  The European Space Agency Rosetta/Philae mission to Comet
67P/Churyumov-Gerasimenko in 2014-2016 is the most complete and diverse
investigation of a comet carried out thus far. Yet, many physical and chemical
properties of the comet remain uncertain or unknown, and cometary activity is
still not a well-understood phenomenon. We here attempt to place constraints on
the nucleus abundances and sublimation front depths of H2O and CO2 ice, and to
reconstruct how the nucleus evolved throughout the perihelion passage. We
employ the thermophysical modelling code 'Numerical Icy Minor Body evolUtion
Simulator', or NIMBUS, to search for conditions under which the observed H2O
and CO2 production rates are simultaneously reproduced before and after
perihelion. We find that the refractories to water-ice mass ratio of relatively
pristine nucleus material is mu~1, that airfall material has mu~2, and that the
molar abundance of CO2 relative H2O near 30 per cent. The dust mantle thickness
is typically < 2 cm. The average CO2 sublimation front depths near aphelion
were ~3.8 m and ~1.9 m on the northern and southern hemispheres, respectively,
but varied substantially with time. We propose that airfall material is
subjected to substantial fragmentation and pulverisation due to thermal fatigue
during the aphelion passage. Sub-surface compaction of material due to CO2
activity near perihelion seems to have reduced the diffusivity in a measurable
way.
