After 13 years rolling across Gale Crater, NASA's Curiosity rover has just turned in arguably the most chemically interesting result of its long career: the most diverse collection of organic molecules ever detected on Mars.
In a study published in Nature Communications, the science team reported that Curiosity's onboard chemistry lab identified seven organic compounds in a single mudstone sample drilled from a dried-out lakebed. Five of those molecules — including trimethylbenzene, methyl benzoate, naphthalene, benzothiophene and methylnaphthalene — had never been observed on Mars before.
"This is the richest organic inventory we have from Mars," said one of the lead researchers from NASA's Goddard Space Flight Center. "It is exactly the kind of complex carbon chemistry you want to find when you are asking whether the planet was once habitable."
The breakthrough came from a first-of-its-kind experiment on the rover. Instead of simply baking the rock sample and sniffing the vapours, as Curiosity normally does, the team commanded the rover to dissolve the powder in a chemical solution carried from Earth. That liquid extraction unlocked molecules that ovens alone cannot pull free, allowing the rover's mass spectrometer to read a much wider chemical fingerprint.
Crucially, the team is careful about what the result does — and does not — mean. The compounds are organic in the chemistry sense, meaning they contain carbon, but that is not the same as evidence of life. Such molecules can be produced biologically, but they can also be delivered by meteorites or formed by purely geological processes. Several of the molecules detected are known to occur in interstellar dust and on asteroids.
What the discovery does establish is that Mars was capable of preserving a rich organic record. The Gale Crater lakebed where Curiosity drilled was once a long-lived body of water with the right pH and chemistry to support microbes, if any existed. Finding intact, complex carbon molecules billions of years later means that, if life did get a foothold there, the chemical fingerprints we would expect to find may still be sitting in the rock.
That is a hugely encouraging signal for the next phase of Mars exploration. NASA's Perseverance rover is already collecting and caching pristine samples from a different ancient delta with the goal of returning them to Earth, where labs can run far more sensitive analyses than any rover can carry. Today's result strengthens the case that those samples will be worth the trip.
The molecules themselves include a class of aromatic hydrocarbons — ring-shaped carbon structures — that scientists were specifically hoping to find. On Earth, similar compounds turn up in everything from petroleum to wood smoke to bacteria. Their presence on Mars suggests a long history of complex organic chemistry, even in the absence of any living thing.
The Curiosity team also noted nitrogen-bearing compounds in the sample. Nitrogen is a key element for biology, and finding it bound up with organic carbon in ancient sediments is exactly the kind of layered chemistry that Earth's earliest microbes would have left behind.
Curiosity was originally designed for a two-year mission. Thirteen years and 35 kilometres of driving later, it is still doing science that no other instrument on the planet can do. NASA engineers say its nuclear-powered generator and core systems are healthy enough to continue working through the rest of the decade, slowly climbing Mount Sharp and reading rock layers that capture more than a billion years of Martian history.
For now, scientists have a richer Martian rock record to chew on, and a stronger reason than ever to think that the answer to "was Mars ever alive?" is hiding in chemistry already in our reach.
