Models of Polarized Light from Oceans and Atmospheres of Earth-like Extrasolar Planets

Specularly reflected light, or glint, from an ocean surface may provide a useful observational tool for studying extrasolar terrestrial planets. Detection of seasurface glints would differentiate ocean-bearing terrestrial planets, i.e. those similar to Earth, from other terrestrial extrasolar planets. Sea-surface glints are localized and linearly polarized. The brightness and degree of polarization of both sea-surface glints and atmospheric Rayleigh scattering are strong functions of the phase angle of the extrasolar planet. The difference of the two orthogonal linearly polarized reflectances may be an important observational signature of Rayleigh scattering or glint. The difference attributable to Rayleigh scattering peaks near quadrature, i.e. near maximum elongation of a circular orbit. The difference attributable to glint peaks at crescent phase, and in crescent phase the total (unpolarized) reflectance of the glint is also near maximum. We present analytic and physical models of sea-surface glints. We modify analytic expressions for the bi-directional reflectances previously validated by satellite imagery of the Earth to account for the fractional linear polarization of sea-surface reflections and of Rayleigh scattering in the atmosphere. We compare our models with Earth’s total visual light and degree of linear polarization as observed in the ashen light of the Moon, or Earthshine. We predict the spatially-integrated reflected light and its degree of polarization as functions of the diurnal cycle and orbital phase of Earth and Earth-like planets of various imagined types. The difference in polarized reflectances of Earth-like planets may increase greatly the detectability of such planets in the glare of their host star. Finally, sea-surface glints potentially may provide a practical means to map the boundaries between oceans and continents on extrasolar planets. Space Telescope Science Institute, 3700 San Martin Dr., Baltimore MD 21218