Thermal‐Infrared Emission Spectroscopy of Graybody Minerals (Sulfide): Implications for Extraterrestrial Exploration for Magmatic Ore Deposits

Abstract

AbstractGraybody materials exhibit systematically low emissivity across their spectrum. This characteristic violates the key assumption of unit emissivity at some wavenumber in the spectrum used to calibrate thermal‐infrared emission data. This assumption makes graybody materials “appear colder” than their actual physical temperature and imparts a slope in emission spectra that is non‐physical in nature, both of which affect interpretations of planetary surfaces. Pyrrhotite derived from the Stillwater Complex's J‐M Reef in Montana, USA exhibits systemic graybody behavior across its mid‐infrared spectrum and thus has a steep negative spectral slope from high to low wavenumbers when calibrated using conventional methods. A new measurement technique is introduced for deriving the absolute emissivity of graybody materials using reference samples with known Christiansen Frequencies during calibration. The reference temperature method significantly reduces the spectral slope of and provides a more accurate estimation of the absolute emissivity of graybody materials. After correcting the temperature of pyrrhotite using results from a series of reference experiments, we conclude that the emission spectrum of pyrrhotite is spectrally featureless and has a maximum emissivity of ∼0.7. If sulfide mineral deposits are exposed on Mars, they will not be identified using spectral features found in the mid‐infrared (5–40 μm). However, they could be located by identifying basaltic terrain that appears colder than their surroundings and with apparent emissivity spectra that exhibit negative spectral slopes from high to low wavenumbers and are spectrally neutral.