Looking at Comet 67P Sub-surface in the Vicinity of Abydos

Abstract : While amazing surface features of comet 67P have been observed and revealed since the beginning of the Rosetta mission by a number of cameras onboard Rosetta’s main spacecraft (OSIRIS and NAVCAM) and Philae lander (CIVA and ROLIS), information below the surface has also been collected by the CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission) experiment that can help constrain the nucleus formation and evolution. CONSERT is a bistatic radar with receivers and transmitters on-board both Rosetta’s main spacecraft and Philae lander. It has been successfully operating during the descent and First Science Sequence (FSS) right after Philae’s final landing at Abydos. The nucleus in the vicinity of Abydos has been actually sounded by CONSERT’s electromagnetic waves at 90 MHz with a spatial resolution around 10m (over lengths ranging from approximately 200 to 800 m and maximum depths of about one hundred of meters). The data collected provide information about the permittivity values inside the sounded volume and allows us to retrieve some constraints about the internal structures of the nucleus inside the sounded volume. In this paper, we specifically focus on local variations in the nucleus subsurface permittivity simulated over spatial scales ranging from tens to hundreds of meters. A number of propagation simulations corresponding to the CONSERT operations have been performed for a variety of subsurface permittivity models. The effect of local vertical and horizontal variations of the permittivity values around the landing site as well as comparison with CONSERT’s experimental data collected in the same configurations will be presented and discussed. Possible interpretations of the results will be presented as well as potential consequences for the nucleus structure in connection with observations instruments.