CONSERT probing of 67P/C G nucleus during the ROSETTA mission, operations and results
The structure and composition of cometary nuclei is one of the major unknowns in cometary science. These were the scientific objectives of the Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) aboard ESA spacecraft Rosetta. The knowledge of the interior is important to understand the formation and evolution of comets. CONSERT was a bi static radar composed of two parts, one on the lander Philae on the comet's surface, and another on Rosetta spacecraft (Kofman et al. 2007) CONSERT operated during 9 hours after the landing and made measurements through the small lobe (head) of 67P/ C G. The analysis and interpretation have been done using measurements of the propagation time of the signal through the comet, the shape of the received signals and then 3D modeling of the propagation through the comet. The modelling and the propagation time inside the comet permit to derive the bulk permittivity value 1.27) of the cometary interior (Kofman et al, 2015) Permittivity values for porous mixture of ices and dusts were compared with laboratory experimental values in order to obtain constraints on the possible constituents of the comet nucleus (ices, silicates and organics) and its porosity (70 85% The minimum required content of carbonaceous material is 75 in volume. This suggests that comets represent a massive carbon reservoir (Kofman et al. 2015, Herique et al. 2016) The shape of the signal, very close to the shape of the calibration one, shows that the scattering by inhomogeneities in the medium is not observable. This indicates that the interior is homogenous at the scale of few wavelengths (Kofman et al. 2015) 3D simulations of the signal propagation in non homogeneous media have been run to define the sensitivity of CONSERT to inhomogeneities and to find constrains on the internal structures in terms of size and composition at a scale commensurate with the wavelength. Given the high bulk porosity of about 75% inside the sounded part of the nucleus, a likely model would be obtained by a mixture, at this 3 m size scale, of voids (vacuum) and blobs with material made of ices and dust with a porosity larger than 60% The absence of any pulse spreading due to scattering allows us to exclude heterogeneity with higher contrast (0.25) and larger size (3m) (but remaining on the few wavelengths scale, since larger scales can be responsible for multipath propagation) (Ciarletti et al. 2017) Properties of meterscale inhomogeneities inside the comet are essential to understand cometary formation. The knowledge of the precise position of Philae on the comet, since the September 2016, permitted to improve the determination of the propagation paths inside the comet and therefore to describe better the interior. The influence of the close environment on antennas lobes and polarization was calculated using the Digital Terrain Model of the landing site and used to study the received signal power. We will describe shortly measurements that explored the interior of the comet, discuss results, their interpretation in terms of the internal structure and composition. References [1] Ciarletti et al, CONSERT constrains the internal structure of 67P at a fewmetre size scale, MNRAS, 469 (Suppl. 2) pp.S805 S817, 2017 [2] Herique et al, Cosmochemical implications of CONSERT permittivity characterization of 67P/CG, MNRAS 462, S516 S532, 2016 [3] Kofman et al, The Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) Space Science Reviews, Volume 128, Issue 1 4, 413 432, 2007 [4] Kofman et al, Properties of the 67P/Churyumov Gerasimenko interior revealed by CONSERT radar, Science, 349, 6247 aab0639, 2015.