Near-surface thermal gradients and mid-IR emission spectra: A new model including scattering and application to real data

We model the radiative and conductive heat transfer in the top few millimeters of a particulate medium in order to investigate near-surface thermal gradients and their effects on mid-IR emission spectra for different planetary environments. The model extends our previous work by including scattering in the radiative heat transfer. Our results show that significant thermal gradients will form in the top few hundred microns of particulate materials on the surfaces of the Moon and Mercury. Their presence alters spectral contrast and creates emission maxima in the transparent regions of the spectra. The results also show that thermal gradients cause the wavelength position of the Christiansen emission peak to shift by as much as 0.5 mu m with variations in thermal conductivity and grain size, in agreement with previous laboratory investigations. These wavelength shifts are due to increased emission in the transparent regions of the spectrum which are superimposed upon the emissivity signature. The results are applied to telescopic spectra of the surfaces of the Moon and Mercury and can account for certain features seen in these data. Additional calculations show that thermal gradients will be minor on the surface of Mars and negligible for Earth.