Detecting Ce3+ as a biosignature mimicker using UV time-resolved laser-induced fluorescence and Raman spectroscopy: Implications for planetary missions

Abstract

Combined UV Raman and laser-induced fluorescence (LIF) spectroscopy instruments will soon be launched onboard missions to planetary surfaces, including Mars, to search for biosignatures. However, the rare earth element Ce3+, found in many common and Mars-relevant minerals, can produce fluorescence features within the spectral window usually attributed to organic compounds in a LIF spectrum. This study explored the detection of Ce3+ as a biosignature mimicker using UV Raman-LIF mission instruments. We assessed how LIF spectra of a suite of synthetic CePO4 samples compare with those of organics, how varying concentrations of both Ce3+ and organics in Martian regolith simulant affect this comparison, and whether two additional data sets obtainable by combined UV Raman-LIF instruments, including time-resolved fluorescence decay lifetimes and Raman spectra, can distinguish Ce3+-containing samples from organics. Results showed that the dominant LIF features of Ce3+ (320 and 338 nm) are similar to those of the aromatic amino acid tryptophan (325 and 340 nm), even when Ce3+ samples were mixed in a Martian regolith simulant at a range of concentrations. Lifetimes were revealed to be 2–9 ns in Ce3+-containing samples, typical for organic fluorophores. These results support the erroneous interpretation that LIF spectra and lifetime values obtained on these samples constitute potential organic signatures. Raman spectroscopy results suggested that with UV laser excitation, Raman is unlikely to identify Ce-bearing samples due to strong absorption of Raman scattered energy by Ce3+. We conclude that biosignature searches using UV LIF and Raman spectroscopy instrumentation may encounter challenges with unambiguously distinguishing spectra of organic compounds from Ce-bearing compounds.