Development of Raman Spectroscopic Methods for Detection of Molecules Indicating Life in Extraterrestrial Environments

Mineral analysis is of great importance to the understanding of the world around us and worlds beyond. Geology, chemistry, and environmental studies all benefit from characterization of the structure and properties of minerals. While various techniques have been applied towards the study of minerals, we propose that Raman spectroscopy is specifically suited for the detection and study of minerals under various conditions, including for terrestrial and space applications.

In our terrestrial studies, we explored the polymorphs of calcium carbonate within freshwater mollusk shell matrices with Raman spectroscopy. We found that aragonite was the main calcium carbonate polymorph present in the bivalve shells, which was expected. Unexpectedly, vaterite, a rare polymorph that indicates shell damage and repair, was identified in several shells. Principal Components Analysis combined with Linear Discriminant Analysis (PCA-LDA) was implemented to identify differences in the shell matrices of different species of bivalves and provided insight to shell growth patterns.

For extraterrestrial applications, we developed Raman spectroscopic methods for detection of molecules that would indicate life in space. We developed a Martian soil analog as the substrate within which we would detect the biomolecules of interest. The analog was doped with aromatic amino acids (tyrosine and tryptophan) to establish the Raman spectroscopic limits of detection (LOD). We determined with normal Raman spectroscopy that the LODs for these biosignatures is 0.25% by weight. We then incorporated surface enhanced Raman spectroscopy (SERS), to push the LOD further.

Other molecules that could indicate life and have been previously found on Mars are thiophene, 2-methylthiophene, and 3-methylthiophene. We detected the with normal Raman signatures for these molecules as neat liquids and as slushes, which is the form they are most likely to be found in on Mars.

We are developing spatially offset Raman spectroscopy (SORS), a technique that allows for detection of sub-surface molecules in multilayer samples, for detection of molecules buried in ice or under soil on Mars. Our initial method development includes detection of a strong Raman scatterer, polytetrafluoroethylene (PTFE), buried under the Martian soil analog. We established SORS detection of the PTFE through 15 mm of Martian analog.