Volatile organic sulfur compounds as biomarkers complementary to methane: Infrared absorption spectroscopy of CH3SH enables insitu measurements on Earth and Mars

As universal products of biological processes, volatile organic sulfur compounds such as methyl mercaptan (CH3SH) may be essential in the search for signs of life on Mars and in exoplanet atmospheres. Methyl mercaptan is implicated in the origin of life at sites of low-temperature hydrothermal activity driven by serpentinization. Serpentinization may occur on Mars, in icy satellite oceans, and in other small wet bodies to a greater extent than on Earth, with important implications for life. We characterized absorption features in pure laboratory sample spectra of CH3SH using the Carbon Isotope Laser Spectrometer (CILS), an infrared (3.27μm) tunable diode laser spectrometer with capabilities nearly identical to those of the Tunable Laser Spectrometer (TLS) instrument on the Mars Science Laboratory. The molecular species proves detectable by CILS and TLS at the sensitivities approaching the level of parts per trillion with pre-concentration. These measurements demonstrate the possibility for detection of methyl mercaptan, with implications for its possible use as an in situ biosignature for Earth-based and extraterrestrial exploration.

Research highlights
► Methyl mercaptan has multiple diagnostic absorption features in the 3056 cm−1 (3.2μm) spectral region probed by the Tunable Laser Spectrometer (TLS) on the Curiosity Mars Science Laboratory rover.
► CILS (and TLS) can detect CH3SH at the tens of parts per billion, well below the approximate odor threshold for detection of 1 ppm.
► Sample pre-concentration using the SAM suite enables detection of CH3SH at tens of parts per trillion by TLS.
► Methyl mercaptan features do not impede detection of methane.
► Dimethyl sulfide is not detectable by TLS.