Minerals and microstructure identification using Raman instruments: Evaluation of field and laboratory data in preparation for space mission

Two rovers will be launched in 2020 for robotic exploration missions on Mars: the ESA/Roscosmos ExoMars and the NASA Mars 2020 missions. Both missions will include Raman instruments to characterize the habitable of Mars and to search for molecular evidence of past and present life. In preparation for these missions and to characterize the scientific information that will eventually return from Mars, terrestrial analogue samples of rock formations on Mars are studied in detail. During this study, we compared Raman spectra obtained from different samples of geological matrixes, including specimens of silicate, sulfate, carbonates, and sulfurs, with two Raman instruments. Raman data were obtained in the field with a portable instrument operating with a 785-nm laser. Based on fast data analyses in the field, a selection of samples were collected and were further analyzed using a benchtop confocal instrument operated in the laboratory and with two excitation lasers at 532 and 785 nm. We illustrate the possible interference from sharp fluorescence associated with rare earth element present in trace amount in minerals. In addition, carbonaceous compounds and beta-carotene, a highly Raman-active biomarker, were also detected by Raman spectroscopy in association with specific minerals. In the context of planetary exploration, we discuss the capacity for specific mineral matrixes to sustain life under extreme conditions. We also discuss that Raman imaging, although not ready yet for space application with miniaturized spectroscopy, could significantly help in discussing the possible biotic origin of carbonaceous residues, especially by the association of molecular signatures with the localized microstructure.