  On November 12, 2014, the ESA/Rosetta descent module Philae landed on the
Abydos site of comet 67P/Churyumov-Gerasimenko. Aboard this module, the Ptolemy
mass spectrometer measured a CO/CO2 ratio of 0.07 +/- 0.04 which differs
substantially from the value obtained in the coma by the Rosetta/ROSINA
instrument, suggesting a heterogeneity in the comet nucleus. To understand this
difference, we investigated the physico-chemical properties of the Abydos
subsurface leading to CO/CO2 ratios close to that observed by Ptolemy at the
surface of this region. We used a comet nucleus model that takes into account
different water ice phase changes (amorphous ice, crystalline ice and
clathrates), as well as diffusion of molecules throughout the pores of the
matrix. The input parameters of the model were optimized for the Abydos site
and the ROSINA CO/CO2 measured ratio is assumed to correspond to the bulk value
in the nucleus. We find that all considered structures of water 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 heterogeneity of 67P/Churyumov-Gerasimenko's nucleus is also found
possible only if it is constituted of crystalline ices. If the icy phase is
made of amorphous ice or clathrates, the difference between Ptolemy and
ROSINA's measurements would rather originate from the spatial variations in
illumination on the nucleus surface. An eventual new measurement of the CO/CO2
ratio at Abydos by Ptolemy could be decisive to distinguish between the three
water ice structures.
