  We present the results of the optical spectropolarimetry of Earthshine on the
Moon for Earth phase angles ranging from 49 to 96 degrees. The observations
were conducted on 2011 March 9-13 (UT) using the spectropolarimeter HBS
installed on the 1.88 m telescope at Okayama Astrophysical Observatory. The
wavelength coverage was 450-850 nm with a resolution of 6 nm. The observed
Earthshine polarization degree spectra exhibit decreasing polarization degree
with increasing wavelength at any phase. The overall degree of polarization
increases as the Earth approaches a quadrature phase. The phase dependence
differs with the wavelengths; the maximum polarization for the V band occurs at
a phase angle of ~90 degrees, whereas that for longer wavelengths is reached at
larger phase angles. This is interpreted as indicating that Earthshine
polarization at shorter wavelengths is dominated by atmospheric Rayleigh
scattering, whereas that at longer wavelengths has an increasingly effective
contribution from the Earth surface reflection. The wavelength dependence of
the phase angle of the maximum polarization appears to be unique among the
terrestrial planetary bodies in the Solar System. Therefore this might
constitute important evidence pointing toward a distinctive characteristic of
the Earth: the planet has a scattering but transparent atmosphere above its
surface.
