New laboratory measurements of mid‐IR emission spectra of simulated planetary surfaces

Abstract

We present emission spectra of particulate quartz measured in an environment 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 different, in that our samples were heated at the base by a temperature‐controlled hot plate rather than from above by a solar lamp in order to separate 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 uppermost layer of the material. The contrast of the emission peak is maximized under vacuum conditions, for which it is estimated that a temperature difference of at least 40 K existed within the top emission skin depth. The wavelength location of the emission peak occurs near the Christiansen wavelength at 7.35 μm but has been shifted by approximately 0.2 μm to shorter wavelengths. This result is in agreement with the earlier results of Logan et al. [1973] and points out that the existence of a thermal gradient violates the conditions required by Kirchoff'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 chamber increased the conductivity of the soil, mitigating the thermal gradient and decreasing the contrast of the emission maxima. Although thermal gradients complicate the interpretation of emission spectra of airless bodies, they tend to enhance certain spectral features, and therefore emission spectroscopy should be useful for remote sensing of the surfaces of the Moon and Mercury.