After more than six years sniffing the red planet’s thin, frigid air, a NASA rover has made a startling discovery: There’s more oxygen gas in the Martian atmosphere than scientists expected, and what’s there is behaving strangely.
In the Martian spring and summer, the red planet’s oxygen levels spike an extra 400 parts per million, or 30 percent above what researchers expected to see based on the behavior of other gases in the planet’s atmosphere. The oxygen spike seems to partially correlate to another gassy mystery: a seasonal ebb and flow of atmospheric methane on Mars. (Find out more about the building blocks of life found so far on Mars.)
“Mars has fooled us again!” says Sushil Atreya, a planetary scientist at the University of Michigan who is part of the team reporting the odd oxygen results in the Journal of Geophysical Research: Planets.
Though it’s tempting to think of photosynthesis when hearing about oxygen in a planet’s atmosphere, non-living processes are known to make oxygen on Mars, and these findings are not necessarily evidence of life. Instead, the results highlight gaps in our understanding of the red planet’s surface chemistry—holes that must be filled if we are to hunt for direct evidence of past or present Martians.
Next summer, four nations will be launching missions to Mars to further that goal, including NASA’s Mars 2020 rover, which will cache samples for future return to Earth. The European Union and Russia have also teamed up on the ExoMars mission, which includes the Rosalind Franklin rover. That robotic explorer will drill more than six feet into the Martian surface, exploring the red planet’s inner chemistry better than ever before. (Here’s how the InSight NASA lander discovered that the planet experiences mysterious magnetic pulses at night.)
“With any new planetary system, [life] has to be the explanation of last resort,” says lead study author Melissa Trainer, a planetary scientist at NASA Goddard Space Flight Center. “We need to make sure we fully understand how Mars operates as a system.”
Gases behaving badly
Much of what we currently know about Mars’s atmosphere comes from measurements by ground-based telescopes or Mars orbiters, which can look for chemical signals to reveal global composition, including oxygen levels. Scientists previously knew that this oxygen can be made through non-biological means.
As ultraviolet light from the sun smashes into carbon dioxide and water vapor in Mars’s atmosphere, it breaks those molecules into their components, creating molecular oxygen, or O2. Eventually, this O2 goes through another set of chemical reactions to form CO2, completing the cycle. In the meantime, an individual O2 molecule can hang around in the Martian atmosphere for at least 10 years, if not several decades. This sunlight-formed O2 makes up about 0.13 percent of the red planet’s modern atmosphere. (Explore Mars’s warmer, wetter past with our red planet interactive.)
Because of its long-term stability, researchers thought that Martian oxygen would essentially behave as a nonreactive gas, ebbing and flowing just like the inert gases argon and nitrogen. But because of obscuring dust and other factors, telescopes can’t get reliable data on the air just above the Martian surface. That’s where the Mars rover Curiosity comes in. It has been rolling across Martian terrain since 2012, and its dataset on the local air is the most thorough ever assembled.
“This is a really unprecedented measurement set,” Trainer says.
Curiosity’s measurements revealed that Mars’s oxygen isn’t so well behaved, after all. Not only do O2 levels spike each Martian year, but the spikes themselves are irregular from one year to the next.
“When we first saw it, my first reaction was, This is totally bizarre,” Atreya says.
What’s more, the oxygen spike appears oddly similar to a seasonal spike in methane, a trace gas in the Martian atmosphere that on Earth is often associated with life. Both gases’ concentrations taper off in the fall and winter and then rise in the spring and summer—but with some key differences. Oxygen starts its climb earlier in the Martian year than methane, and unlike oxygen’s irregular spikes, Mars’s methane peaks are consistent year to year.
“That’s a whole new part of the mystery—we find it extremely intriguing, and we’re very interested to figure out if there’s a true correlation between those two,” Trainer says. “They both potentially could have a source at the surface, [but] it’s not clear that they have the same source.
Fast and curious
For now, there aren’t any obvious suspects for what causes the oxygen spike. The usual sunlight-driven process that makes Martian O2 does not happen fast enough to account for such a rapid rise. So Trainer and her colleagues have focused their gaze on the red planet’s surface, where there are plenty of chemicals that contain oxygen.
One potential suspect is perchlorates, which are stable, toxic salts found in the Martian soil. In principle, cosmic radiation slamming into the red planet could break down perchlorates into more reactive compounds that could, in turn, release O2. But researchers say this process occurs at a millionth of the speed needed to account for the annual spike.
This global mosaic of Mars is centered on Valles Marineris, the solar system's largest canyon range. It extends 4,000 kilometers and is seven kilometers deep in some places.
Another possibility is hydrogen peroxide, water’s unstable cousin. Used on Earth as an antiseptic, hydrogen peroxide gas is also produced continually as sunlight breaks up carbon dioxide and water vapor, ultimately making up a small portion of the Martian atmosphere. Chemical models suggest that this hydrogen peroxide can diffuse into the Martian soil and stick to particles as deep as 10 feet underground, forming a buried oxygen reservoir of sorts. (Some data show that pockets of subsurface water on Mars may hold enough dissolved oxygen to support life.)
But even in the best case, which assumes that hydrogen peroxide can stay in the soil for 10 million years at a time, Trainer’s team says that this process accounts for just a tenth of the oxygen molecules needed to account for the spike.
The team also reexamined results from the Viking landers of the 1970s, which found that humidifying Martian soil made it release a surprising amount of oxygen gas. But Trainer and her colleagues don’t think this ties directly to their observations. For one, the Viking experiment was done at 50 degrees Fahrenheit, which is far warmer than Mars’s average surface temperature. And just because Martian soil can release oxygen all at once doesn’t address how the oxygen spike shows up year after year, with no obvious method of replenishment.
“[Viking] doesn’t give us a suspect—it’s another crime, I suppose,” says study coauthor Timothy McConnochie, a postdoctoral researcher at the University of Maryland.
Trainer and her colleagues are still brainstorming possible answers. Atreya, for instance, is keen to look more at how high-energy particles zooming through the galaxy could trigger chemical reactions within the first few feet of Martian soil. Bethany Ehlmann, a Caltech planetary scientist who wasn’t involved with the study, notes that Mars’s soils are more reactive than our home loam.
“We still don’t completely understand what the Martian soil is made out of; it’s very clear that it’s quite exotic, relative to Earth, in terms of being very rich in iron-bearing minerals, sulfur-bearing minerals,” she says. “They seem to have interesting properties.”
Future missions might be able to help, especially if they can take more atmospheric measurements. Because of the many science demands on Curiosity, Trainer’s team obtained only 19 data points across the Martian seasons. While this gives them a sense of the long-term pattern, they can’t see any shorter-term changes. What would researchers find if they could take daily, or even hourly, oxygen and methane readings from Mars?
“That’d be way, way more useful—that’d be amazing,” says study coauthor Germán Martínez, a staff scientist at the Lunar and Planetary Institute.
With each new study, scientists will get a better sense of what and how much non-living reactions are contributing to the Martian air supply—better preparing them, and us, to draw a line between geology and biology.
“On Earth, all those processes are really overprinted with the effects of our biosphere,” Trainer says. “Going to Mars, we’re surprised by the behavior of the oxygen. That tells us there’s a lot more going on—a lot more digging in, so to speak.”