Calibration and analysis of the telluric O2-bands: A spectropolarimetric approach for aerosol and cloud analysis

Earthshine observations are a classical benchmark for the studies of the atmospheres of exo-earth type planets. For this kind of observations, background subtraction is a critical and challenging task to disentangle the effects of atmospheric scattering from atmospheric transmission of molecular lines. In fact, although usually neglected, the background itself carries information on the composition of our atmosphere, and further crucial information may be extracted from the analysis of spectropolarimetric observations of the O2 lines at sunset. In this talk we will first discuss the surprisingly high variability of telluric lines in Earthshine spectra. With full vector radiative transfer calculations of the Earth\u2019s atmosphere we show the impact of aerosol and cloud properties on the spectropolarimetric appearance of the O2 band. They demonstrate that high spectral resolution observations allow to constrain high clouds and aerosols with high sensitivity. We suggest to use HARPS in polarimetric mode to observe the sky at zenith during sunset various times in order to obtain a larger sample of different sky polarisation spectra at high resolution in the O2B spectral band for further modelling.