  We propose that astronomers will be eventually be able to discriminate
between extrasolar Earth-like planets with surface oceans and those without
using the shape of phase light curves in the visible and near-IR spectrum. We
model the visible light curves of planets having Earth-like surfaces, seasons,
and optically-thin atmospheres with idealized diffuse-scattering clouds. We
show that planets partially covered by water will appear measurably brighter
near crescent phase (relative to Lambertian planets) because of the efficient
specular reflection (i.e., glint) of starlight incident on their surfaces at a
highly oblique angle. Planets on orbits within 30 degrees of edge-on
orientation (half of all planets) will show pronounced glint over a sizeable
range of orbital longitudes, from quadrature to crescent, all outside the glare
of their parent stars. Also, water-covered planets will appear darker than a
Lambertian disk near full illumination. Finally, we show that planets with a
mixed land/water surface will polarize the reflected signal by as much as 30-70
percent. These results suggest several new ways of directly identifying water
on distant planets.
