Burgdorf et al. 2000
presented a solution for the emissivity of Mars at wavelengths from
50 um to 180 um from ISO
observations (see also
Sidher et al. 2000).
The emissivity as a function of wavelength is shown in
Figure 6 of that paper, shown below (click on it for a bigger version).
This emissivity can be roughly fit by a linear which goes from 0.98
at 50 um to 0.90 at 200 um. From the results of
Rudy et al. 1987
we also know that the emissivity at radio wavelengths
is around 0.90 (because the derived dielectric was in the range
2.25-2.5).
Given the emissivity we can back out the dielectric, which is then
used as input to the model.