Amorphous iron oxides investigated by portable and bench-top Raman spectrometers: An extraterrestrial analytical perspective

One of the current issues in the Mars regolith investigation is the performance evaluation of the instruments that are to be sent on the next missions, and commercial portable Raman instruments have been used in experiments aiming to simulate the capabilities of the Mars mission prototypes. In this study, several types of iron oxides with low crystallinity, some of them containing other metals as impurities, were synthetized and characterized using a bench-top research grade Raman microscope and a portable instrument, both with excitation at 785 nm. The obtained results clearly indicated that luminescence is an issue in such type of samples, even with excitation at 785 nm, and this is particularly more important with the portable instrument which lacks confocality; furthermore, the higher laser power density, necessary in this kind of instrument, is likely to cause phase transitions and de-hydration. A comparison with excitation at 514.5 nm has shown that, as expected, these effects are more marked at higher laser frequency. Laser excitation and power density are, therefore, two important issues to overcome for a successful remote characterization of the Mars regolith.