摘要：Context.An exoplanet’s habitability will depend strongly on the presence of liquid water. Flux and/or polarization measurements of starlight that is reflected by exoplanets could help to identify exo-oceans.Aims.We investigate which broadband spectral features in flux and polarization phase functions of reflected starlight uniquely identify exo-oceans.Methods.With an adding-doubling algorithm, we computed total fluxesFand polarized fluxesQof starlight that is reflected by cloud-free and (partly) cloudy exoplanets, for wavelengths from 350 to 865 nm. The ocean surface has waves composed of Fresnel reflecting wave facets and whitecaps, and scattering within the water body is included.Results.Total fluxF, polarized fluxQ, and degree of polarizationPof ocean planets change color from blue, through white, to red at phase anglesαranging from ~134° to ~108° forF, and from ~123° to ~157° forQ, with cloud coverage fractionfcincreasing from 0.0 (cloud-free) to 1.0 (completely cloudy) forF, and to 0.98 forQ. The color change inPonly occurs forfcranging from 0.03 to 0.98, with the color crossing angleαranging from ~88° to ~161°. The total fluxFof a cloudy, zero surface albedo planet can also change color, and forfc= 0.0, an ocean planet’sFwill not change color for surface pressuresps≿ 8 bars. Polarized fluxQof a zero surface albedo planet does not change color for anyfc.Conclusions.The color change ofPof starlight reflected by an exoplanet, from blue, through white, to red with increasingαabove 88°, appears to identify a (partly) cloudy exo-ocean. The color change of polarized fluxQwith increasingαabove 123° appears to uniquely identify an exo-ocean, independent of surface pressure or cloud fraction. At the color changing phase angle, the angular distance between a star and its planet is much larger than at the phase angle where the glint appears in reflected light. The color change in polarization thus offers better prospects for detecting an exo-ocean.
关键词：Key wordsenradiative transferpolarizationplanets and satellites: terrestrial planetsplanets and satellites: oceanstechniques: polarimetricplanets and satellites: atmospheres
