NASA’s Curiosity rover recently discovered various classes of organic matter in the Martian sediments. One intriguing group of detected organic compounds were thiophenes, which typically occur on Earth in kerogen, coal, crude oil, stromatolites, microfossils and oddly enough, in white truffles, the mushroom beloved by epicureans and wild pigs. Astrobiologists Dirk Schulze-Makuch and Jacob Heinz think the presence of thiophenes would be consistent with the presence of early life on Mars.
An artist’s impression of a habitable Mars. Image credit: Daein Ballard / CC BY-SA 3.0.
In the study, Dr. Schulze-Makuch and Dr. Heinz explored some of the possible pathways for thiophenes’ origins on Mars.
Their work suggests that a biological process, most likely involving bacteria, may have played a role in the organic compound’s existence in the Martian soil.
“We identified several biological pathways for thiophenes that seem more likely than chemical ones, but we still need proof,” said Dr. Schulze-Makuch, a researcher at Washington State University.
“If you find thiophenes on Earth, then you would think they are biological, but on Mars, of course, the bar to prove that has to be quite a bit higher.”
Thiophene molecules have four carbon atoms and a sulfur atom arranged in a ring, and both carbon and sulfur, are bio-essential elements.
Yet Dr. Schulze-Makuch and Dr. Heinz, a scientist at the Technische Universität Berlin, could not exclude non-biological processes leading to the existence of these compounds on Mars.
Meteor impacts provide one possible abiotic explanation.
Thiophenes can also be created through thermochemical sulfate reduction, a process that involves a set of compounds being heated to 120 degrees Celsius (248 degrees Fahrenheit) or more.
In the biological scenario, bacteria, which may have existed more than 3 billion years ago when Mars was warmer and wetter, could have facilitated a sulfate reduction process that results in thiophenes.
There are also other pathways where the thiophenes themselves are broken down by bacteria.
While Curiosity has provided many clues, it uses techniques that break larger molecules up into components, so scientists can only look at the resulting fragments.
Further evidence should come from the next rover, the Rosalind Franklin, which is expected to launch in July 2020.
It will be carrying the Mars Organic Molecule Analyzer (MOMA), which uses a less destructive analyzing method that will allow for the collection of larger molecules.
The team recommends using the data collected by the next rover to look at carbon and sulfur isotopes.
“Organisms are lazy. They would rather use the light isotope variations of the element because it costs them less energy,” Dr. Schulze-Makuch said.
“Organisms alter the ratios of heavy and light isotopes in the compounds they produce that are substantially different from the ratios found in their building blocks — a telltale signal for life.”
“Yet even if the next rover returns this isotopic evidence, it may still not be enough to prove definitively that there is, or once was, life on Mars.”
“As Carl Sagan said ‘extraordinary claims require extraordinary evidence’.”
“I think the proof will really require that we actually send people there, and an astronaut looks through a microscope and sees a moving microbe.”
The team’s paper was published in the journal Astrobiology.
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Jacob Heinz & Dirk Schulze-Makuch. Thiophenes on Mars: Biotic or Abiotic Origin? Astrobiology, published online February 24, 2020; doi: 10.1089/ast.2019.2139
