  In this work, we present a new model for the heat conductivity of porous dust
layers in vacuum, based on an existing solution of the heat transfer equation
of single spheres in contact. This model is capable of distinguishing between
two different types of dust layers: dust layers composed of single particles
(simple model) and dust layers consisting of individual aggregates (complex
model). Additionally, we describe laboratory experiments, which were used to
measure the heat conductivity of porous dust layers, in order to test the
model. We found that the model predictions are in an excellent agreement with
the experimental results, if we include radiative heat transport in the model.
This implies that radiation plays an important role for the heat transport in
porous materials. Furthermore, the influence of this new model on the Hertz
factor are demonstrated and the implications of this new model on the modeling
of cometary activity are discussed. Finally, the limitations of this new model
are critically reviewed.
