Spectral and temporal variability of Earth observed in polarization

Context. Earthshine, i.e., sunlight scattered by Earth and back-reflected from the lunar surface to Earth, allows observations of the total flux and polarization of Earth with ground-based astronomical facilities on timescales from minutes to years. Like flux spectra, polarization spectra exhibit imprints of the atmospheric and surface properties of Earth. Earth's polarization spectra may prove an important benchmark to constrain expected biosignatures of Earth-like planets observed with future telescopes. Aims. We derive the polarimetric phase curve of Earth from a statistically significant sample of Earthshine polarization spectra. The impact of changing Earth views on the variation of polarization spectra is investigated. Methods. We present a comprehensive set of spectropolarimetric observations of Earthshine as obtained by FORS2 at the Very Large Telescope for phase angles from 50° to 135° (Sun-Earth-Moon angle), covering a spectral range from 4300 to 9200 Å. The degree of polarization in the B, V, R, I passbands, the differential polarization vegetation index, and the equivalent width of the O2-A polarization band around 7600 Å are determined with absolute errors around 0.1% 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 on the order of 1% in the degree of polarization. Results. Distinct viewing sceneries such as observing the Atlantic or Pacific side in Earthshine yield statistically different phase curves. The equivalent width defined for the O2-A band polarization is found to vary from -50 to +20 Å. 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. Conclusions. The overall behavior of polarization of planet Earth in the continuum and in the O2-A band can be explained by existing models. Biosignatures such as the O2-A band and the VRE are detectable in Earthshine polarization with a high degree of significance and sensitivity. An in-depth understanding of the temporal and spectral variability of Earthshine requires improved models of Earth's biosphere, as a prerequisite to interpreting possible detections of polarized biosignatures in Earth-like exoplanets in the future.