Organic compounds have been discovered on Mars in recent years – but how did they get there? A new study examines the different ways that one type in particular, thiophene, may have formed, and intriguingly one of the most plausible scenarios involves ancient microbial life.

Thiophene molecules are made up of four carbon atoms and a sulfur atom arranged in a ring shape. Here on Earth, these organic compounds are found in coal, crude oil and even white truffles. And perhaps surprisingly, NASA’s Curiosity rover dug them up on the Red Planet a few years ago.

So are these compounds signs that life was once present on Mars? Thiophenes could form in a number of different ways, but the new study from researchers at Washington State University and the Berlin Institute of Technology leans more towards the life explanation.

“We identified several biological pathways for thiophenes that seem more likely than chemical ones, but we still need proof,” Dirk Schulze‑Makuch said. “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.”

Thiophenes are often created through a sulfate reduction process, which can be triggered in a few ways. It may be a thermochemical process, where precursor compounds are heated to 120° C (248° F) and thiophenes are produced as a result. These conditions could be created on Mars through things like meteorite impacts.

But, the team says, bacteria can also perform sulfate reduction under milder circumstances. Mounting evidence suggests that billions of years ago, Mars was much more habitable than it is today, with a thicker atmosphere, warmer temperatures and plenty of water. Bacteria could have been common in this Earth-like world, producing thiophines that still linger today.

The researchers say that this scenario is simpler and more likely than others, but of course it alone doesn’t prove that life does or ever has existed on Mars. That said, there are ways to check. If these thiophines really were produced by organisms, their carbon and sulfur atoms would have completely different isotopes than if they were produced non-biologically.

Curiosity isn’t equipped to look for these differences, but future Red Planet rovers, such as ESA’s Rosalind Franklin, will be. Launching this July, the rover will be decked out with the Mars Organic Molecule Analyzer (MOMA), which will be able to study larger molecules and determine which isotopes are present.

The research was published in the journal Astrobiology.

Source: Washington State University