THE DETECTION OF BIOMARKERS IN SALT MIXTURES BY LASER RAMAN SPECTROSCOPY

Biomarkers targets during the next Mars surface explorations (NASA’s 2020 rover mission to Mars) are organic compounds with structures diagnostic for biological processes, particularly those that are geologically stable under harsh environmental conditions. Ancient terrestrial biomarkers that have survived extensive diagenesis and catagenesis are suggestive of the compound classes that might be most recalcitrant. We have conducted preliminary tests to determine the feasibility of detecting biomarker classes at low concentrations inrobotic surface explorations.

Laser Raman spectroscopy (LRS) was been proposed to be a powerful tool for in situ characterization of molecular species in surface and subsurface materials on Mars, for inorganic (minerals) and organic compounds. In this study, LRS was used to detect seven organic compounds representative of biomarker classes, mixed in the powder of hydrous Ca-sulfates (CaSO4.2H2O, gypsum, found with large quantity and wide spread on Mars) at various concentrations. The starting concentration of biomarkers in mixtures that we made was 1 mole%. A linear scan of 100 spot measurements was made on each of seven mixtures, using a microbeam Raman spectrometer with ~ 9 mw of 532 nm excitation beam (diameter ~ 6 µm) without auto-focusing at each sampling spot. After first set of LRS measurements, the mixtures were further diluted, step by step, until no Raman peaks of biomarkers can be detected. We found that all seven biomarkers have their own finger-print Raman spectra that can be used to identify from each others. In addition, we found that octadecane was detected down to 0.1 mole %, cholesterol was detected down to 0.01 mole%, and beta-carotene was detected down to 0.001 mole% in gypsum mixtures.

A microbeam Raman system with line scan capability will be proposed for NASA 2020 rover mission to Mars, by a team of scientists from Washington University in St. Louis and engineers from Jet propulsion Laboratory. The use of microbeam for in situ Raman measurements will ensure the Raman signals from the minor and trace species in the sampled area of planetary materials (rock and soil) not to be concealed by the signals of bulk components. The use of Raman line scan will increase the probability to encounter the needle (biomarkers) in hay stack (rock and soil).