Quantification of fluorescence emission from extraterrestrial materials and its significance for planetary Raman spectroscopy

Abstract

AbstractFluorescence emission has been a serious impediment in the application of Raman spectroscopy to terrestrial geologic samples. It is largely unknown how significant the fluorescence problem would affect Raman analyses during a planetary surface exploration mission. We have completed a set of two experimental studies to quantitatively evaluate the fluorescence emission excited by multiple wavelengths from a set of 21 stony meteorites that represent 98.9% (by types) of all stony meteorites collected, named, and classified worldwide. Our data revealed a general trend, that is, the non‐Raman emission intensities of extraterrestrial materials are two to three orders of magnitude lower than those of typical terrestrial clays and the soils from hyperarid Mars‐analog region (Atacama Desert). Based on >4,500 Raman spectra (excited by continuous wave [continuous wave] 532‐nm laser line) obtained by auto‐scans on these 21 meteorites, we found that the average percentage of informative Raman spectra is nearly 98%, with a standard deviation of 2% and the lowest value of 93%. We also found that the information on major and minor mineral phases revealed by our multispots Raman measurements are consistent with those published in Meteoritic Bulletin database for the examined meteorites. In conclusion, our experimental results demonstrated that the fluorescence interference that sometimes concealed the Raman signals of terrestrial samples would not be a threat to planetary Raman spectroscopy that uses CW 532‐nm laser line for excitation. This conclusion is consistent with the in situ fluorescence observations made during two missions on Mars and with many Raman spectroscopic studies of lunar rocks and soils returned by Apollo missions.