  We present a comprehensive set of spectropolarimetric observations of
Earthshine as obtained by FORS2 at the VLT for phase angles from 50degree to
135degree (Sun-Earth-Moon angle), covering a spectral range from 430nm to
920nm. The degree of polarization in BVRI passbands, the differential
polarization vegetation index, and the equivalent width of the O2A polarization
band around 760nm are determined with absolute errors around 0.1 percent in the
degree of polarization. Earthshine polarization spectra are corrected for the
effect of depolarization introduced by backscattering on the lunar surface,
introducing systematic errors of the order of 1 percent in the degree of
polarization. Distinct viewing sceneries such as observing the Atlantic or
Pacific side in Earthshine yield statistically different phase curves. The
equivalent width defined for the O2A band polarization is found to vary from
-5nm to +2nm. A differential polarized vegetation index is introduced and
reveals a larger vegetation signal for those viewing sceneries that contain
larger fractions of vegetated surface areas. We corroborate the observed
correlations with theoretical models from the literature, and conclude that the
Vegetation Red Edge(VRE) is a robust and sensitive signature in polarization
spectra of planet Earth. The overall behaviour of polarization of planet Earth
in the continuum and in the O2A band can be explained by existing models.
Biosignatures such as the O2A band and the VRE are detectable in Earthshine
polarization with a high degree of significance and sensitivity. An in-depth
understanding of Earthshines temporal and spectral variability requires
improved models of Earths biosphere, as a prerequisite to interpret possible
detections of polarised biosignatures in earthlike exoplanets in the future.
