Big news from Mars today: NASA's Curiosity rover found ancient traces of organic matter embedded in Martian rocks and detected a "seasonal variation" in atmospheric methane on the Red Planet — an annual pulse of the gas, almost as if something out there were breathing.
These are exciting findings, published as twin papers in the journal Science today (June 7). But they aren't proof of life on Mars, or even necessarily strong evidence that there's anything living, or anything that used to be alive, out there. The organic compounds aren't even the first molecules of their kind found on Mars, though they are the oldest.
"We can explain both of these things with geological processes," said Inge Loes ten Kate, an astrobiologist at Utrecht University in the Netherlands who wrote a commentary for Science accompanying the two papers. [7 Most Mars-like Places on Earth ]
Finding organic compounds — which are substances that contain carbon and are considered necessary components of life — in 3.5-billion-year-old rocks on Mars is a big deal, ten Kate told Live Science, and so is the discovery of the seasonal methane (CH4) variation in the atmosphere.
Living things produce a lot of organic molecules. And life as we know it requires organic molecules to exist. So the Martian traces of organic matter do hint that the basic conditions for life to form were present on Mars at around the same time they existed on Earth. (Curiosity has already shown that water flowed in Gale Crater, the same place the rover found these organic compounds, billions of years ago.)
And the seasonal methane pulse is perhaps, maybe, possibly — but far, far from certainly — the sort of signal Curiosity might detect if life did form back then and was still around somewhere, ten Kate said. On Earth, living things (especially bacteria) produce lots of methane, though the gas also has plenty of non-living sources.
But scientists found ancient organic matter! On Mars! Why isn't that a bigger deal?
One big reason, ten Kate said, is that it's not actually that surprising. "Organic matter" in this context doesn't mean anything we'd recognize from our lives on Earth. These aren't tufts of grass, or bits of flesh, or dead cells. "Organic matter" includes a whole host of compounds with carbon atoms in them. They're considered necessary for life to form, but there are plenty of places with lots of organic compounds but no life. In this case, Curiosity found molecules with names like "thiophene" (C4H4S) and "dimethylsulfide" (C2H6S) that aren't all that rare in the solar system.
There's enough ambient carbon and hydrogen in the solar system that they react to form basic organic compounds pretty frequently, even without biology involved, ten Kate said.
"Even nowadays on Earth, we see a large influx of extraterrestrial [organic] material in the form of interplanetary dust and meteorites," ten Kate said.
That stuff is thought to be spread throughout the solar system, she said. And scientists already expected that, in the early, more turbulent days of the solar system, organic compounds would rain down on Mars. (We can find organic material on the Jupiter's moon for the same reason, and Curiosity first spotted organic compounds on Mars back in 2014, though in less-ancient rocks.)
These newly-found ancient organics, ten Kate said, serve to confirm that the basic conditions for life to form really did exist on Mars 3.5 billion years ago, and that there wasn't any outside force (say, ultraviolet light) powerful enough to destroy them entirely.
The authors of the two studies in Science agree with her, writing that there's no way to tell what produced the molecules And certain features of the molecules show they aren't the direct, unchanged remnants of anything living.
"[Curiosity's] molecular observations do not clearly reveal the source of the organic matter in [Gale Crater]. Biological, geological and meteoritic sources are all possible," they wrote.
Part of the problem, the researchers wrote, is that the molecules have changed a great deal in the eons since they originally formed. Whatever chemical structure they once had might have offered clues as to their origin, but it's long since been lost.
For those reasons, ten Kate said, the methane variation is the more exciting finding. Certainly, there are geological processes that could make methane levels change over the Martian year, she said. A likely candidate: "serpentinization," where water and minerals react, releasing methane. It's possible, ten Kate said, that this could happen on Mars. And the reaction might speed up and slow down over the course of the year as the planet warms and cools, producing the pulse without any living source.
To figure out the source of the methane flux, ten Kate said, scientists need to determine how widespread it is on Mars. (So far, it's been detected only in Gale Crater, where Curiosity hangs out.) They also need to figure out how old it is and its specific chemistry; Curiosity's sensors didn't reveal whether the methane molecules are ancient or new, or whether they include similar isotopes of carbon to methane released by life on Earth.
The answers to those questions will require more equipment and more measurement hours, ten Kate said. But these findings do, at the very least, point the way forward in the Martian hunt for life.
Originally published on Live Science.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.