Earth-like, potentially habitable exoplanets are prime targets in the search for extraterrestrial life. Information about their atmosphere and surface can be derived by analyzing light of the parent star reflected by the planet.
We investigate the influence of the surface albedo As, the optical thickness bcloud and altitude of water clouds, and the mixing ratio η of biosignature O2 on the strength of the O2 A-band (around 760 nm) in flux and polarization spectra of starlight reflected by Earth-like exoplanets. Our computations for horizontally homogeneous planets show that small mixing ratios (η < 0.4) will yield moderately deep bands in flux and moderate to small band strengths in polarization, and that clouds will usually decrease the band depth in flux and the band strength in polarization. However, cloud influence will be strongly dependent on their properties such as optical thickness, top altitude, particle phase, coverage fraction, horizontal distribution.
Depending on the surface albedo, and cloud properties, different O2 mixing ratios η can give similar absorption band depths in flux and band strengths in polarization, in particular if the clouds have moderate to high optical thicknesses. Measuring both the flux and the polarization is essential to reduce the degeneracies, although it will not solve them, in particular not for horizontally inhomogeneous planets. Observations at a wide range of phase angles and with a high temporal resolution could help to derive cloud properties and, once those are known, the mixing ratio of O2 or any other absorbing gas.
Thomas Fauchez, Loic Rossi, Daphne M. Stam
(Submitted on 20 Apr 2017)
Comments: 21 pages, 20 figures, accepted for publication in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1704.06247 [astro-ph.EP] (or arXiv:1704.06247v1 [astro-ph.EP] for this version)
From: Daphne Stam
[v1] Thu, 20 Apr 2017 17:47:04 GMT (7166kb,D)
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