NIR Earthshine spectropolarimetry: observing strategy, data calibration and high-resolution line identification

Earthshine is a powerful benchmark to test predictions from models which will be fundamental, in the near future, to interpret reflected light observations of rocky exoplanets. Its already disk-integrated flux mimics single-pixel observations and allows the study of the Earth’s reflected light with the largest ground-based telescopes. In this thesis, four half nights of high and low-resolution NIR spectropolarimetric Earthshine observations were planned and carried out with CRIRES+ and FORS2 at the VLT. A robust observing strategy was developed and tested and will serve as a precious reference for future observations. A novel reduction procedure was then developed to extract the faint signal of an extended source from the CRIRES+ spectropolarimetric raw data. The spectra were analysed, adopting a new and successful approach: the difference between spectra taken from a position inside the Moon and on the adjacent sky was compared with state-of-the-art 3D Earthshine models modified for the atmospheric transmission above the observatory. The spectral analysis led to the identification (at ≥ 5σ) of more than 120 true Earthshine H2O spectral lines in both Y and J bands, together with a few O2 lines in the J band. A statistical detection of the presence of CO2 in the Earthshine spectrum was also performed by combining all its spectral lines in J band and the obtained significance was in good agreement with model predictions. Finally, the polarization spectra and the current limitations of CRIRES+ polarimetry were discussed. These results demonstrate that individual molecular lines of the Earth’s reflected light spectrum can be measured at high resolution with ground-based telescopes. This paves the way for the realization of a phase-resolved, high-resolution Earthshine template, precious in view of future reflected light observations of rocky exoplanets.