Martian chronicled: By Raman

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  • Published: Oct 1, 2015
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Martian chronicled: By Raman

Mars rover

Samples of tephra from two Icelandic volcanoes were prepared and analysed by Raman spectroscopy, XRD and FTIR spectroscopy.  Credit: J Raman Spectrosc/Wiley

Seven samples of Icelandic volcanic fragments, tephra, from the Katla volcano and Eldgjá, have been analysed using X-ray diffraction (XRD), Raman spectroscopy and Fourier transform infrared (FTIR) as a proxy for Martian rocks to show how a comprehensive library of spectra might be built up ahead of a fact-finding mission to the Red Planet.

Emily Bathgate, Linda Xiao, Barbara Stuart, of the School of Chemistry and Forensic Science, at the University of Technology Sydney, in Broadway, Helen Maynard-Casely of the Bragg Institute, at the Australian Nuclear Science and Technology Organisation, in Kirrawee, New South Wales, Graziella Caprarelli of the Division of IT, Engineering and the Environment (DITEE), at the University of South Australia, Adelaide, Australia, Kate Smith of the University of Exeter, Penryn, Cornwall, UK, and Ross Pogson of the Australian Museum, Sydney, New South Wales, Australia, suggest that libraries of Raman spectra will be essential to future exploratory missions to Mars, running water or not. Indeed, Raman instruments will most likely be aboard the European Space Agency's 2018 ExoMars Rover mission and the National Aeronautics and Space Administration's 2020 Rover.

Spectroscopic library

A spectroscopic library based on known minerals will allow data from samples tested on Mars by such rovers to be interpreted in situ. Raman and FTIR are often complementary as characteristic peaks present in one might be absent in the other, while X-ray diffraction provides almost definitive data on crystalline materials. Working together these three can provide highly detailed information for mineral identification.

There are, the team explains in the Journal of Raman Spectroscopy three primary Raman modes used in planetary science: remote Raman, also known as stand-off Raman, in situ Raman, and laboratory microspectroscopy. The latter is perhaps off-limits until we have human technicians on the planet. Remote Raman works on a sample distant from the device from one metre to 20 m, but this gives rise to interference and fluorescence effects. A more rugged instrument is needed for in situ Raman but analyses a sample at just 2 to 5 millimetres using optical fibres. Both of these techniques, the team says are close in methodology to the types of analytical instrumentation and procedures they expect to be on board planetary landers.

Conditions on the Red Planet will not be optimal for any analysis, with many mixed mineral particles some of which will be of an inadequate size for good results. There will be minerals of volcanic origin and other confounding factors. There is thus a need for a robust and substantial comparison library of minerals to which analysts have access here on Earth. "The Martian surface is known to be mostly covered by basalts," the team says. "Only a few instances of evolved siliceous rocks have been documented." As such, volcanic matter from Iceland, which have many similarities to Martian rocks can often be used as a proxy, or analogue, for the kinds of samples a future ESA or NASA rover might be expected to analyse on the Red Planet.

Mineral ID

The team has demonstrated that if they can avoid fluorescence interferences, then Raman spectroscopy has a 90% identification success rate (compared with 77% success with that problem), which is slightly better than the X-ray diffraction success rate of 89% but without the complexities of the latter technique.

The team concludes that, "Overall, our investigation reinforces previous work findings that Raman spectroscopy and XRD are a powerful combination to obtain information across the spectrum of mineral sizes likely to exist in the Martian rocks and regolith." They add that the XRD results from the CheMin instrument aboard Curiosity on Mars, which we have mentioned previously on SpectroscopyNOW, show numerous key minerals on the surface of the planet, many of which were also present in the samples examined in the present study. They add that, "Essential to the effectiveness of Raman spectrometers deployed to Mars is the collection in tandem of data from other techniques, such as the XRD."

Related Links

J Raman Spectrosc 2015, online: "Raman, FTIR and XRD study of Icelandic tephra minerals: implications for Mars"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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