PSI’s Ashley Murphy presented an abstract and poster titled “Comparison of Organic Carbon Under Deep Ultraviolet Light and Visible Raman Spectroscopy” at the 54th Lunar and Planetary Science Conference in The Woodlands, Texas. PSI’s Aileen Yingst was a co-author.
Raman spectroscopy is useful in biosignature (life detection) research because it allows for the detection and characterization of minerals and organics within a spatial context. The SHERLOC (Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals) instrument aboard the Mars Perseverance rover is equipped with a deep ultraviolet Raman spectrometer to detect and classify minerals and organics in Jezero crater, Mars. The SuperCam instrument utilizes a visible Raman spectrometer, and many terrestrial biosignature studies are based on Raman spectra collected under this excitation wavelength.
In extremely old rocks on Earth, organic carbon is often the only evidence of past microbial life. Since early Mars may have been more similar to Earth, we may anticipate finding organic carbon in old Martian rocks too. Raman spectroscopy is a technique that uses laser excitation to examine a sample. The laser light excites molecules in a sample, resulting in a spectral signal that, like a unique spectral fingerprint, is used to classify or identify minerals and organics within the sample. The Raman spectra of organic carbon include a D band (disorder band at ~1300 cm-1) and a G band (graphite band at ~1600 cm-1). In VIS Raman studies, D and G band parameters, such as the intensity ratio (D/G) or bandwidth, are often used to determine the maximum temperature of the host rock because band shape and intensity change with increasing temperature (Fig 1). Although VIS Raman is commonly used for mineral identification and D and G band thermometry, DUV Raman is well suited for organics which may fluoresce under VIS excitation and obscure the signal. DUV Raman thermometry results have yet to be reported in the literature, and since the Raman spectra of carbon depend on the excitation wavelength used, comparing results from different Raman excitation wavelengths may reveal more information related to where the organic carbon originated (biotic [from life] vs abiotic [not related to life]), which is important for early Earth biosignature and astrobiology research.
In this study, we characterize VIS and DUV Raman spectra from coals and carbonate rocks called stromatolites. Stromatolites are thinly layered rocks that form when microbial communities trap loose rocky materials in their sticky biofilm. Our goal is to understand how Raman D and G band parameters vary between DUV and VIS excitation and what implications this has for life detection under DUV excitation and the search for potential biosignatures on Mars. Coal and stromatolite samples were analyzed on a custom-built SHERLOC analog instrument (248.6 nm laser excitation) and a commercial Raman spectrometer (532 nm laser excitation) at NASA Jet Propulsion Laboratory. The contrast in DUV and VIS Raman results for coal (Fig 1), and stromatolite (Fig 2) may be due to different organic carbon structures and compositions related to different sources of organic material. Additionally, this may be due to different alteration settings related to environmental factors such as temperature, pressure, and chemistry. Raman D and G bands were deconvolved into five bands (D1, D2, D3, D4, and G), and max temperature estimates were calculated using the D2 or G peak according to VIS Raman thermometry methods from literature. Preliminary thermometry results indicate that current VIS Raman thermometry methods may not be easily applied to DUV Raman and may result in a higher max temperature. We continue to analyze samples from various geologic settings to understand how structural and compositional differences, geologic settings, and excitation wavelength influence Raman D and G band spectra.