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 environment s. The model is one-dimensional and uses a finite difference scheme fo r approximate to 10-mu m layers. Calculations are performed for sample s heated at the base and from above by sunlight. Our results indicate that near-surface radiative cooling creates significant thermal gradie nts in the top few hundred microns of surfaces in which radiation is a n important heat transfer mechanism. The effect is maximized in evacua ted, underdense particulate media with sufficiently high temperatures. Near-surface thermal gradients will be significant in fine-grained pa rticulate surfaces on the Moon (40-60 K/100 mu m) and Mercury (approxi mate to 80 K/100 mu m), increasing spectral contrast and creating emis sion maxima in the transparent regions of the spectra. They will be of lesser importance on the surface of Mars, with a maximum value of aro und 5 K/100 mu m in areas of low thermal inertia, and will be negligib le on planets with more substantial atmospheres (<1 K/100 mu m). We co nclude that the effects that thermal gradients have on mid-LR emission spectra are predictable and do nor negate the utility of emission spe ctroscopy for remote determination of planetary surface composition.