J L Grenfell

Potential biosignatures in super-Earth atmospheres - I. Spectral appearance of super-Earths around M dwarfs

Atmospheric temperature and mixing ratio profiles of terres trial planets vary with the spectral energy flux distribution for di fferent types of M-dwarf stars and the planetary gravity. We investigate the resulting effects on the spectral appearance of molecular absorption bands, which are relevant as indicators for potential planetary habitability during primary and secondary eclipse for transiting terrestrial planets with Earth-like biomass emissi ons. Atmospheric profiles are computed using a plane-parallel, 1D climate model coupled with a chemistry model. We then calculate simulated spectra using a line-by-line radiative transfer model. We find that emission spectra during secondary eclipse show i ncreasing absorption of methane, water, and ozone for planets orbiting quiet M0-M3 dwarfs and the active M-type star AD Leo compared with solar-type central stars. However, for planets orbiting very cool and quiet M dwarfs (M4 to M7), increasing temperatures in the mid-atmosphere lead to reduced absorption signals, which impedes the detection of molecules in these scenarios. Transmission spectra during primary eclipse show strong absorption features of CH4, N2O and H2O for planets orbiting quiet M0-M7 stars and AD Leo. The N2O absorption of an Earth-sized planet orbiting a quiet M7 star can even be as strong as the CO2 signal. However, ozone absorption decreases for planets orbiting these cool central stars owing to chemical effects in the atmosphere. To investigate the effect on the spectroscopic detection of absorption bands with potential future satellite missions, we compute signal-to-noise-ratios (SNR) for a James Webb Space Telescope (JWST)-like aperture telescope.

Clouds in the atmospheres of extrasolar planets. I. Climatic effects of multi-layered clouds for Earth-like planets and implications for habitable zones

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 Earths atmosphere on the surface temperatures and Bond albedos of Earth-like planets orbiting different types of main sequence dwarf stars.

Spectral features of Earth-like planets and their detectability at different orbital distances around F, G, and K-type stars

Context. In recent years, more and more transiting terrestrial extrasolar planets have been found. Spectroscopy already yielded the detection of molecular absorption bands in the atmospheres of Jupiter and Neptune-sized exoplanets. Detecting spectral features in the atmosphere of terrestrial planets is the next great challenge for exoplanet characterization. Aims. We investigate the spectral appearance of Earth-like exoplanets in the habitable zone (HZ) of different main sequence (F, G, and K-type) stars at different orbital distances. We furthermore discuss for which of these scenarios biomarker absorption bands and related compounds may be detected during primary or secondary transit with near-future telescopes and instruments. Methods. Atmospheric profiles from a 1D cloud-free atmospheric climate-photochemistry model were used to compute primary and secondary eclipse infrared spectra. The spectra were analyzed taking into account different filter bandpasses of two photometric instruments planned to be mounted to the James Webb Space Telescope (JWST). We analyzed in which filters and for which scenarios molecular absorption bands are detectable when using the space-borne JWST or the ground-based European Extremely Large Telescope (E-ELT). Results. Absorption bands of carbon dioxide (CO2), water (H2O), methane (CH4) and ozone (O3) are clearly visible in both highresolution spectra as well as in the filters of photometric instruments. However, only during primary eclipse absorption bands of CO2, H2 Oa nd O 3 are detectable for all scenarios when using photometric instruments and an E-ELT-like telescope setup. CH4 is only detectable at the outer HZ of the K-type star since here the atmospheric modeling results in very high abundances. Since the detectable CO2 and H2O absorption bands overlap, separate bands need to be observed to prove their existence in the planetary atmosphere. In order to detect H2O in a separate band, a ratio S /N > 7 needs to be achieved for E-ELT observations, e.g. by co-adding at least 10 transit observations. Using a space-borne telescope like the JWST enables the detection of CO2 at 4.3 μm, which is not possible for ground-based observations due to the Earth’s atmospheric absorption. Hence combining observations of space-borne and groundbased telescopes might allow to detect the presence of the biomarker molecule O3 and the related compounds H2 Oa nd CO 2 in a planetary atmosphere. Other absorption bands using the JWST can only be detected for much higher S/Ns, which is not achievable by just co-adding transit observations since this would be far beyond the planned mission time of JWST.

The extrasolar planet Gliese 581d: a potentially habitable 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 CO2, 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 CO2-dominated atmospheres (95% CO2 concentration with 5, 10, and 20 bar surface pressure) and a high-pressure CO2-enriched atmosphere (5% CO2 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 CO2-dominated scenarios confirm very recent model results by Wordsworth et al. (2010). However, our model calculations imply that also atmospheres that are not CO2-dominated (i.e., 5% vmr instead of 95% vmr) could result in habitable conditions for Gl 581d.

Spectroscopic characterization of the atmospheres of potentially habitable planets: GL 581 d as a model case study

Context. Were a potentially habitable planet to be discovered, the next step would be the search for an atmosphere and its characterization. Eventually, surface conditions, hence habitability, and biomarkers as indicators for life would be assessed. Aims. The super-Earth candidate Gliese (GL) 581 d is the first potentially habitable extrasolar planet so far discovered. Therefore, GL 581 d is used to illustrate a hypothetical detailed spectroscopic characterization of such planets. Methods. Atmospheric profiles for a wide range of possible one-dimensional (1D) radiative-convective model scenarios of GL 581 d were used to calculate high-resolution synthetic emission and transmission spectra. Atmospheres were assumed to be composed of N2 ,C O 2 ,a nd H 2O. From the spectra, signal-to-noise ratios (SNRs) were calculated for a telescope such as the planned James Webb Space Telescope (JWST). Exposure times were set to be equal to the duration of one transit. Results. The presence of the model atmospheres can be clearly inferred from the calculated synthetic spectra thanks to strong water and carbon-dioxide absorption bands. Surface temperatures can be inferred for model scenarios with optically thin spectral windows. Dense, CO2-rich (potentially habitable) scenarios do not enable us to determine the surface temperatures nor assess habitability. Degeneracies between CO2 concentration and surface pressure complicate the interpretation of the calculated spectra, hence the determination of atmospheric conditions. Still, inferring approximative CO2 concentrations and surface pressures is possible. In practice, detecting atmospheric signals is challenging because the calculated SNR values are well below unity in most of the cases. The SNR for a single transit was only barely larger than unity in some near-IR bands for transmission spectroscopy. Most interestingly, the false-positive detection of biomarker candidates such as methane and ozone might be possible in low resolution spectra because CO2 absorption bands overlap biomarker spectral bands. This can be avoided, however, by observing all main CO2 IR bands instead of concentrating on, e.g., the 4.3 or 15 μm bands only. Furthermore, a masking of ozone signatures by CO2 absorption bands is shown to be possible. Simulations imply that such a false-negative detection of ozone would be possible even for rather high ozone concentrations of up to 10 −5 .

Atmospheric constraints for the CO2 partial pressure on terrestrial planets near the outer edge of the habitable zone

Context. In recent years, several potentially habitable, probably t errestrial exoplanets and exoplanet candidates have been discovered. The amount of CO2 in their atmosphere is of great importance for surface conditions and habitability. In the absence of detailed information on the geochemistry of the planet, this amount could be considered as a free parameter. Aims. Up to now, CO2 partial pressures for terrestrial planets have been obtain ed assuming an available volatile reservoir and outgassing scenarios. This study aims at calculating the allowed maximum CO2 pressure at the surface of terrestrial exoplanets orbiting near the outer boundary of the habitable zone by coupling the radiative effects of the CO2 and its condensation at the surface. These constraints might limit the permitted amount of atmospheric CO2, independent of the planetary reservoir. Methods. A 1D radiative-convective cloud-free atmospheric model was used to calculate surface conditions for hypothetical ter restrial exoplanets. CO2 partial pressures are fixed according to surface temperatur e and vapor pressure curve. Considered scenarios cover a wide range of parameters, such as gravity, central star type and orbital distance, atmospheric N2 content and surface albedo. Results. Results show that for planets in the habitable zone around K-, G-, and F-type stars the allowed CO2 pressure is limited by the vapor pressure curve and not by the planetary reservoir. The maximum CO2 pressure lies below the CO2 vapor pressure at the critical point of pcrit =73.8 bar. For M-type stars, due to the stellar spectrum being shifted to the near-IR, CO2 pressures above pcrit are possible for almost all scenarios considered across the habitable zone. This implies that determining CO2 partial pressures for terrestrial planets by using only geological models is prob ably too simplified and might over-estimate atmospheric CO 2 towards the outer edge of the habitable zone.

The extrasolar planet GL 581 d: A potentially habitable 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 CO2, 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 CO2-dominated atmospheres (95% CO2 concentration with 5, 10, and 20 bar surface pressure) and a high-pressure CO2-enriched atmosphere (5% CO2 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 CO2-dominated scenarios confirm very recent model results by Wordsworth et al. (2010). However, our model calculations imply that also atmospheres that are not CO2-dominated (i.e., 5% vmr instead of 95% vmr) could result in habitable conditions for Gl 581d.