NEAR-SURFACE THERMAL-GRADIENTS AND THEIR EFFECTS ON MIDINFRARED EMISSION-SPECTRA OF PLANETARY SURFACES

We model the heat transfer by radiation and conduction in the top few millimeters of a planetary surface to determine the magnitude of near-surface (approximate to 100 mu m) thermal gradients and their effects on mid-infrared emission spectra for a number of planetary environments. The model is one-dimensional and uses a finite difference scheme for approximate to 10-mu m layers. Calculations are performed for samples heated at the base and from above by sunlight. Our results indicate that near-surface radiative cooling creates significant thermal gradients in the top few hundred microns of surfaces in which radiation is an important heat transfer mechanism. The effect is maximized in evacuated, underdense particulate media with sufficiently high temperatures. Near-surface thermal gradients will be significant in fine-grained particulate surfaces on the Moon (40-60 K/100 mu m) and Mercury (approximate to 80 K/100 mu m), increasing spectral contrast and creating emission maxima in the transparent regions of the spectra. They will be of lesser importance on the surface of Mars, with a maximum value of around 5 K/100 mu m in areas of low thermal inertia, and will be negligible on planets with more substantial atmospheres (<1 K/100 mu m). We conclude that the effects that thermal gradients have on mid-LR emission spectra are predictable and do nor negate the utility of emission spectroscopy for remote determination of planetary surface composition.