Miniature high-speed, low-pulse-energy picosecond Raman spectrometer for identification of minerals and organics in planetary science.

Abstract

The motivation behind time-resolved Raman spectroscopy for planetary surface exploration is (1) to provide comprehensive identification of minerals (nearly all rock-forming minerals and weathering products) and many organics of prime importance including fossilized carbonaceous materials; (2) to do so ensuring that it is possible to characterize even the most sensitive materials that would be altered by current state-of-the-art pulsed lasers (e.g., dark minerals, organics). These goals are accomplished here using a lightweight, high-speed (MHz) pulsed (<100ps) Raman spectrometer based on a high-speed microchip laser combined with a single photon avalanche diode detector array. Using a Mars analog sample set and an automated grid sampling technique, we demonstrate consistent identification of major minerals and kerogen detection at ∼≥1% by volume, without losses typically associated with the two biggest problems: fluorescence interference and sample damage. Despite improvements, we find that time-resolved Raman spectroscopy is still limited by the availability of a suitable laser and detector. As technology advances and such devices become available, we expect that this technique will hold an important place in Raman spectroscopy for both commercial and planetary science applications. We also discuss the utility of Raman point mapping for planetary science (e.g., in comparison with other common techniques like infrared reflectance spectroscopy) and conclude that the choice of technique must be planetary mission-specific; one must consider whether incurring the time to map single microscopic points is worthwhile, and how many points would be sufficient to gain the required information to characterize the surface.