In 2020, NASA plans to launch a Mars rover that will probe the Red Planet for remnants of ancient microbial life, let alone Matt Damon's abandoned scientist Mark Whatney.
But seriously, the rover will collect samples of rocks and soil, and store them on the Martian surface. At some time in the distant future, the samples would be returned to Earth and analyzed for signs of present or former extraterrestrial life.
Scientists with the Massachusetts Institute of Technology (MIT) have developed a technique that will help the rover quickly and non-invasively identify sediments that are unaltered, or which have maintained much of their original composition. Geological processes such as excessive heating or radiation damage can wipe clean the histories of the rocks, and thus any chance for identifying signs of former life.
SPECTROSCOPY ON MARS
Raman spectroscopy is a spectroscopic technique used to observe vibration, rotational and other low-frequency modes in a system, according to Sinha Guarav, a research analyst with BCC Research.
The Raman effect was discovered over 70 years ago but Raman spectroscopy was not used as an analytical tool until the invention of the laser. The technique of the MIT's team involves 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.
According to Jennifer Chu, Raman spectroscopy measures the minute vibrations of atoms within the molecules of a given material. She cites graphite as an example. Composed of a very orderly arrangement of carbon atoms, graphite contains bonds between its carbon atoms that vibrate naturally, at a frequency that scientists can measure when they focus a laser beam on graphite’s surface.
To determine if Mars ever sustained any forms of life, NASA has outfitted its 2020 Mars rover with SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals). The instrument that will acquire Raman spectra from samples on or just below the Martian surface.
Raman spectroscopy enables scientists to identify aspects of a sample’s chemical composition by the various vibrational frequencies of atoms and molecules and what they are bound to. According to Chu, the technique can determine whether a sample contains carbonaceous matter—a first clue that the sample may contain signs of life.
CHALLENGES OF RAMAN SPECTRA
Challenges loom with using Raman spectroscopy, however. For example, a Raman spectrum acquired from a piece of coal on Earth might look like that of an organic particle in a meteorite originally formed in space, Chu notes. In other words, distinguishing between organic matter of biological in origin versus organic matter originating from another chemical process may be problematic.
But, 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.
Researchers estimated the ratio of hydrogen to carbon atoms from the substructure of the peaks in Raman spectra, Chu writes. This feature is key "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, thus identifying the most pristine, ancient samples of rocks for further study.
“We’re interested in the oldest organic matter preserved on the planet that might tell us something about the physiologies of Earth’s earliest forms of cellular life,” Roger Summons of MIT says. “We’re hoping to understand, for example, when did the biological carbon cycle that we have on the Earth today first appear? How did it evolve over time? This technique will ultimately help us to find organic matter that is minimally altered, to help us learn more about what organisms were made of, and how they worked.”
The technique was published in Carbon.