-carotene as a Potential Biomarker in Martian Evaporitic Rocks: Evaluation of Raman Microspectrometric Analysis

Introduction: Based on the latest results of Martian exploration [1-3], evaporitic rocks on the Martian surface are proposed as potential habitats for microbial (halophilic) life on Mars [4,5]. At least, there is a reasonable possibility that these rocks may sustain molecular remnants (biomarkers) as evidence for the presence of extinct or extant living organisms. It has been shown that Raman spectrometry is a powerful tool for the characterization of various biomarkers which are produced by halophilic or halotrophic microbes as part of their survival strategy [6,7] and in this respect carotene is considered a suitable biomarker. In this work, Raman microspectrometry was tested as a nondestructive method of determining the lowest detectable -carotene content in experimentally prepared evaporitic matrices – namely, gypsum, halite and epsomite. Here, 514,5 nm excitation was adopted because of the Resonance Raman enhancement in the carotenoid analysis, and a 785 nm laser source as a more universal wavelength which is now much used in the detection of biomolecules terrestrially. Powdered mixtures were measured directly as well as with a laser beam penetrating the crystals of gypsum and epsomite. The number of registered -carotene Raman bands differed depending on the particular mineral matrix and the excitation wavelength. Concentrations of βcarotene of about one order of magnitude higher were identified when analyzed through single crystals of gypsum and epsomite, compared to the analysis of the mixtures alone.