Effect of meter-scale heterogeneities inside 67P nucleus on CONSERT data

Abstract : Since their arrival at comet 67P in August 2014, a number of instruments onboard Rosetta’s main spacecraft and Philae lander have been observing the surface of the nucleus and revealed details of amazing surficial structures (hundreds of meters deep pits and cliffs, surface roughness of the order of a couple of meters in size, non-continuous apparent layers on both lobes of the comet). After two years of observations, the activity of the comet has also been better constrained, while the origin of sporadic jet activities remains debated. This surficial information is complemented by relevant measurements assessing the nucleus internal structure that have been collected by the CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission) experiment in order to constrain the nucleus formation and evolution. The CONSERT experiment is a bistatic radar with receivers and transmitters on-board both Rosetta’s main spacecraft and the Philae lander. The instrument transmits electromagnetic waves at 90 MHz (10 MHz bandwidth) between Philae and Rosetta. The signal propagated through the small lobe of 67P over distances ranging from approximately 200 to 800 meters depending on the spacecraft location and probed a maximum depth of about one hundred meters in the vicinity of the final landing site Abydos. The CONSERT data have been used to obtain an estimate of the permittivity mean value. Thanks to the 10 MHz frequency bandwidth of the signal used by the instrument, a spatial resolution around 10m is obtained inside the sounded volume of the nucleus. In this work, we analyze the effect of internal heterogeneities of 67P on the CONSERT data by simulating the propagation of the signal through a fractal model of the comet interior. We considered for the simulations a range of realistic permittivity values and characteristic sizes of the material heterogeneities. The different parameters values used have an impact on the width of the signal propagating through the modeled nucleus. Comparison with the values measured by CONSERT will allow us to determine the possible permittivity variations and heterogeneities size compatible with 67P internal structure.