Effect of Solar radiation on the Distribution of Raman Biosignatures in Salt Nodules from the Atacama Desert

Kurzfassung

Raman instruments are part of the next two rover missions to Mars: ESA/Roscosmos’s ExoMars2020 and NASA’s Mars2020. A better understanding of potential habitable environments and putative biosignatures, using analogue environments such as the Atacama Desert, is of paramount importance to support and interpret future spectroscopic data correctly. In the Atacama Desert, one of the driest places on Earth, life has developed adaptive strategies to decreasing amounts of water: from refuges inside or below rocks as endoliths or hypoliths to inside salts in hygroscopic niches [1]. In the hyperarid core one of the last refuges for life are inside salt crusts using deliquescence as a water source or being in the subsurface waiting for transitory episodes of increased moisture [2]. These adaptive strategies might also apply to putative Martian life which endured a transition from a water rich past to the very harsh surface conditions of the present giving us clues on where to best look for traces of life on the Red Planet. Salt crusts and salt nodules are particularly interesting targets in this regard because they reside on or very near the surface and are thus easily accessible to future robotic missions. In the Atacama, salt nodules have been shown to host photosynthetic organisms containing easily identifiable pigments by Raman spectroscopy such as carotenoids and scytonemin. These salt nodules are mostly composed of halite and are associated with polygonated soils, but their formation processes are still not fully understood. Salt nodules occur in varying morphologies which can control micro-environmental conditions and possibly microbial colonization (habitation of “micro-niches”). One of the most damaging factors for life and its remains, both in the Atacama and on Mars, is solar radiation. To investigate the spatial distribution of potential Raman signatures in micro-niches we generated georeferenced 3D-reconstructions of the sampling areas using photogrammetry techniques and plotted the dose received according to the nodules’ orientation. We then analysed salt nodule along dry cut north-south thick sections using Raman mapping to infer any relations between the amount of light received and the presence of detectable signal. Preliminary data show an increased presence of carotenoids signal on the nodules oriented toward the south, which are the-more-protected sections of the nodules. [1] A.F. Davila and D. Schulze-Makuch, Astrobiology, 16,159-168, (2016). [2] D. Schulze-Makuch, D. Wagner, S.P. Kounaves, K. Mangelsdorf, K.G. Devine, J.-P. de Vera, P. Schmitt-Kopplin, H.-P. Grossart, V. Parro, M. Kaupenjohann, A. Galy, B. Schneider, A. Airo, J. Frösler, A.F. Davila, F.L. Arens, L. Cáceres, F.S. Cornejo, D. Carrizo, et al., PNAS, 115, 2670–2675 (2018).