Detecting Oceans on Exoplanets with Next-Generation Telescopes

Detecting oceans on terrestrial exoplanets is pivotal to understanding the prevalence of habitable worlds and searching for life beyond the Solar System, and it is therefore a key goal for next-generation telescopes. Rotational mapping and specular reflection (glint) are two proposed methods to directly detect liquid water on the surface of habitable exoplanets. However, false positives for both methods may prevent the unambiguous detection of exoplanet oceans. We report on realistic simulations of Earth as an exoplanet to introduce a combination of multi-wavelength, multi-phase, time-series direct imaging observations and accompanying analyses that may improve the robustness of exoplanet ocean detection by spatially mapping the ocean glint signal. As the planet rotates, the glint spot appears to "blink" as Lambertian scattering continents interrupt the specular reflection from the ocean. This manifests as a strong source of periodic variability in crescent phase disk-integrated reflected lightcurves. We invert these lightcurves to derive probabilistic constraints on the longitudinal slice map and apparent albedo of multiple surfaces at both quadrature and crescent phases. At crescent phase the retrieved apparent albedo of ocean-bearing longitudinal slices is increased by a factor of 5 compared to similar results at quadrature phase due to the contribution from glint. The land-bearing slices exhibit no significant change in apparent albedo with phase. The presence of forward scattering clouds in our simulated observations increases the overall reflectivity towards crescent, but we find that clouds do not correlate with any specific surfaces, thereby allowing for the phase-dependent glint effect to be interpreted as distinct from cloud scattering. Retrieving the same longitudinal map at quadrature and crescent phases can be used to tie changes in the apparent albedo with phase back to specific geographic surfaces (or longstanding atmospheric features), although this requires nearly edge-on, low obliquity planetary systems. We estimate that crescent-phase, time-dependent glint measurements are feasible for between 1 - 10 habitable zone exoplanets orbiting the nearest G, K, and M dwarfs using a space-based, high-contrast, direct imaging telescope with a diameter between 6 - 15 meters.