Two cups. That was all Curiosity had. Two sealed metal cups holding a strong alkaline fluid called TMAH, ferried across 225 million kilometres of space precisely for a moment like this one, and if the team on the ground picked the wrong rock, the whole experiment was gone.
They picked Mary Anning. A patch of finely laminated sandstone in a place called Glen Torridon, named after the 19th-century English palaeontologist who, as a child, started pulling fossils out of the Dorset cliffs and more or less founded a science. Apt, that. Because on sol 2879, Curiosity drilled into the 3.5-billion-year-old bedrock, tipped the powdered fines into one of those precious cups, and heated the lot to 550 degrees. Then it waited to see what came off. What came off were organic molecules. More than 20 of them.
The findings, reported today in Nature Communications by a team led by Amy Williams of the University of Florida, describe the first thermochemolysis experiment ever performed on another world, a wet chemistry trick that coaxes complex molecules out of rock by breaking them apart and methylating the fragments until they’re light enough to float into the rover’s mass spectrometer. Among the compounds teased out of Mary Anning: benzothiophene, a sulphur-bearing double-ringed aromatic that’s turned up in carbonaceous meteorites for decades but had never been firmly pinned down on Mars. Methyl benzoate. Naphthalene. Methylnaphthalene. Trimethyl- and tetramethylbenzenes. And, tantalisingly, a peak the team can’t quite identify but whose mass spectrum matches closely with a class of nitrogen-bearing ring molecules known as N-heterocycles, the same structural family that includes the bases of DNA and RNA. “We think we’re looking at organic matter that’s been preserved on Mars for 3.5 billion years,” says Williams.
A Planetary Radiation Chamber
Which is the crux, really. Mars is a planetary radiation chamber. No magnetic field worth the name, a wafer-thin atmosphere, cosmic rays hammering the surface for aeons. Any delicate organics ought to have been fried into featureless char long ago. And yet the Knockfarrill Hill sandstone, which was laid down on the shore of an ancient lake when the crater floor was fluvial and wet, has somehow kept hold of these molecules through more than three billion years of diagenesis and radiation. The secret, Williams’ team reckons, is the clay. Smectite clays in particular are brilliant at hanging on to organic matter, trapping it between their layered sheets, shielding it from the worst of the chemistry that would otherwise pull it apart.
The whole point of SAM, the Sample Analysis at Mars instrument suite, is chemistry like this. And TMAH is its crown jewel.
Tetramethylammonium hydroxide (to give it its full name) doesn’t just volatilise what’s already loose in a sample. It hydrolyses chemical bonds; it pries apart macromolecular carbon, the sort of huge, tarry, insoluble stuff that makes up most of the organic matter in meteorites and, apparently, in Martian sediments too. Which is why only two cups went up with the rover: the reagent is corrosive, it’s hard to store, and every gram of propellant mattered at launch. Jennifer Eigenbrode at NASA Goddard, who helped build the experiment and is a co-author on the new paper, has spent much of the past decade nudging Curiosity towards the right rock to spend one of them on. When the time came, the team chose a drill site rich in clay minerals, low in the pesky perchlorates that can destroy organics during heating, and sitting squarely in a former lakebed. Good bet. It paid out.
Where Did the Molecules Come From?
One cup left, then. And some genuine questions about where this stuff actually came from, because the experiment can’t tell you. The molecules in Mary Anning could be endogenous, cooked up by water-rock chemistry in hydrothermal veins or produced abiotically through the interaction of carbon dioxide with iron-rich rocks. They could be exogenous, delivered by the same meteorites that have been peppering the inner solar system since the Hadean. Or, conceivably, they could have come from something that was once alive. The shapes and structures alone can’t discriminate between those three options. “The same stuff that rained down on Mars from meteorites is what rained down on Earth, and it probably provided the building blocks for life as we know it on our planet,” says Williams, and the comment cuts in both directions. If the Martian organics are meteoritic, they’re cousins of the molecules that built us. If they’re not, they could be something stranger still.
There are caveats worth noting. A known leak of another reagent, MTBSTFA, inside the instrument has muddied some of SAM’s chemistry in the past, and parts of the flight run didn’t go entirely to plan, with an internal standard lost and one of the recovery markers slipping past the detector. Sixteen of the 28 compounds seen at Mary Anning also show up in benchtop experiments on the Murchison meteorite, which raises the distinct possibility that much of what Curiosity picked up is, in the end, just space dust that fell on a very old lakebed.
None of which dims the main result. A chemical test never before tried off Earth has worked. The Martian surface, it turns out, can preserve big, complex, structurally diverse organic molecules for billions of years, including some that share the architecture of life’s most fundamental machinery. “It’s really useful to have evidence that ancient organic matter is preserved, because that is a way to assess the habitability of an environment,” Williams says. “And if we want to search for evidence of life in the form of preserved organic carbon, this demonstrates it’s possible.”
Coming Soon to Mars and Titan
The TMAH trick is going farther afield. ESA’s Rosalind Franklin rover, now set to land on Mars in 2028, will carry a version of the same wet chemistry aboard its MOMA instrument, with a drill that goes two metres down rather than Curiosity’s few centimetres, reaching rock that’s been shielded from radiation for even longer. And Dragonfly, the nuclear-powered quadcopter heading to Saturn’s moon Titan, will take TMAH along too, ready to sample the hydrocarbon dunes of an alien world where the chemistry of pre-life might still be happening in real time.
Whether any of the Martian molecules turn out to be biosignatures is a question for the samples Perseverance has been tubing up in Jezero crater, assuming NASA and its partners can work out how to get them home. For now, the Mary Anning rocks have said about as much as they’re going to. They said it in 20-odd molecules, lifted from clay that was once mud on the floor of a lake, on a world that might, once, have been a lot like this one.
Source: Nature Communications, DOI: 10.1038/s41467-026-70656-0
Frequently Asked Questions
What exactly did Curiosity find at Mary Anning?
More than 20 organic molecules, including naphthalene, methyl benzoate, and benzothiophene, a sulphur-bearing two-ringed aromatic that had never been firmly confirmed on Mars before. The team also flagged a molecule whose fingerprint resembles an N-heterocycle, the structural class that includes the building blocks of DNA and RNA. All came from clay-rich sandstone in Gale crater roughly 3.5 billion years old.
Does this mean Mars had life?
Not on its own. The experiment can’t distinguish between organic matter delivered by meteorites, produced abiotically by water-rock chemistry, or generated by something once alive. Definitively identifying biological signatures would require returning Martian rock samples to Earth, which is the job waiting on the Perseverance rover’s cached tubes in Jezero crater.
How does the TMAH experiment actually work?
TMAH is a strong alkaline reagent that breaks apart large, insoluble macromolecular carbon, the kind of tarry gunk that makes up most of the organic matter in meteorites and, apparently, Martian rocks. It hydrolyses chemical bonds and attaches methyl groups to the fragments, making them volatile enough to be separated and identified by gas chromatography and mass spectrometry onboard the rover.
Why does preservation matter so much?
Mars has no global magnetic field and almost no atmosphere, so the surface takes a constant beating from cosmic rays that destroy delicate organic molecules. The fact that complex aromatics survived in clay-rich rock for 3.5 billion years means future missions have a realistic chance of finding preserved biosignatures, if any ever existed, in similar sediments.
What’s the next test of this chemistry?
ESA’s Rosalind Franklin rover, due to land on Mars in 2028, carries the MOMA instrument with its own TMAH experiment, plus a drill that reaches two metres below the surface instead of Curiosity’s few centimetres. After that, NASA’s Dragonfly quadcopter will haul the same trick to Saturn’s moon Titan, where an entirely different kind of prebiotic chemistry may still be in progress.
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