  The standard approach to obtaining knowledge about the properties of the
surface layer of a comet from observations of gas production consists of two
stages. First, various thermophysical models are used to calculate gas
production for a few sets of parameters. Second, a comparison of observations
and theoretical predictions is performed. This approach is complicated because
the values of many model characteristics are known only approximately.
Therefore, it is necessary to investigate the sensitivity of the simulated
outgassing to variations in the properties of the surface layer. This problem
was recently considered by us for aggregates up to tens of microns in size. For
millimetre-size aggregates, a qualitative extension of the method used to model
the structural characteristics of the layer is required. It is also necessary
to study the role of radiative thermal conductivity, which may play an
important role for such large particles. We investigated layers constructed
from large aggregates and having various thicknesses and porosity and evaluated
the effective sublimation of water ice at different heliocentric distances. For
radiative conductivity, approximate commonly used models and the complicated
model based on the Dense Medium Radiative Transfer theory were compared. It was
shown that for millimetre-size aggregates careful consideration of the
radiative thermal conductivity is required since this mechanism of energy
transfer may change the resulting gas productivity by several times. We
demonstrate that our model is more realistic for an evolved comet than simple
models parameterising the properties of the cometary surface layer, yet
maintains comparable computational complexity.
