Organic features in the spectrum of 67P/ Churyumov-Gerasimenko from the improved calibration of VIRTIS-M-IR

Abstract

The VIRTIS spectra of 67P/CG in the region between 2.9 and 3.5 micron display a wide absorption band, which has been associated to the presence of organic compounds. However, several instrumental effects have hindered, so far, the detailed interpretation of the molecules and compounds contributing to this band. In this work we revised the in-flight calibration of the VIRTIS-M-IR instrument onboard Rosetta spacecraft with the aim to improve: a) the detection of low-contrast spectral features and b) the radiometric accuracy. This updated calibration involves all 432 spectral channels and 256 spatial samples. It includes: 1) the removal of artifacts associated to calibration residuals and/or incomplete flat field correction; 2) the reduction of the non-poissonian noise introduced by the readout electronics of the instrument; 3) a newer version of the radiometric calibration using stellar sources. Furthermore, we have modeled the thermal emission to remove the nucleus contribution at wavelengths in excess of 3.0 micron and we have also modeled and removed, by means of the Hapke model, the contribution of water ice absorption to isolate the organic features within the spectral region 2.9 - 3.5 micron. These spectral features have been compared with laboratory measurements and observations of diffuse interstellar medium to provide indications on the relative contribution of the Aromatic and Aliphatic components.