Biosignature detection in Mars Analogue Aqueous Alteration Minerals Using Raman Spectroscopy

The Mars Exploration Program Analysis Group states in their documentation regarding science goals, objectives, investigations and priorities, that the number one goal of Mars exploration is to determine if Mars ever supported life. A Raman spectrometer is suggested to be a prime candidate instrument for the detection of organic compounds, the building blocks of terrestrial life. Though Raman spectroscopy has no space heritage, both NASA and ESA have Raman instruments planned for their next Mars rovers. The ideal place to search for organic matter which could provide evidence for past or present life would be in the presence of minerals that were formed in water. This study was carried out to determine if a molecular biosignature can be detected in a mixture with either a phyllosilicate or a hydrated sulphate. We present this data at UV excitation wavelength using a prototype in-situ stand off Raman system intended to represent an instrument for a rover platform. Data is also presented for the same samples using green excitation wavelength and a commercial Raman microscope. The spectral resolution of the short focal length prototype in-situ UV Raman system leads to less numerous bands than the green Raman microscope system. Ultraviolet electronic absorption transitions in the samples containing transition metals, either as part of a mineral's crystal structure or as impurities, result in no Raman spectra from the minerals and poor or limited detection of the organic. Only the synthetic calcium sulphate dihydrate, which contains no transition metals, allowed for both acquisition of the mineral spectrum and detection of the embedded biosignature at all concentrations tested using either excitation wavelength. Certain samples allowed for the detection of an organic through the observation of a C-H stretching vibration, but did not provide enough clear peaks to define which organic was present.