Raman Detection Threshold Measurements for Acetic Acid in Martian Regolith Simulant JSC-1 in the Presence of Hydrated Metallic Sulfates

Abstract

Several measurements, including the data from the laser spectrometer in the Sample Analysis at Mars instrument suite on the Curiosity Rover in Gale crater, have measured seasonal variations in atmospheric methane at the 0.7 ppbv (parts per billion volume) level. As a result, models have been proposed to understand the methane production including novel chemical, geological, meteorological, and biological mechanisms. Biological models often rely on methanogenic extremophile archaea which might be similar to some Earth based organisms which can be studied exhaustively in a laboratory setting. Such organisms might thrive in a subsurface ecosystem involving water and methane and as such could leave a unique biosignature in the Martian regolith that could be preserved over extended periods of time. The resulting mixture of carboxyl, acetyl and hydroxyl groups blended with the metallic sulfates and regolith constituents could produce a Raman signature detectable on the next rover mission if the concentration is high enough. Here we measure the 532 nm and the 780 nm Raman spectra of a variety of molar concentrations of several hydrated metallic sulfates mixed with JSC-1 Martian regolith simulant to identify the signature concentration thresholds for the isolated C=O Raman peak from acetic acid. We find a Raman peak removed from most fragmented molecular Raman bands near 1608 cm-1 using the 532 nm laser for the Fe and Mg sulfates to yield a threshold in the 120 - 160 ppmv range for a cutoff S/N of 2.