  Stable surface liquid water is a key indicator of exoplanet habitability.
However, few approaches exist for directly detecting oceans on potentially
Earth-like exoplanets. In most cases, specular reflection of host starlight
from surface bodies of water -- referred to as ocean glint -- proves to be an
important aspect of liquids that can enable detection of habitable conditions.
Here, we propose that spectral principal component analysis (PCA) applied to
orbital phase-dependent observations of Earth-like exoplanets can provide a
straightforward means of detecting ocean glint and, thus, habitability. Using
high-fidelity, orbit-resolved spectral models of Earth, and for instrument
capabilities applicable to proposed exo-Earth direct imaging concept missions,
the extreme reddening effect of crescent-phase ocean glint is demonstrated as
the primary spectral component that explains phase-dependent variability for
orbital inclinations spanning 60--90 degrees. At smaller orbital inclinations
where more-extreme crescent phases cannot be accessed, glint can still
significantly increase planetary brightness but reddening effects are less
pronounced and, as a result, glint is not plainly indicated by phase-dependent
spectral PCA. Using instrument models for future exoplanet direct imaging
mission concepts, we show that brightness enhancements due to glint could be
detected across a wide range of orbital inclinations with typical exposure
times measured in hours to weeks, depending on system distance and mission
architecture. Thus, brightness increases due to glint are potentially
detectable for Earth-like exoplanets for most system inclinations and
phase-dependent spectral PCA could indicate reddening due to glint for a subset
of these inclinations.
