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Numerical simulations of starlight that is reflected by Earth-like exoplanetspredict habitability signatures that can be searched for with futuretelescopes. We explore signatures of water oceans in the flux and polarizationspectra of this reflected light. With an adding-doubling algorithm, we computethe total flux F, polarized flux Q and degree of polarization P of starlightreflected by dry and ocean model planets with Earth-like atmospheres and patchyclouds. The oceans consist of Fresnel reflecting surfaces with wind-ruffledwaves, foam and wave shadows, above natural blue seawater. Our results arepresented as functions of wavelength (from 300 to 2500 nm with 1 nm resolution)and as functions of the planetary phase angle from 90 to 170 degrees. The oceanglint increases F, |Q| and P with increasing phase angle at non-absorbingwavelengths, and causes the spectra of F and |Q| for the various phase anglesto intersect. In the near-infrared, Q is negative, i.e. the direction ofpolarization is perpendicular to the plane through the star, planet, andobserver. In the P-spectra, the glint leaves dips (instead of peaks) in gaseousabsorption bands. All those signatures are missing in the spectra of dryplanets. The dips in P, and the negative Q in the near-infrared, can besearched for at a phase angle of 90 degrees, where the planet-star separationis largest. Those ocean signatures in polarized light do not suffer from falsepositive glint signals that could be due to clouds or reflecting dry surfaces.For heavily cloudy planets, ocean detection is possible when the glint is(partially) cloud-free. When modelling signals of planets with oceans, usinghorizontally inhomogeneous cloud covers is thus crucial. Observations spreadover time would increase the probability of catching a cloud-free glint anddetecting an ocean.

Ocean signatures in the total flux and polarization spectra of Earth-like exoplanets