The role of Clouds in Emitted, Reflected and Transmitted Spectra of Terrestrial Exoplanets

Details The role of Clouds in Emitted, Reflected and Transmitted Spectra of Terrestrial Exoplanets The role of Clouds in Emitted, Reflected and Transmitted Spectra of Terrestrial Exoplanets

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005dps....37.3117t&link_type=abstract

American Astronomical Society, DPS meeting #37, #31.17; Bulletin of the American Astronomical Society, Vol. 37, p.1566

Biology

Scientific paper

Two objectives of the NASA-Terrestrial Planet Finder and ESA-Darwin missions are to characterize the environments of terrestrial planets outside of our solar system and to search for life on these planets. These objectives will be met by measuring the disk-averaged spectra of the radiation reflected or emitted from these planets. Clouds play a significant role in determining these spectra. For Earth, water clouds can reduce the infrared emission by up to 50 and increase the visible reflectance by up to 400%. The disk-averaged spectra of a cloudy planet are also very sensitive to the observed planetary phase. For Earth, we see up to 40% increases of the solar albedo from the gibbous phase to the fully illuminated phase. Moreover, clouds strongly modify the strength of absorption features due to tropospheric trace gases and may impact the detectability of surface biosignatures in the visible (Tinetti et al.,2005). Stellar occultation might provide another effective method for probing the atmospheres of Earth-size extrasolar planets in the not too distant future. In the transmission spectra of terrestrial planets in transit, clouds act, to a first order approximation, as an optically thick layer at a given altitude. A uniform cloud layer will effectively increase the apparent radius of the planet and yield information only about atmospheric components existing above the clouds. The altitude where the cloud deck occurs, changes for Venus-like, Earth-like or highly-condensable-volatile rich planets (Ehrenreich et al.,2005). The radiative properties of clouds are strongly dependent on the chemical species that condense or freeze (e.g. water for present-day Earth, methane for Titan etc.), the particle size distributions present and particle shapes. Therefore, an understanding of aerosol and cloud microphysics on extra-solar terrestrial planets is necessary to properly interpret the spectra of terrestrial planets, emitted, reflected or transmitted. This work was supported by NASA-Astrobiology Institute/Caltech/CNRS.

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