Investigations by the many landers and rover on Mars have uncovered the presence of several organic compounds on the Red Planet. A newly discovered class, known as thiophenes, has caught the attention of researchers at the University of Washington. The scientists believe the molecules found in samples scooped up by Curiosity in 2018 were created by biological processes rather than by chemical ones, hinting at ancient life on Mars.
Thiophenes are molecules made of four carbon atoms and a sulfur atom forming a pentagon-shaped ring. On Earth, these molecules are found in coal and crude oil – made up of dead plants and organisms, respectively – and white truffles. This compound is thought to form through a thermochemical process, but bacteria can create it too. Could ancient bacteria have created it on Mars? That is the focus of a new paper published in the journal Astrobiology.
"We identified several biological pathways for thiophenes that seem more likely than chemical ones, but we still need proof," co-author Dirk Schulze-Makuch said in a statement. "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."
If it was a biological reaction, the researchers say, it would have involved ancient bacteria that either facilitated the assembling of these carbon and sulfur rings, or actually breaks down these molecules. If the compounds were formed through an abiotic reaction, meaning physical rather than biological, so not requiring lifeforms, it's a bit more extreme.
Thiophenes can be created through thermochemical sulfate reduction, which requires a temperature of 120°C (240°F) to work. Mars had volcanos and other related geological processes, so they could have produced these substances, as could meteorite impacts. The team, however, doesn’t think that these processes can explain the amount of thiophenes detected by Curiosity.
Currently, we still can't prove definitively if there has ever been life on Mars. Despite this new discovery, which appears to point to ancient bacteria being the more likely answer, further evidence is needed. A stronger case for it would come by studying the isotopic composition of the molecules. Isotopes are a version of the same chemical elements that have slightly different weights. Lifeforms have preferences when it comes to the isotopes they use and we might soon detect them in these molecules, which would indicate with more clarity if they are of biological origin or not.
"Organisms are 'lazy'. They would rather use the light isotope variations of the element because it costs them less energy," Schulze-Makuch explained.
Curiosity can’t run this analysis with the instruments on board, but ESA and Roscosmos' Rosalind Franklin rover, which will launch this summer, can. When Curiosity dug the sample out of the mud of the Murray Formation in Gale Crater in 2018, it used a technique to analyze it that requires heating the samples to 500°C. Rosalind Franklin will be carrying an instrument that is much less destructive, so we might be finding more about thiophenes and their origin soon.