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.