Beta-carotene-A possible biomarker in the Martian evaporitic environment: Raman micro-spectroscopic study

Due to the discovery of the evaporitic environment on the Martian surface, there is a reasonable possibility that evaporites served (or still serve) as habitats for microbial life if ever present on Mars. At the very least, if no signatures of extant life exist within these rocks, it may sustain molecular remnants as evidence for living organisms in the past. beta-Carotene, among other carotenoids, could be such a suitable biomarker. In this study, Raman micro-spectroscopy was tested as a nondestructive method of determining the presence of beta-carotene in experimentally prepared evaporitic matrices. Samples prepared by mixing beta-carotene with powdered gypsum (CaSO4 center dot 2H(2)O), halite (NaCl) and epsomite (MgSO4 center dot 7H(2)O) were analyzed using a 785 nm excitation source. Various concentrations of beta-carotene in the matrices were investigated to determine the lowest beta-carotene content detectable by Raman microspectroscopy. Mixtures were also measured with a laser beam permeating the crystals of gypsum and epsomite in order to evaluate the possibility of identifying beta-carotene inside the mineral matrix. We were able to obtain a clear beta-carotene signal at the 10 mg kg(-1) concentration level-the number of registered beta-carotene Raman bands differed depending on the particular mineral matrix. Spectral signatures of beta-carotene were detected even when analyzing samples containing 1 mg kg(-1) of this molecule. The 10-100 mg kg(-1) of beta-carotene in mineral matrices (halite, epsomite) was detected when analyzed through the monocrystal of gypsum and epsomite, respectively. These results will aid both in-situ analyses on Mars and sample analyses on Earth. (C) 2008 Elsevier Ltd. All rights reserved.