NASA’s Curiosity rover, for three Martian years—nearly six years to us Earthlings—has been sniffing the air above Mars’ Gale Crater, its near-equatorial exploration site. Using its Sample Analysis at Mars (SAM) portable chemistry lab, the rover has ascertained not only what the surface atmosphere is made of, but also how its gases change with the seasons.

Many of Mars’ gases “are very well behaved,” says Melissa Trainer, a planetary scientist at NASA and a team member on the SAM experiment. One, however, appears to be behaving in a decidedly unexpected and altogether bizarre manner: oxygen.

Scientists have long known that carbon dioxide on Mars, which makes up 95 percent of the planet’s atmosphere, freezes out over the poles in winter, and sublimates back into a gas in summer. In the thin air around Gale Crater, Curiosity’s measurements have shown tiny amounts of inert argon and nitrogen periodically rising and falling as expected, due to this seasonal cycling of carbon dioxide.

Curiosity’s instruments also registered atmospheric oxygen rising and falling at similar times but in amounts that defy easy explanation. There was far more of it during the spring and summer, and less of it in the winter, than the seasonal whooshing back and forth of other gases would predict.

That suggests something is making or unleashing stores of oxygen in the warmer months and trapping or swallowing it up during frigid ones. It could be a geological, chemical, atmospheric or, perhaps even a biological process, but right now, no one has the foggiest as to what the culprit actually is. And although the oxygen’s trampolining certainly appears to be a local feature, it might be a regional or even global peculiarity.

François Forget, a planetary scientist at the French National Center for Scientific Research, says that this finding is surprising, weird and mysterious. Jon Telling, a geochemist and geomicrobiologist at Newcastle University, says he and other experts are understandably “flummoxed.”

An unanticipated challenge has suddenly been laid out before the scientific community. It is unclear when, or even if, the case of the overzealous oxygen will be cracked. Already, says Paul Niles, a planetary geologist and analytical geochemist at NASA, it is abundantly clear that “Mars is a lot more alien than we thought.”

Curiosity Killed the Stats

In situ measurements of the pressure, temperature and composition of Mars’ atmosphere date back nearly a half century, from the Viking landers in the 1970s through to the Spirit, Opportunity and now Curiosity rovers. Curiosity’s SAM suite, however, has painstakingly tracked how Martian atmospheric gas amounts change through the seasons, thereby providing scientists with a game-changing, precise chronicle of the planet's atmosphere.

Oxygen’s too-high spikes and too-low nadirs during the warmer and colder months, respectively, came as a shock. Curiosity’s scientists could conceive of only two possibilities: either a mysterious creator and destroyer of oxygen existed on Mars that scientists were unaware of, or the measurements were wrong. Trainer, lead author of the study reporting the discovery in the Journal of Geophysical Research: Planets, emphasizes that this detection and analysis took many years, with all possible false positive triggers ruled out.

“I think they’ve done their due diligence,” Niles says. Plenty can go wrong with these interplanetary science experiments, from equipment malfunctions to contamination. Regardless, he says, “I don’t see any reason to have any doubt in the oxygen measurements.”

“I hope it’s real,” Forget says, because an extraterrestrial oxygen enigma is far more fun than a glitch.

A true enigma would force researchers to go back to basics, says Manish Patel, a planetary scientist at the Open University. “We must first interrogate our understanding of the known processes for creating oxygen, before we invoke any new, or controversial, processes.”

Trainer and her colleagues did just that. But they still came up short. Solar radiation could be breaking up oxygen molecules and blowing them away into space, but this process appears to be too slow and inefficient to account for the seasonal dips seen by Curiosity. Perhaps carbon dioxide’s slow breakdown in the atmosphere could have released oxygen, causing a summertime spike—but again, this process would take too long to produce the observed peaks.

Martian soil is rich in oxygen-containing hydrogen peroxide and perchlorates. The Viking landers demonstrated that warm, damp air could free this oxygen, but those conditions do not prevail across enough of the planet—let alone Gale Crater—to suffice for the SAM data. Soil bombardment by ionizing radiation from cosmic rays and solar storms might do the trick, but is estimated to require a million years to create the oxygen peak seen during one solitary spring.

We simply do not know enough about Mars to get a grip on this particular puzzle, says Niles. So much about its chemistry—how gases are transported above and within the planet, what sources and sinks they may have—remains deeply uncertain. For all we know, he says, events in Mars’ past could have conspired to lock away vast amounts of oxygen belowground, which is now, for some reason, surging back into the atmosphere.

If the answer is not to be found in Mars’ lifeless air and rocks, could some cryptic, alien form of biology be to blame? On Earth, photosynthesis and respiration by living things cause tiny fluctuations in our planet’s otherwise steady oxygen concentration. We shouldn’t expect this on Mars, though. “That’s far out,” Telling says: Mars appears too inhospitable for a critical mass of life capable of sustaining either process. “It’s almost certainly going to be a nonbiological chemical reaction.”

Trainer herself does not rule out a biological explanation, but nevertheless underscores its unlikeliness. “People in the community like to say that it will be the explanation of last resort, because that would be so monumental,” she says. There are abiotic mechanisms aplenty, both known and unknown, to rule out first before leaping to any more sensational claims.

Riding the Methane Rollercoaster

This oxygen conundrum is reminiscent of Mars’ mischievous methane, another of the Red Planet’s long-standing mystery. Although Martian air contains a persistent low background level of methane, for years multiple independent groups of scientists have claimed detections of dramatic, unpredictable spikes in the gas’ atmospheric abundance. Curiosity spotted one such spike in 2013, and another substantially greater one in 2019. Puzzlingly, many ground-based methane detections have not been corroborated by atmosphere-probing spacecraft high above the Martian surface, including ESA’s Trace Gas Orbiter and Mars Express.

Methane has a wide range of sources and sinks on Earth. Some are geological, but many are biological. On Mars, airborne methane should break down quickly, so any spikes are presumed to have been generated shortly before a detection is made; this fact has fueled speculation over an extant Martian microbial progenitor as the cause of Curiosity’s observed spikes. But as with oxygen, “methane is completely puzzling as well,” Forget says. Scientists can say little about it with any certainty, including whether it crops up on a local, regional or global scale, and why.

If Mars’ methane spikes are genuine, and caused by some as-yet-unknown chemical process, scientists posit this should have knock-on effects for other gases in the atmosphere—but nothing of the sort has ever been observed. Until now, that is: Curiosity’s SAM readings suggest that oxygen levels, across the seasons, sometimes rise and fall with Mars’ methane concentrations.

“It’s certainly not a perfect match,” Trainer says; for each gas, the timings and extremities of the spikes and troughs diverge, which suggests both gases are controlled by different combinations of processes. If, however, the implied imperfectly synchronized dance between the two gases is real, then understanding one’s behavior may help us comprehend the other.

With so many questions in hand, where do we go from here? Sniffing more of the air down at Mars’ surface with SAM would always be welcome, Trainer says; that data can be fed into models and laboratory experiments that may unravel the mystery of Mars’ manic oxygen.

Curiosity, though, is only huffing gas in one single area, so its data are ill-suited for confirming whether this oxygen roller-coaster is a local or global occurrence, says Patel. The Trace Gas Orbiter, however, could be a big help. “I will really bet a lot that if this oxygen variation is real, it has to be quite global,” says Forget—and this orbiter is well placed to determine that.

It is too early to predict if this will be a colossal or miniature phenomenon, one with a rudimentary explanation or a far more revolutionary root. At present, Trainer says, all we can confidently say about Mars’ wild oxygen levels is that “we’re pulling on all the current understanding we have and saying, gosh, it just doesn’t add up.”