Raman spectroscopy for Mars : an interdisciplinary approach

The Raman Laser Spectrometer (RLS) aboard the Rosalind Franklin ExoMars rover is among the first instances of Raman spectroscopy, a technique well-established in the study of mineralogy and molecular structures, being used in the search for life on Mars. This thesis is focused on increasing the scientific return of the ExoMars RLS in the particular context of (presumed) challenging mixed mineralogy samples of exobiological interest including lacustrine sediments and evaporite minerals; more specifically, clay (phyllosilicate) minerals and nitrate salts. To accomplish this goal the thesis provides an end-to-end assessment of the capabilities of the RLS, with insights into how interdisciplinary approaches can increase its scientific return. The interdisciplinary approach adopted sought to maintain continuity between field observation, sample processing and spectral analysis using terrestrial analogues and flight equivalent hardware. This approach also forms an accessible framework for replicating ExoMars RLS processes and includes an in-depth review of calibrating an RLS equivalent repositioning state. The stage can be positioned to within 20μm and return to a previous position within 5μm. Constructed and tested using off-the-shelf hardware, it is well suited to research groups seeking to add positioning capabilities to analogue ExoMars systems. The Tecopa Basin, a former terminal basin in the Mojave Desert, California, was selected as sufficiently analogous to the proposed Oxia Planum rover landing site. The location provided samples with smectite clays which are abundant at Oxia Planum and subsurface evaporates, notably biologically accessible nitrogen which is a prerequisite for known forms of life but yet to be directly observed on Mars. Fourteen well documented and contextualised samples were taken from the basin with efforts made to first understand their stratigraphic contexts and relations to other samples in the data set. Sample contents analysed with RLS flight equivalent hardware, and ExoMars sample handling processes were identified in reference to spectral databases, in-situ field observations, laboratory standard minerals and cross-sample referencing. It was demonstrated that the RLS can differentiate geochemical changes across stratigraphic horizons within lacustrine sediments, including evaporite solubility gradients. However, detrital phyllosilicates and low concentrations of nitrogen compounds are extremely challenging to detect using RLS measurement protocols. This is due to weak Raman scattering of phyllosilicate minerals requiring sampling strategies distinct from those used for evaporites, a high variability in the contents of natural crushed mixtures and the nominal Raman sampling strategies being insufficient for the detection of nitrate at putative Martian concentrations. Whilst there is increasing scientific interest in phyllosilicates, this project would also advocate that additional resources both in the build up to and during the mission should be allocated to identification of nitrates on Mars given the unique capability of the ExoMars rover to sample the shallow subsurface and the importance of a robust subsurface nitrogen inventory for tracing Martian habitability and the evolution of the planet's atmosphere.