Mars’ Valles Marineris canyon, pictured, spans as much as 600km across and delves as much as 8km deep. The image was created from over 100 images of Mars taken by Viking Orbiters in the 1970s. (Image credit: NASA)
A new Mars rover is likely to be launched by NASA in 2020 with the aim to explore a region that may contain remains of ancient microbial life according to scientists.
The rover will be programmed to gather soil and rock samples and stock them. It will be transported back to Earth in the distant future for researchers to investigate them for possible signs of current or past extraterrestrial life.
MIT scientists have developed a method that will assist the rover to rapidly and non-invasively spot sediments that are fairly unaltered, and that retains most of its original composition. The research has been reported in the journal Carbon.
Scientists will benefit greatly from such unspoiled samples as they will have a better chance at identifying signs of former life, if they are present. They would not gain much from rocks whose histories have been wiped off by geological processes such as radiation damage or excessive heating.
Spectroscopy on Mars
The team’s method focuses on a new approach to interpret the results of Raman spectroscopy, a common, non-destructive process used by geologists to discover the chemical composition of ancient rocks.
The 2020 Mars rover will be equipped with SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals), a scientific tool that will obtain Raman spectra from samples just below or on the Martian surface. This tool will be crucial in establishing whether life on Mars ever existed.
Raman spectroscopy measures the tiny vibrations of atoms inside the molecules of a particular material. For example, graphite is made up of a very systematic arrangement of carbon atoms. The bonds between these carbon atoms vibrate naturally, at a frequency that researchers can measure when they center a laser beam on the surface of graphite.
As molecules and atoms vibrate at a variety of frequencies based on what they are bound to, Raman spectroscopy allows researchers to detect main features of the chemical composition of a sample.
The method could also establish if a sample has carbonaceous matter - an initial clue that the sample could also hold signs of life.
However, Roger Summons, professor of earth, atmospheric, and planetary sciences at MIT, states that so far the chemical picture that researchers have using Raman spectroscopy has been quite vague.
For instance, a Raman spectrum obtained from a piece of coal on Earth may resemble an organic particle in a meteorite that was initially created 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.
Roger Summons, Professor, MIT
However, Nicola Ferralis, a research scientist in MIT’s Department of Materials Science and Engineering, spotted hidden properties in Raman spectra that can provide a more comprehensive imagery of a sample’s chemical composition.
In particular, the researchers could predict the ratio of hydrogen to carbon atoms from the substructure of the peaks in Raman spectra.
This is vital because the more heating a rock undergoes, the chances of the organic matter becoming altered is higher, particularly due to the loss of hydrogen in the form of methane.
The enhanced method enables researchers to more accurately understand the meaning of current Raman spectra, and speedily assess the ratio of hydrogen to carbon, thus spotting the most pure, ancient rocks samples for additional study. Summons says this might also assist researchers and engineers working with the SHERLOC tool on the 2020 Mars rover to detect ideal Martian samples from the other materials.
This may help in deciding what samples the 2020 rover will archive. It will be looking for organic matter preserved in sediments, and this will allow a more informed selection of samples for potential return to Earth.
Roger Summons, Professor, MIT
Seeing the Hidden Peaks
A Raman spectrum signifies the vibration of a molecule or atom, in relation to laser light. A classic spectrum for a sample comprising organic matter looks like a curve with two main peaks - one broad peak, and a sharper, more narrow peak.
Researchers have formerly labeled the broad peak as the D (disordered) band, as vibrations in this area compare with carbon atoms that have a disordered composition, linked to any number of other elements.
The narrow second peak is the G (graphite) band, which is normally linked to more ordered arrangements of carbon, such as is present in graphitic materials.
Ferralis, working with ancient residue samples being tested in the Summons’ lab, spotted substructures within the main D band that are directly linked to the quantity of hydrogen in a sample.
In other words, the higher the sub-peaks, the more hydrogen exists - a sign that the sample has been altered less and its unique chemical makeup has been better preserved.
To examine this new interpretation, the team chose to use Raman spectroscopy, and their analytic technique, on samples of sediments whose chemical composition was previously known.
They acquired extra samples of ancient kerogen - fragments of organic matter in sedimentary rocks - from a team in the University of California at Los Angeles, who had used chemical techniques in the 1980s to accurately establish the ratio of hydrogen to carbon.
First applying Raman spectroscopy to produce spectra of the varied kerogen samples, and then using their technique to interpret the peaks in each spectrum, the MIT team was able to swiftly predict the same ratio. The ratios of hydrogen to carbon demonstrated by the team directly matched the original ratios.
“This means our method is sound, and we don’t need to do an insane or impossible amount of chemical purification to get a precise answer,” Summons says.
Mapping a Fossil
The researchers then tried to figure out if they could apply their method to chart the chemical composition of a microscopic fossil, which usually would have very little carbon that it would be untraceable by conventional chemistry methods.
“We were wondering, could we map across a single microscopic fossil and see if any chemical differences were preserved?” Summons says.
To answer that query, the team acquired a microscopic fossil of a protest, which is an ancient, single-celled organism that could signify a simple alga or its predator. The researchers suggest that such fossils were biological in origin, merely from their appearance and their resemblance to several other patterns in the fossil record.
The atomic vibrations across the fossil were measured by the team at a sub-micron resolution using Raman spectroscopy, and then they tested the resultant spectra using their new analytic method. A chemical map was derived from their analysis.
The fossil has seen the same thermal history throughout, and yet we found the cell wall and cell contents have higher hydrogen than the cell’s matrix or its exterior. That to me is evidence of biology. It might not convince everybody, but it’s a significant improvement than what we had before.
Roger Summons, Professor, MIT
Summons says that, apart from identifying potential samples on Mars, the team’s method will assist paleontologists in understanding the Earth’s biological evolution.
“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,” Summons 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.”
Shell Oil Company and Schlumberger through the X-Shale Consortium under the MIT-Energy Initiative, and Extramural Research by Shell Innovation Research and Development, The Simons Foundation Collaboration on the Origins of Life, the NASA Astrobiology Institute, and the Max Planck Society supported this research.