Avoiding misidentification of bands in planetary Raman spectra

Raman spectroscopy has been identified as a powerful tool for astrobiology and remote robotic planetary exploration. It can be used to identify and characterise rock matrices, mineral inclusions and organic molecules and is demonstrably effective at identifying biomarkers, or indicators of biological activity. The ExoMars rover, jointly operated by the European and Russian Federal Space Agencies, will carry the first Raman spectrometer into space when it launches in 2018 and two further Raman instruments have recently been announced as part of the payload onboard the National Aeronautics and Space Administration's Mars 2020 rover. Each of these spectrometers however will, by necessity, have poorer resolution than the most sophisticated laboratory instruments because of mass, volume and power constraints and the space readiness of the requisite technologies. As a result, it is important to understand the minimum instrument specification required to achieve the scientific objectives of a mission, in terms of parameters such as spectral resolution and laser footprint size. This requires knowledge of the target minerals and molecules between which there may be ambiguity when identifying bands in spectra from geological samples. Here, we present spectra from a number of Mars analogue samples that include a range of such molecules, highlighting where such confusion may occur and identifying the most useful bands for differentiation. It is recommended that a Raman spectrometer achieves a resolution of at least 3cm^-1 and covers a spectral range from 100 to 4000cm^-1 in order to differentiate between all of the target molecules presented here.