NEW LABORATORY MEASUREMENTS OF MID-IR EMISSION-SPECTRA OF SIMULATED PLANETARY SURFACES

We present emission spectra of particulate quartz measured in an envir onment chamber designed to simulate the conditions on actual planetary surfaces. The goal was to investigate near-surface thermal gradients and their effects on emission spectra for other planetary environments . Our experiment parallels that of Logan et al. [1973] but is differen t, in that our samples were heated at the base by a temperature-contro lled hot plate rather than from above by a solar lamp in order to sepa rate infrared surface cooling from solar heating effects. Our spectra show prominent emission peaks which are attributed to the presence of near-surface thermal gradients created by infrared cooling of the uppe rmost layer of the material. The contrast of the emission peak is maxi mized under vacuum conditions, for which it is estimated that a temper ature difference of at least 40 K existed within the top emission skin depth. The wavelength location of the emission peak occurs near the C hristiansen wavelength at 7.35 mu m but has been shifted by approximat ely 0.2 mu m to shorter wavelengths. This result is in agreement with the earlier results of Logan et al. [1973] and points out that the exi stence of a thermal gradient violates the conditions required by Kirch off's law, and therefore care should be taken when spectra of surfaces on airless bodies are interpreted using emissivity spectra converted from reflectance data. Increasing the atmospheric pressure in the cham ber increased the conductivity of the soil, mitigating the thermal gra dient and decreasing the contrast of the emission maxima. Although the rmal gradients complicate the interpretation of emission spectra of ai rless bodies, they tend to enhance certain spectral features, and ther efore emission spectroscopy should be useful for remote sensing of the surfaces of the Moon and Mercury.