  Unmixing the disk-integrated spectra of exoplanets provides hints about
heterogeneous surfaces that we cannot directly resolve in the foreseeable
future. It is particularly important for terrestrial planets with diverse
surface compositions like Earth. Although previous work on unmixing the spectra
of Earth from disk-integrated multi-band light curves appeared successful, we
point out a mathematical degeneracy between the surface colors and their
spatial distributions. Nevertheless, useful constraints on the spectral shape
of individual surface types may be obtained from the premise that albedo is
everywhere between 0 and 1. We demonstrate the degeneracy and the possible
constraints using both mock data based on a toy model of Earth, as well as real
observations of Earth. Despite the severe degeneracy, we are still able to
recover an approximate albedo spectrum for an ocean. In general, we find that
surfaces are easier to identify when they cover a large fraction of the planet
and when their spectra approach zero or unity in certain bands.
