Laser alteration on iron sulfides under various environmental conditions

Upcoming lander missions to planetary bodies require new innovative techniques for the in situ investigation of their surfaces and near-surface materials. In recent years, Raman spectroscopy has been developed to become an excellent laboratory tool for fast petrological and mineralogical investigation of terrestrial and extraterrestrial rocks. Consequently, Raman spectroscopy has successfully been proposed for operation on planetary surfaces. For example, the Raman Laser Spectrometer of the joint European Space Agency and Roscosmos mission ExoMars will, for the first time in space, identify minerals and organic compounds in Martian surface rocks and soils by using Raman spectroscopy. In preparation for space missions, we investigate iron sulfides to measure the influence of various environmental conditions and the effect of laser irradiation itself on the obtained Raman spectra. Iron sulfides are chosen because they are widely disseminated on the Martian surface as known from Martian meteorites, lander missions and remote sensing. They are also present in almost all meteorites as well as on Moon's surface, and they might be present in the rocks of Jupiter's satellites Ganymede and Callisto. We selected the iron di-sulfides pyrite, marcasite, and chalcopyrite, and the iron mono-sulfides troilite, and pyrrhotite. Because on Earth in ambient air these iron sulfides are very sensitive to laser irradiation, it is necessary to know how these minerals react during the measurements under conditions that are similar to space environments. Therefore, spectra were taken with various laser power and in various environmental conditions relevant for measurements on Mars, the Moon, and asteroids. These conditions are ambient air, vacuum down to 10(-6)mbar, 8mbar CO2 atmosphere, and temperatures ranging between room temperature (293 K) and 10K. We found that Raman spectra of different iron di-sulfides are stable under different environmental conditions and with increasing laser power. In contrast, iron sulfides are very sensitive to increasing laser power in ambient air and convert into hematite and magnetite. However, most important for space missions, no effects were found with increasing laser power in vacuum. In addition, a CO2 atmosphere does not affect the Raman spectra for the investigated minerals, a finding particularly important for missions to Mars. Copyright (c) 2017 John Wiley & Sons, Ltd.