Rotational mapping and 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
use simulations of Earth as an exoplanet to introduce a combination of
multiwavelength, multiphase, 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 itself as a strong
source of periodic variability in crescent-phase reflected light curves. We
invert these light curves to constrain the longitudinal slice maps and apparent
albedo of two surfaces at both quadrature and crescent phase. At crescent
phase, the retrieved apparent albedo of ocean-bearing longitudinal slices is
increased by a factor of 5, compared to the albedo 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 observation increases the overall reflectivity toward
crescent, but 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 may be used to tie changes in the apparent albedo with phase
back to specific geographic surfaces, although this requires ideal geometries.
We estimate that crescent-phase time-dependent glint detections 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 >6 m.
