In a new paper published in the journal Astrobiology, Jacob Heinz, my colleague at the Technical University of Berlin, and I discuss one of the more important findings of the Curiosity rover on Mars. The rover’s discovery of sulfur-containing organic compounds was a game-changer in the search for life on the Red Planet. Organics had been detected before, but the data from Curiosity finally convinced the scientific community that they were indigenous, and not contamination brought from Earth.
Some of the discovered compounds belong to the group known as thiophenes—ring-like compounds containing four carbon atoms and one sulfur atom. They are known to occur naturally in coal and crude oil. In the paper, Heinz and I discuss possible abiotic (non-living) and biotic explanations for the presence of thiophenes on Mars.
Interestingly, the first organism we stumbled on that synthesizes thiophenes was a fungus prized by gourmet cooks—the white truffle. Lots of microbes also produce thiophenes. The most feasible pathway for doing this, called microbial sulfate reduction, even works in the sub-zero temperatures on Mars, and certainly would have worked in the Martian past, when environmental conditions were even more benign.
As usual when looking for a biological “smoking gun,” however, there’s a complication. An abiotic pathway for thiophenes can’t be excluded, because another process—called thermochemical sulfate reduction—can occur at temperatures higher than 120o C. And the organic material needed for this process could, for example, come from meteorites. No biology required.
It would be great if Curiosity could tell the difference between organics derived from biology and those that aren’t. But there’s a problem with the analysis method used by the rover. Called pyrolysis, this technique heats the sample to temperatures above 500o C, resulting in fragments of organic molecules that might originally have been much larger before pyrolysis.
Europe’s ExoMars rover, due to be launched to Mars in July, has an instrument on board called MOMA that uses a nondestructive method to analyze organic compounds. The hope is that it will be able to detect some of the suspected larger organic molecules that will help scientists determine whether the thiophenes are of biological origin.
What else can we do? The isotopic composition of the thiophenes may provide an important clue. Microbes tend to prefer the lighter version of chemical elements, as these require less work to process. The isotopic signature of sulfur-containing sediments found by Curiosity on Mars is very similar to the signature seen in rocks from the Haughton impact crater in the High Canadian Arctic, which are thought to result from biological sulfate reduction.
The isotopic signature of carbon in Martian rocks is likely an even more important clue, but as of today is still unknown. Further missions are needed. Meanwhile, research continues on Earth to explore the possible biological production of thiophene under simulated Martian conditions, in specially designed test chambers.
So stay tuned. More revelations are likely coming soon.