  The European Space Agency's (ESA) Rosetta mission has returned a vast data
set of measurements of the inner gas coma of comet 67P/Churyumov-Gerasimenko.
These measurements have been used by different groups to determine the
distribution of the gas sources at the nucleus surface. The solutions that have
been found differ from each other substantially and illustrate the degeneracy
of this issue. It is the aim of this work to explore the limitations that
current gas models have in linking the coma measurements to the surface. In
particular, we discuss the sensitivity of Rosetta's ROSINA/COPS, VIRTIS, and
MIRO instruments to differentiate between vastly different spatial
distributions of the gas emission from the surface. We have applied a state of
the art 3D DSMC gas dynamics code to simulate the inner gas coma of different
models that vary in the fraction of the surface that contains ice and in
different sizes of active patches. These different distributions result in jet
interactions that differ in their dynamical behaviour. We have found that
ROSINA/COPS measurements by themselves cannot detect the differences in our
models. While ROSINA/COPS measurements are important to constrain the regional
inhomogeneities of the gas emission, they can by themselves not determine the
surface emission distribution of the gas sources to a spatial accuracy of
better than a few hundred metres (400 m). Any solutions fitting the ROSINA/COPS
measurements is hence fundamentally degenerate, be it through a forward or
inverse model. Only other instruments with complementary measurements can
potentially lift this degeneracy as we show here for VIRTIS and MIRO. Finally,
as a by-product, we have explored the effect of our activity distributions on
lateral flow at the surface that may be responsible for some of the observed
aeolian features.
