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Graybody 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. Plain Language Summary: An integral step to off‐Earth human settlement is identifying the minerals with the critical metals required to support a new form of human life in space that would be heavily dependent on robotics and technology. This work investigates whether remote thermal infrared emission spectroscopy can be used to identify extraterrestrial sulfide mineral deposits. For this purpose, we acquired emissivity measurements of a 100% pure sulfide (pyrrhotite; Fe(1−x)S) derived from the Stillwater Complex's J‐M Reef. We show that when deriving emissivity using the conventional calibration routine, pyrrhotite's spectrum exhibits a steep spectral slope and is nearly featureless. The slope results from an incorrect estimation of pyrrhotite's temperature. The temperature error stems from pyrrhotite exhibiting systematically low emissivity, which violates the key assumption that somewhere in its spectrum that the emissivity is equal to that of a blackbody at the same temperature. A new laboratory procedure was developed to constrain the temperature error and produce a more accurate emissivity spectrum of pyrrhotite, revealing that pyrrhotite exhibits low emissivity in the mid‐infrared. Rather than relying on absorption features for prospecting, temperature errors, the presence of a spectral slope, and a featureless spectrum can be used to prospect for sulfide mineral deposits across the solar system. Key Points: We developed a new calibration and measurement technique for deriving the absolute emissivity of graybody materials in the mid‐infraredPyrrhotite is spectrally neutral and exhibits systematic graybody behavior, with a maximum absolute emissivity of ∼0.7–0.6 from 5 to 40 μmAnomalously cold brightness temperatures and sloped, featureless emissivity spectra may indicate the presence of sulfide deposits on Mars [ABSTRACT FROM AUTHOR]