More than 20 organic compounds found on Mars – many for the first time

An international team of researchers has identified more than 20 organic molecules in clay-bearing sandstones on Mars that are about 3.5 billion years old. The compounds discovered include benzothiophene – a known component of meteoritic macromolecular carbon that represents the largest confirmed underivatised aromatic molecule to be identified as indigenous to the Red Planet. The ester methyl benzoate was also detected, along with naphthalene, as well as single and dicyclic aromatic molecules. Finding these substances on Mars, including several widely considered building blocks for the origin of life on Earth, suggests that prebiotic chemistry could have existed there once.

The Curiosity rover detected aromatic and cyclic molecules with methyl and ester/carboxylic acid functional groups, and sulfur-, oxygen-, and nitrogen-bearing organics, including the first discovery of a possible N-heterocycle on Mars. The rover identified these organic molecules using its suite of on-board instruments.

Of the more than 20 molecules detected, the team was able to confirm the identity of seven, most of which had never been detected on Mars previously. Team leader Amy Williams, a geobiologist and organic geochemist at the University of Florida, says that a nitrogen heterocycle Curiosity found is ‘really intriguing’ because of its apparent similarity to a molecule considered a precursor for RNA and DNA. Spectra from Curiosity are consistent with dimethylindole.

These complex chemicals could have been generated geologically or biologically on Mars, or they could have been formed on meteorites and been delivered to the planet, Williams explains. ‘The story our findings tell is that there is complex organic carbon preserved in the very near subsurface of Mars, where we thought radiation would have destroyed all of that,’ she says. ‘The organics that rained down on Mars might have been present very early on in Mars’s history, even at the time when life was originating on Earth.’

Janice Bishop, a chemist and planetary scientist with the SETI Institute who was not involved in this research, is enthusiastic about its findings. ‘Detection of a variety of complex organics on Mars is really exciting because it implies the presence of molecules that could be important for prebiotic chemistry or life,’ she states. ‘Observation of N-complexes is important because these can lead to formation of nucleic acids and amino acids, which are the building blocks of life as we know it on Earth,’ Bishop adds.

These promising results come as space future missions are planned that will search for organic compounds. The European Space Agency’s Rosalind Franklin mission to Mars, which is expected to land on Mars in November 2030, has a newer, more powerful mass spectrometer than Curiosity and there are plans to drill much deeper into the subsurface of the planet. ‘That gives you a whole different suite of organics you can search for that probably aren’t destroyed by radiation,’ Williams says.

Nasa’s Dragonfly expedition to Saturn’s moon Titan is slated to touchdown in 2034 and will also be searching for prebiotic chemistry there. ‘Even though we’re not searching for life on Titan, it is a world that is dominated by organic molecules,’ Williams explains. She is not currently on either of those two missions but hopes to get involved closer to their launches.

Mars is special because it provides a window into an ancient era that is no longer recorded on Earth, Williams says. ‘You are starting to see the things that were maybe raining down on the planet that were the feedstock for prebiotic chemistry and life as we know it,’ she says. ‘And you’re getting to see that paused in time in the rocks on Mars.’ On Earth, those rocks have largely been recycled due to plate tectonics, Williams explains, and those that still exist are heavily metamorphosed.

Nevertheless, Williams is clear that while her team has found the building blocks of life on Mars she cannot say whether they derive from past life. To provide this clarity, multiple lines of evidence must come together to support such an interpretation. ‘Sample return from Mars would enable us to investigate these samples in a far more robust way … and perhaps we could detect those more complex molecules if they existed because we have cutting-edge instruments on Earth,’ Williams states.

In the meantime, she suggests that scientists could look for large, complex molecules on other planets that could not possibly form abiotically, including hopanoids, a class of terpenoid, and sterols. ‘It makes sense that if there were life on Mars potentially its biochemistry would be similar, if not the same, to terrestrial life,’ Williams says. ‘But we only have one data point.’