PROSPECTING FOR MARTIAN SULFIDE ORE DEPOSITS USING THERMAL-INFRARED EMISSION SPECTROSCOPY

On Earth, magmatic sulfide deposits are host to resources such as Ni, Cu, and the platinum-group elements, critical to the global economy. Despite their economic significance, very little research has been conducted on potential technologies or techniques to prospect for such resources on other planetary bodies. Therefore, an experiment was conducted to test the viability of thermal-infrared emission spectroscopy as a remote or in-situ prospecting tool for extraterrestrial magmatic sulfide ore deposits.

Thermal-infrared emission measurements of two suites of physically constructed 500-710 μm silicate/sulfide mixtures were acquired from 2000–250 cm^-1 (5–40 μm) using a Nicolet iS50 FTIR modified for emission. Emissivity spectra were calibrated using the conventional calibration method used in remote sensing applications and a new reference calibration method developed specifically for low-emissivity materials such as pyrrhotite. Our results suggest that several indicator variables can be used in tandem to prospect for extraterrestrial sulfide deposits, including: (1) the apparent brightness temperature, (2) the appearance of a negative slope from high to low wavenumbers in the emissivity spectrum calibrated using the conventional calibration routine, (3) a decrease in the emissivity value at the Christiansen Frequency of the silicate endmember when calibrated using the reference temperature method, and (4) a lack of transparency features in “intraband” regions of the spectrum coupled with (5) a reduction in spectral contrast at the Reststrahlen Bands and other spectral absorptions of the silicate component. The deconvolution results confirm that linear mixing is a valid assumption for infrared emission spectra containing a spectrally featureless component.

Additionally, a spectral deconvolution technique was applied to each mixture spectrum to determine if the abundance of sulfide in our physically constructed mixtures could be accurately predicted. The spectroscopically determined modal abundance of sulfide is within ~12 ± 7 vol % of the measured abundances using the conventional calibration technique and ~6 ± 4 vol % when using the reference temperature calibration technique. Regardless of the calibration method, the detectability limit is ~10% for the sulfide endmember.