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

• Main Authors: Trees, Victor J. H, Stam, Daphne M
• Format: Journal Article
• Language: English
• Published: 11-05-2022
• Subjects: Physics - Atmospheric and Oceanic Physics; Physics - Earth and Planetary Astrophysics
• DOI: 10.48550/arxiv.2205.05669

A&A 664, A172 (2022) Numerical simulations of starlight that is reflected by Earth-like exoplanets
predict habitability signatures that can be searched for with future
telescopes. We explore signatures of water oceans in the flux and polarization
spectra of this reflected light. With an adding-doubling algorithm, we compute
the total flux F, polarized flux Q and degree of polarization P of starlight
reflected by dry and ocean model planets with Earth-like atmospheres and patchy
clouds. The oceans consist of Fresnel reflecting surfaces with wind-ruffled
waves, foam and wave shadows, above natural blue seawater. Our results are
presented 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 ocean
glint increases F, |Q| and P with increasing phase angle at non-absorbing
wavelengths, and causes the spectra of F and |Q| for the various phase angles
to intersect. In the near-infrared, Q is negative, i.e. the direction of
polarization is perpendicular to the plane through the star, planet, and
observer. In the P-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 P, and the negative Q in the near-infrared, can be
searched for at a phase angle of 90 degrees, 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 modelling 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.