Thermal infrared emissivity of felsic-rich to mafic-rich analogues of hot planetary regoliths

Abstract

Inner Solar System bodies with absent or extremely tenuous atmospheres such as Mercury and several high-eccentricity Near Earth Objects (NEOs) are subject to a wide range of surface temperatures. Here we show how the thermal expansion induced by strong diurnal insolation variations affects the thermal infrared spectra of felsic-rich to mafic-rich regoliths. 1400-700 cm−1 spectra of granular plagioclase-pyroxene mixtures were measured in the laboratory at four temperatures between 425 K and 725 K, simulating different daily surface temperatures within the Solar System. At the same time, we describe for the first time how two minerals with different thermal expansion coefficients combined in the same granular sample appear in a non-ambient (i.e., non-298 K) thermal infrared spectrum. The spectra of plagioclase-rich analogues appear to be dominated by felsic features regardless of the daily surface temperature, and do not exhibit any of the short-wavelength absorptions of pyroxenes. When detectable, on the other hand, the 870-cm−1 pyroxene absorption, which is not affected by thermal variations, functions instead as a proxy for the solid-solution composition (i.e., the Mg-Fe relative contents). In contrast, indications of the plagioclase solid solution occurring at ambient temperatures (i.e., Ca-Na relative contents) disappear at temperatures over 525 K. Christiansen feature position and spectral contrast, which show a clear dependency on composition even at high temperatures, require an accurate reproduction of the shallow thermal gradient in order to be used as indices. In general, in order to decipher planetary hyperspectral data, surface temperature acquisition seems to be a prerequisite, as well as a comparison with analogue spectra obtained under realistic environmental conditions.