NASA’s Curiosity Rover Discovers Mysterious Oxygen Spikes on Mars

After more than six years of roaming the red planet of our solar system, NASA’s Curiosity rover has granted scientists at home a new mystery: the bizarre discovery of oxygen spikes in the Martian atmosphere.

Curiosity uses its Sample Analysis at Mars (SAM) portable chemistry lab to determine not only the composition of the surface atmosphere but also how its gases change with the seasons. The data generated by SAM revealed that carbon dioxide comprises 95 percent of the Martian atmosphere. The carbon dioxide freezes during the winter and sublimates back into a gas during the summer. Other gases in the Martian atmosphere, such as nitrogen and argon, follow predictable patterns according to this seasonal cycling of carbon dioxide.

Since oxygen molecules are relatively stable compounds, scientists expected oxygen levels to remain constant throughout the Martian atmosphere. However, Curiosity found that atmospheric oxygen levels rose by as much as 30 percent during the spring, which was far higher than what scientists had predicted. “This was a very unexpected result, an unexpected phenomenon. There’s a lot we don’t know about the oxygen cycle on Mars. That’s become apparent,” said Melissa Trainer, a planetary scientist at NASA.

Researchers have been working toward finding the culprit for the mysterious oxygen spikes, but there is no clear explanation. A possible source of oxygen could be the chemicals in the Martian dirt, such as hydrogen peroxide and perchlorates. “It’s pretty clear you need a flux from the surface,” Sushil Atreya, a professor of climate and space sciences at the University of Michigan, said. Hydrogen peroxide is produced as sunlight breaks up carbon dioxide and water vapor, and can stay in the ground for over 10 million years. However, this process would only account for one-tenth of the oxygen molecules needed to explain the rise in oxygen levels. Another possibility lies with perchlorates, which are toxic salts found in the soil. In theory, cosmic radiation could break down the perchlorates into compounds that would release oxygen, but this process is far too slow to adequately explain the oxygen spike.

The oxygen mystery is comparable to the erratic behavior of another gas found on the red planet: methane. Martian air contains a relatively low level of this gas, but for years groups of independent scientists have reported dramatic spikes of methane levels in the atmosphere. In June, Curiosity measured a substantial amount of methane—21 parts per billion by volume (ppbv)—relative to the usual range of 0.24 ppbv to 0.65 ppbv. But a few days later, methane levels dropped to less than one ppbv, confusing NASA scientists even more. "With our current measurements, we have no way of telling if the methane source is biology or geology, or even ancient or modern,” SAM principal investigator Paul Mahaffy said.

Martian atmospheric methane spikes could be related to its oxygen levels: Curiosity’s SAM readings have shown that oxygen and methane concentrations sometimes rise and fall at the same time. However, “it’s certainly not a perfect match,” Dr. Trainer said.

But if there is indeed a correlation between the two gases, then closely observing the behavior of one could help scientists better understand the other. "We're beginning to see this tantalizing correlation between methane and oxygen for a good part of the Mars year," Atreya said. "I think there's something to it. I just don't have the answers yet. Nobody does."

In order to crack the oxygen mystery, Dr. Trainer plans to examine more of the air on Mars’s surface with SAM. But since Curiosity only collects gas from one single area, its data cannot confirm whether this oxygen problem occurs only in one area or on the rest of the planet as well.

While it is too early to draw any definitive conclusions about the mysterious oxygen levels, Dr. Trainer can confidently say that “we’re pulling on all the current understanding we have and saying, gosh, it just doesn’t add up.”

So why are scientists so intrigued by this phenomenon? First of all, part of the fascination behind this discovery is the mystery surrounding it: there is no clear cause behind the fluctuations. The sudden changes in oxygen levels could even point to signs of life on Mars—a prospect scientists have tried and failed to find for decades. While this theory may be far-fetched today, observing these changes in oxygen levels will undoubtedly give scientists a better understanding of our planetary neighbor.