides the normal equilibrium condensation, FastChem Cond can also be used with the rainout approximation that is commonly employed in atmospheres of brown dwarfs or (exo)planets. FastChem Cond is available as open-source code, released under the GPLv3 licence. In addition to the C++ code, FastChem Cond also offers a Python interface. Together with the code update we also add about 290 liquid and solid condensate species to FastChem.Comment: submitted to MNRAS, code available at https://github.com/exoclime/FastChe

Biomarker Response to Galactic Cosmic Ray-Induced NOx and the Methane Greenhouse Effect in the Atmosphere of an Earthlike Planet Orbiting an M-Dwarf Star

Planets orbiting in the habitable zone (HZ) of M-Dwarf stars are subject to high levels of galactic cosmic rays (GCRs) which produce nitrogen oxides in earthlike atmospheres. We investigate to what extent this NOx may modify biomarker compounds such as ozone (O3) and nitrous oxide (N2O), as well as related compounds such as water (H2O) (essential for life) and methane (CH4) (which has both abiotic and biotic sources) . Our model results suggest that such signals are robust, changing in the M-star world atmospheric column by up to 20% due to the GCR NOx effects compared to an M-star 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. CH4 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 CH4 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 M-star world. We calculate a CH4 greenhouse heating effect of up to 4K. O3 photochemistry in terms of the smog mechanism and the catalytic loss cycles on the M-star world differs considerably compared with the Earth

Warming the early Earth - CO2 reconsidered

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 CO2 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 CO2 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 CO2 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 CO2 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.Comment: 53 pages, 4 tables, 11 figures, published in Planetary and Space Scienc

Infrared radiative transfer in atmospheres of Earth-like planets around F, G, K, and M stars. II. Thermal emission spectra influenced by clouds

Context: Clouds play an important role in the radiative transfer of planetary atmospheres because of the influence they have on the different molecular signatures through scattering and absorption processes. Furthermore, they are important modulators of the radiative energy budget affecting surface and atmospheric temperatures. Aims. We present a detailed study of the thermal emission of cloud-covered planets orbiting F-, G-, K-, and M-type stars. These Earth-like planets include planets with the same gravity and total irradiation as Earth, but can differ significantly in the upper atmosphere. The impact of single-layered clouds is analyzed to determine what information on the atmosphere may be lost or gained. The planetary spectra are studied at different instrument resolutions and compared to previously calculated low-resolution spectra. Methods. A line-by-line molecular absorption model coupled with a multiple scattering radiative transfer solver was used to calculate the spectra of cloud-covered planets. The atmospheric profiles used in the radiation calculations were obtained with a radiative-convective climate model combined with a parametric cloud description. Results. In the high-resolution flux spectra, clouds changed the intensities and shapes of the bands of CO2, N2O, H2O, CH4, and O3. Some of these bands turned out to be highly reduced by the presence of clouds, which causes difficulties for their detection. The most affected spectral bands resulted for the planet orbiting the F-type star. Clouds could lead to false negative interpretations for the different molecular species investigated. However, at low resolution, clouds were found to be crucial for detecting some of the molecular bands that could not be distinguished in the cloud-free atmospheres. The CO2 bands were found to be less affected by clouds. Radiation sources were visualized with weighting functions at high resolution. Conclusions. Knowledge of the atmospheric temperature profile is essential for estimating the composition and important for avoiding false negative detection of biomarkers, in both cloudy and clear-sky conditions. In particular, a pronounced temperature contrast between the ozone layer and surface or cloud is needed to detect the molecule. Fortunately, the CO2 bands allow temperature estimation from the upper stratosphere down to the troposphere even in the presence of clouds

FastChem 2 : an improved computer program to determine the gas-phase chemical equilibrium composition for arbitrary element distributions

The computation of complex neutral/ionized chemical equilibrium compositions is invaluable to obtain scientific insights of, for example, the atmospheres of extrasolar planets and cool stars. We present FASTCHEM 2 , a new version of the established semi-analytical thermochemical equilibrium code FASTCHEM. Whereas the original version is limited to atmospheres containing a significant amount of hydrogen, FASTCHEM 2 is also applicable to chemical mixtures dominated by any other species, such as CO2 or N2. The new C++ code and an optional PYTHON module are publicly available under the GPLv3 license. The program is backward compatible so that the previous version can be easily substituted. We updated the thermochemical data base by adding HNC, FeH, TiH, Ca−, and some organic molecules. In total 523 species are now in the thermochemical data base including 28 chemical elements. The user can reduce the total number of species to, for example, increase the computation performance or can add further species if the thermochemical data are available. The program is validated against its previous version and extensively tested over an extended pressure–temperature grid with pressures ranging from 10−13 up to 103bar and temperatures between 100 and 6000K⁠. FASTCHEM 2 is successfully applied to a number of different scenarios including nitrogen-, carbon-, and oxygen-dominated atmospheres and test cases without hydrogen and helium. Averaged over the extended pressure–temperature grid FASTCHEM 2 is up to 50 times faster than the previous version and is also applicable to situations not treatable with version 1

Factors affecting surface air composition on earthlike extrasolar planets

The motivation of this study is to investigate changes in surface air pollutants across the Habitable Zone (HZ) hence estimate a more refined definition of the HZ in terms of a “Breathable Air Zone” (BAZ). This implies life as on Earth. We have performed sensitivity experiments using a tropospheric photochemical model in which the flux was increased to a Venus orbit (inner HZ run) and decreased to a Mars orbit (outer HZ run) relative to a pre-industrial Earth atmosphere control run. We then repeated the inner HZ run but with a prescribed temperature increase of 50K, which we denote as the warm inner HZ run. The original control run was also repeated but with modified stellar radiation corresponding to K2V and an F2V stars at 0.53 AU and 1.69 AU respectively [Segura et al., 2003]. The model calculated among other chemical species surface ozone (O 3) - (an important pollutant) and hydroxyl (an important pollutant remover) concentrations. An increase in solar flux stimulated O3 production via the established photochemical "smog" mechanism [Haagen-Smit et al., 1952] and also stimulated OH production via enhanced water photolysis. O3 increased by a factor 6 in going from the outer to the inner HZ. Absolute levels depended sensitively upon methane (and other volatile organics), NOx and UV, required by the smog mechanism. Levels of O3 toxic to humans (i.e. a few hundred of ppbv, [Horstman et al., 1990]) were not reached for the runs shown here but would be approached if surface methane, NOx and UV increase by a factor of 50 to 100, maybe attainable earlier in Earth's history

Potential of Ozone Formation by the Smog Mechanism to shield the surface of the Early Earth from UV Radiation

We propose that the photochemical smog mechanism produced substantial ozone (O3) in the troposphere during the Proterozoic period, which contributed to ultraviolet (UV) radiation shielding, and hence favoured the establishment of life. The smog mechanism is well established and is associated with pollution hazes that sometimes cover modern cities. The mechanism proceeds via the oxidation of volatile organic compounds such as methane (CH4) in the presence of UV radiation and nitrogen oxides (NOx). It would have been particularly favoured during the Proterozoic period given the high levels of CH4 (up to 1000 ppm) recently suggested. Proterozoic UV levels on the surface of the Earth were generally higher compared with today, which would also have favoured the mechanism. On the other hand, Proterozoic O2 required in the final step of the smog mechanism to form O3 was less abundant compared with present times. Furthermore, results are sensitive to Proterozoic NOx concentrations, which are challenging to predict, since they depend on uncertain quantities such as NOx source emissions and OH concentrations. We review NOx sources during the Proterozoic period and apply a photochemical box model having methane oxidation with NOx, HOx and Ox chemistry to estimate the O3 production from the smog mechanism. Runs suggest the smog mechanism during the Proterozoic period can produce approximately double the present-day ozone columns for NOx levels of 1.53×10-9 by volume mixing ratio, which was attainable according to our NOx source analysis, with 1% of the present atmospheric levels of O2. Clearly, forming ozone in the troposphere is a trade-off for survivability – on the one hand, harmful UV radiation is blocked, but on the other hand ozone is a respiratory irratant, which becomes fatal at concentrations exceeding about 1 ppmv

Habitable Zones for Planets with Pre-Industrial Earthlike Biospheres orbiting Main Sequence Stars

We investigate the general case of a "natural" Earth without manmade influence. To do this we switch off anthropogenic sources in our model and investigate the biomarkers and Habitable Zone (HZ) limits for three main sequence stars. The HZ around a star is usually defined as the range of distances over which liquid water can exist on the surface of an orbiting planet. This assumes to be a requirement for the formation of life as we know it. In this work we define the HZ more conservatively to be suitable for complex life with a surface temperature range from 0°C to 30°C. With these constraints we determine the width of the HZ around main sequence stars (Sun, F2V and K2V) in which an earthlike biosphere would exist, i.e. we do not consider here terrestrial planets with enhanced CO2 atmospheres. We neglect anthropogenic influences and take a pre-industrial composition (1850) of the Earth atmosphere as starting conditions for our model. We have used a coupled radiative-convective photochemical column model to investigate the effect on atmospheric biomarkers across the HZ