Subsurface characterization of 67P/Churyumov-Gerasimenko's abydos site
Résumé
On November 12, 2014, the ESA/Rosetta descent module Philae landed on the Abydos site ofcomet 67P/Churyumov-Gerasimenko. Aboard this module, the Ptolemy mass spectrometer measureda CO/CO2ratio of 0.07±0.04 which differs substantially from the value obtained in the coma bythe Rosetta/ROSINA instrument, suggesting a heterogeneity in the comet nucleus. To understandthis difference, we investigated the physico-chemical properties of the Abydos subsurface leading toCO/CO2ratios close to that observed by Ptolemy at the surface of this region. We used a cometnucleus model that takes into account different water ice phase changes (amorphous ice, crystallineice and clathrates), as well as diffusion of molecules throughout the pores of the matrix. The inputparameters of the model were optimized for the Abydos site and the ROSINA CO/CO2measured ratiois assumed to correspond to the bulk value in the nucleus. We find that all considered structures ofwater ice are able to reproduce the Ptolemy observation with a time difference not exceeding∼50 days,i.e. lower than∼2% on 67P/Churyumov-Gerasimenko’s orbital period. The suspected heterogeneityof 67P/Churyumov-Gerasimenko’s nucleus is also found possible only if it is constituted of crystallineices. If the icy phase is made of amorphous ice or clathrates, the difference between Ptolemy andROSINA’s measurements would rather originate from the spatial variations in illumination on thenucleus surface. An eventual new measurement of the CO/CO2ratio at Abydos by Ptolemy could bedecisive to distinguish between the three water ice structures.