Spectra of Earth-like exoplanets Ocean signatures

In the near-infrared, Q is negative, that is, the direction of polarization is perpendicular to the plane through the star, planet, and observer. In the Ps spectra, the glint leaves dips (instead of peaks) in gaseous absorption bands. All those signatures are missing in the spectra of dry planets.

The dips in Ps and the negative Q in the near-infrared can be searched for at a phase angle of 90{deg}, where the planet-star separation is largest. Those ocean signatures in polarized light do not suffer from false positive 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 modeling signals of planets with oceans, using horizontally inhomogeneous cloud covers is thus crucial. Observations spread over time would increase the probability of catching a cloud-free glint and detecting an ocean.