Characteristic Raman bands of amino acids and halophiles as biomarkers in planetary exploration

Abstract

Evaporitic environments that could provide habitats for life exist on present day Mars, in addition to Recurring Slope Lineae (RSL), which have been observed suggesting a briny liquid in the subsurface with a seasonal occurrence at the surface. Life detection is the focus for future missions, including use of the Raman Laser Spectrometer (RLS) instrument on board the ESA ExoMars rover for this purpose. Martian simulation chambers were used to expose a subset of the α-amino acids (AA) and halophilic (‘salt-loving’) microbes to the surface and near surface environment. Post-exposure, the detectability of biological molecules with Raman spectroscopy (where the Raman spectrum is the biomarker) was examined, with the intention to inform future missions. Results from this work suggest that a statistical method (independent of band intensity) should be used to rigorously define a set of characteristic Raman bands to unambiguously identify AA, a technique applicable to all biomolecules. Following exposure, both AA and halite entombed archaeal halophiles survived a combination of simulated martian conditions, e.g. freeze-thaw cycling resulted in a maximum of 20 % biomarker signal loss for AA. Halophilic microbes survived UV exposure up to 3.5 sols, though with a complete biomarker signal loss. However, cell counts and hence, survival in near surface, freeze-thaw conditions (indicative of RSL conditions) remained similar to the control sample. Furthermore, the Raman biomarker signal remained intact and detectable, regardless of a reduced intensity. Importantly, for AA doped onto crushed basalt the Raman band biomarkers were lost, which has major implications for the ExoMars rover which will crush rock samples before examination with the RLS instrument. Future missions should be designed to examine samples with the Raman spectroscopy instrument prior to any crushing to avoid destroying the biomarker signal that could be used to imply the presence of life.