  We collect observational evidence that supports the scheme of mass transfer
on the nucleus of comet 67P/Churyumov-Gerasimenko. The obliquity of the
rotation axis of 67P causes strong seasonal variations. During perihelion the
southern hemisphere is four times more active than the north. Northern
territories are widely covered by granular material that indicates back fall
originating from the active south. Decimetre sized chunks contain water ice and
their trajectories are influenced by an anti-solar force instigated by
sublimation. OSIRIS observations suggest that up to 20 % of the particles
directly return to the nucleus surface taking several hours of travel time. The
back fall covered northern areas are active if illuminated but produce mainly
water vapour. The decimetre chunks from the nucleus surface are too small to
contain more volatile compounds such as CO 2 or CO. This causes a north-south
dichotomy of the composition measurements in the coma. Active particles are
trapped in the gravitational minimum of Hapi during northern winter. They are
"shock frozen" and only reactivated when the comet approaches the sun after its
aphelion passage. The insolation of the big cavity is enhanced by self-heating,
i. e. reflection and IR radiation from the walls. This, together with the
pristinity of the active back fall, explains the early observed activity of the
Hapi region. Sobek may be a role model for the consolidated bottom of Hapi.
Mass transfer in the case of 67P strongly influences the evolution of the
nucleus and the interpretation of coma measurements.
