Abstract
Raman spectrometry is a powerful technique for the rapid identification of most minerals and organic chemicals without sample preparation. In this context, the European Space Agency (ESA) and NASA selected a Raman spectrometer in the payload of the future ExoMars and Mars 2020 missions to identify organic compounds and mineral products indicative of biological activity on Mars. Little is known, however, about the effects of Mars atmospheric conditions on instrument performance and on the Raman spectra. The objective of this study was to i) design and construct a versatile simulation chamber to reproduce the atmospheric conditions expected inside a rover on Mars, ii) to test the performance of a previously designed breadboard miniaturized Raman laser spectrometer (RLS) inside the chamber. The Mars Atmosphere Simulation Chamber (MASC) is a temperature and atmosphere controlled chamber. It includes an innovative heating-cooling system to create homogeneous temperatures inside the chamber that can be varied between 243 K and 283 K, while the charged coupled device (CCD) of the Raman spectrometer can be independently cooled (e.g., 233 K). A vacuum and gas control system permits evacuation of the chamber and the subsequent introduction of any (dried) gas mixture at partial pressures between 1 mbar and several bars. The minimum CCD temperature was found to depend on the surrounding MASC temperature and atmosphere. A vertical shift of 3 pixels on the CCD was observed for the Raman signals upon lowering the temperature from 283 to 253 K. We show that the RLS instrument gives reliable Raman spectra over the tested range of temperatures and from a vacuum of 4 x 10−5 mbar to a CO2 atmosphere at pressure relevant to Mars (8 mbar). For example, the Raman spectra of three test minerals, calcite, aragonite and baryte, showed identifiable Raman peaks with Raman shift values within ± 1 cm−1 of those reported in previous works under terrestrial conditions. This confirms that a RLS instrument is useful for the identification of minerals during future missions to Mars; once a necessary detector recalibration was carried out, the system performed well under 8 mbar pressure and 243 – 283 K. The MASC was found to be a versatile instrument; it can provide important information on instrument performance under Martian conditions and other temperatures and atmospheric conditions can also be simulated.
Highlights
Fundamental questions remain about the geological history of the planet Mars [1,2,3]
Design of the charge coupled device (CCD) detector The Raman Laser Spectrometer (RLS) instrument used for this study was an elegant breadboard for the ExoMars mission, originally designed as a combined Raman and Laser Induced Breakdown Spectroscopy (LIBS) instrument [24]
Temperature control inside the Mars Atmosphere Simulation Chamber (MASC) A representative low temperature experiment under Martian conditions inside MASC consists of five stages: (1) loading the sample tray, (2) evacuating the MASC to remove all water (4 × 10−5 mbar), (3) pumping 8 mbar of CO2 into the MASC, (4) setting the Lauda cooling machine to a specific temperature (5) recording the temperature data
Summary
Fundamental questions remain about the geological history of the planet Mars [1,2,3]. One key question is whether free water has been persistent on the Mars surface to support life. The weathering and erosion products are evidence of the presence of water in the early geological history of Mars, during the Noachian era [5,6,7]. Planetary scientists are interested in the accurate in situ determination of the surface mineralogy and mineral composition on Mars by advanced techniques and instruments on board Mars landers and rovers [9,10,11,12,13,14]. About the effects of different atmospheric conditions on the performance of such instruments and on the response of the minerals to be analysed
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