Abstract
Oxidation on Mars is primarily caused by the high influx of cosmic and solar radiation which interacts with the Martian surface. The evidence of this can be seen in the ubiquitous red colouration of the Martian sediment. This radiation will destroy most signals of life in the top few metres of the Martian surface. If organic carbon (one of the building blocks of life) is present within the accessible Martian sediments, it is very likely that it will have experienced some oxidation. ESA׳s ExoMars mission set to fly in 2018, has on board a miniaturised Raman spectrometer. As Raman spectroscopy is sensitive to carbonaceous material and will be primarily used to characterise organics, it is essential that the effect oxidation has on the Raman carbon signal is assessed. Oxidised carbonaceous shales were analysed using Raman spectroscopy to assess this issue. Results show that haematite has a band which occurs in the same frequency as the carbon D band, which cannot be distinguished from each other. This can lead to a misidentification of the carbon D band and a misinterpretation of the carbon order. Consequently, caution must be taken when applying Raman spectroscopy for organic carbon analysis in oxidised terrestrial and extraterrestrial environments, including on Mars.
Highlights
IntroductionDue to an extremely thin atmosphere and lack of a global magnetic field, galactic cosmic rays (GCRs) and solar energy particles (SEPs) penetrate the Martian surface (e.g. Dartnell et al, 2007)
One of the most prominent differences is in the D band morphology, which shows an increase in band intensity and width compared to spectra from non-oxidised samples
This shows that haematite interference is causing the change in spectral parameters in the D band frequency, and that the carbon is not affected by hydrofluoric acid (HF) treatment
Summary
Due to an extremely thin atmosphere and lack of a global magnetic field, galactic cosmic rays (GCRs) and solar energy particles (SEPs) penetrate the Martian surface (e.g. Dartnell et al, 2007) This irradiation reacts with H2O molecules and can generate HO. High concentrations of oxidised iron have been observed by MER Opportunity at Meridiani Planum (Hurowitz et al, 2010) and MSL Curiosity (Blake et al, 2013) at Gale Crater, and more widely at Gale Crater by CRISM (Fraeman et al, 2013) This radiation will likely destroy any signatures of life such as biomarkers, over a thousand million years (Pavlov et al, 2013), as well as cells or spores which lie in the top few metres of the Martian soil (ten Kate et al, 2005; Schuerger et al, 2006)
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