Raman spectroscopy for detection of biological matter in Mars analogue material

Abstract

Introduction Raman spectra will be measured with the Raman Laser Spectrometer (RLS) onboard ExoMars in 2018 to identify organic compounds and mineral products as an indication of former or recent biological activi-ty. Investigation with the same specifications as those onboard the ExoMars mission is conducted to test the potential of identifying biological material on martian analogue material with Raman spectroscopy. Appropriate parameters concerning integration time and number of repititions for the detection of biological matter as well as for the determination of the mineral composition will be derived. In addition, problems are reported on using Raman spectroscopy to discriminate the microorganisms from the mineral background. Biological sample Cyanobacteria and methane producing archaea are chosen to represent potential life on Mars. Prokaryotes like archaea and bacteria appeared on early Earth at least 3.8 to 3.5 billion years ago (Gya). Life might have developed under similar conditions on Mars as on Earth or might have been transferred from Earth (or vice versa). At that time on Mars the climate was more temperate and wet compared to the present day as inferred from geological evidence for liquid water on the ancient martian surface. Methane is known to be present on Mars. A source is still unknown. Methane might originate from geothermal or biological activities nearby the surface of the red planet. Cyanobacteria and prokaryotes using photosystem I use pigments such as scytonemin and beta-carotene as UV protection. Especially beta - carotene emits a strong Raman signal at the applied laser excitation wavelength. Raman measurements are used for detection of coccid, chain, and biofilm forming cyanobacteria Nostoc commune strain 231-06 (Fraunhofer IMBT CCCryo) on the below described Mars analogue mineral mixtures. Nostoc commune is known to be resistant to desiccation, UV B radiation and low temperatures, and thus suitable as a candidate for a potential life form on Mars. Furthermore, the Raman technique is applied on samples of the methane producing archaea candidatus Methanosarcina gelisolum (strain SMA 21) isolated from Siberian permafrost affected soils and on these archaea embedded in the martian analogue material. Martian analogue material In this investigation two different Mars analogue materials prepared from mineral and rock mixtures are used. The (1) Phyllosilicatic Mars Regolith Simulant (P-MRS) and (2) Sulfatic Mars Reg-olith Simulant (S-MRS) reflect the current understanding regarding environmental changes on Mars. Weathering or hydrothermal alteration of crustal rocks and of secondary mineralization during part of the Noachian and Hesperian epoch followed by the prevailing cold and dry oxidising condition with formation of anhydrous iron oxides. The use of two different mixtures accounts for the observations that phyllosilicatic deposits do not occur together with sulphatic deposits. P-MRS and S-MRS serve as the analogue geomaterials in which the cells of cyanobacteria and of methanogenes are embedded. Results Varying periods of measurement time and number of repetitions are used to get optimal Raman spectra for cyanobacteria and methanogenes. If cyanobacteria are present, beta-carotene is the dominant feature in the spectrum. Measurement times need to be adjusted to obtain optimal spectra of the P-MRS and S-MRS with cyanobacteria. Measurements performed with various values of measurement time and number of measurements show clearly the improvement achieved by increasing the time per spectrum from 1s to 20s. But it is desirable to find a set of small values of measurement time and number of repetitions in order to optimize the detection of minerals and biological markers and to reduce the disturbing effect of cosmic rays. A measurement regime is proposed for mineral mixtures with cyanobacteria on the basis of the RLS instrument characteristics: A procedure on ExoMars should start with a measurement time of only a few seconds to identify both biomarkers and minerals. If no biomarkers can be identified the time and number of measurements need to be increased until spectra of minerals are obtained. The measurement time should be selected between 1s (for b-carotene) and 20s (for minerals) for a laser power of 1mW (spot diameter < 2 µm). Future investigations of Raman measurement parameters should consider the different environmental parameters on Mars like atmospheric pressure, composition and temperature. For methanogens a different measurement regime needs to be developed. Raman analytics are capable to identify biosignatures like beta – carotene on a multi-mineral mixture similar to those expected to be encountered during the ExoMars mission