Raman spectroscopy could help identify life in Martian rocks

The 2020 planned NASA launch of a new Mars rover will probe a region of the planet scientists believe could hold remnants of ancient microbial life. As reported in the journal Carbon, Massachusetts Institute of Technology (MIT; Cambridge, MA) scientists have developed a technique that will help the rover quickly and non-invasively identify sediments that are relatively unaltered and maintain much of their original composition—"pristine" samples that could give scientists the best chance for identifying signs of former life, if they exist, as opposed to rocks whose histories have been wiped clean by geological processes such as excessive heating or radiation damage.

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The team's technique centers on a new way to interpret the results of Raman spectroscopy, a common, non-destructive process that geologists use to identify the chemical composition of ancient rocks. Among its suite of scientific tools, the 2020 Mars rover includes SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals), an instrument that will acquire Raman spectra from samples on or just below the Martian surface.

As atoms and molecules vibrate at various frequencies depending on what they are bound to, Raman spectroscopy enables scientists to identify key aspects of a sample’s chemical composition. More importantly, the technique can determine whether a sample contains carbonaceous matter—a first clue that the sample may also harbor signs of life.

But Roger Summons, professor of earth, atmospheric, and planetary sciences at MIT, says the chemical picture that scientists have so far been able to discern using Raman spectroscopy has been somewhat fuzzy. For example, a Raman spectrum acquired from a piece of coal on Earth might look very similar to that of an organic particle in a meteorite that was originally made in space.

"We don't have a way to confidently distinguish between organic matter that was once biological in origin, versus organic matter that came from some other chemical process," Summons says. However, 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. Summons says this may also help scientists and engineers working with the SHERLOC instrument on the 2020 Mars rover to zero in on ideal Martian samples.

The team applied Raman spectroscopy, and their analytic technique, to samples of sediments whose chemical composition was already known. They obtained additional samples of ancient kerogen—fragments of organic matter in sedimentary rocks—from a team based at the University of California at Los Angeles, who in the 1980s had used meticulous, painstaking chemical methods to accurately determine the ratio of hydrogen to carbon.

The team quickly estimated the same ratio, first using Raman spectroscopy to generate spectra of the various kerogen samples, then using their method to interpret the peaks in each spectrum, finding that the ratios of hydrogen to carbon closely matched the original ratios. Going a step further, the researchers wondered whether they could use their technique to map the chemical composition of a microscopic fossil, which ordinarily would contain so little carbon that it would be undetectable by traditional chemistry techniques. Raman analysis of the sample showed that cell wall and cell contents had higher hydrogen than the cell’s matrix or its exterior—evidence of biology.

The researchers say that their work will ultimately help find organic matter that is minimally altered and will help them learn more about what the organisms were made of, and how they worked.

SOURCE: MIT; http://news.mit.edu/2016/new-technique-detect-martian-life-0816