Raman spectroscopy is a powerful, non-destructive technology used by geologists and mineralogists to analyze rock and mineral samples here on Earth, and now, according to MIT News, it will be used to help identify rocks on Mars as well.
In 2020, NASA plans to launch a new Mars rover to collect samples of rocks and soil that could hold remnants of ancient microbial life. MIT scientists have developed a way to help the rover select the most likely rocks to study for signs of life using Raman spectroscopy.
The rover will be looking for sediments that maintain much of their original composition because they offer the best chance for identifying signs of former life, as opposed to rocks that have been altered by geological processes such as excessive heating or radiation damage. The MIT article explains the new role of Raman spectroscopy in this search:
Nicola Ferralis, a research scientist in MIT’s Department of Materials Science and Engineering, discovered hidden features in Raman spectra that can give a more informed picture of a sample’s chemical makeup. Specifically, the researchers were able to estimate the ratio of hydrogen to carbon atoms from the substructure of the peaks in Raman spectra. This is important because the more heating any rock has experienced, the more the organic matter becomes altered, specifically through the loss of hydrogen in the form of methane. The improved technique enables scientists to more accurately interpret the meaning of existing Raman spectra, and quickly evaluate the ratio of hydrogen to carbon — thereby identifying the most pristine, ancient samples of rocks for further study.
Raman spectroscopy is an established analytical technique for the analysis a variety of different types of geological samples because it offers the following advantages:
- Raman spectroscopy is fast, non-destructive, and requires little or no sample preparation.
- Raman spectroscopy identifies both the chemical composition and molecular structure of specific materials. Ever expanding databases of Raman spectra aid in the positive identification of minerals, while Raman spectroscopy can be used to evaluate various phases of the same or very similar chemical composition.
- Confocal Raman micro-spectroscopy can be used to characterize inclusions within samples without the need to expose the inclusions. Different solid, liquid, or gaseous phases within the inclusion can be studied this way. This type of analysis can also be useful for analyzing archived samples that have been mounted under glass cover slips.
- Raman spectroscopy can even be used to evaluate things like residual pressure and stress within a sample by looking at shifts in the Raman peaks.
Raman is a form of vibrational spectroscopy that is known to exhibit excellent selectivity for the purposes of material identification. In Raman spectroscopy, an unknown sample of material is illuminated with monochromatic (single wavelength or single frequency) laser light, which can be absorbed, transmitted, reflected, or scattered by the sample. Light scattered from the sample is due to either elastic collisions of the light with the sample’s molecules (Rayleigh scatter) or inelastic collisions (Raman scatter). Whereas Rayleigh scattered light has the same frequency (wavelength) of the incident laser light, Raman scattered light returns from the sample at different frequencies corresponding to the vibrational frequencies of the bonds of the molecules in the sample.
Since the bonds for every molecule are different, the Raman scattering for every molecule is also different. Thus, a Raman spectral “fingerprint” can be generated by recording the intensity of light as a function of the frequency difference between the laser and Raman scattered light.
(Editor’s Note: No, that photo above of a person walking on Mars is not real…though maybe one day it could be!)