Yet Another Biosignature Gas Appears to Change With the Seasons on Mars

NASA’s Curiosity rover has found that the oxygen levels in the air in the Mars’s Gale Crater change along with the seasons on the Red Planet, rising in the spring and summer and falling during the winter. This follows Curiosity’s discovery that Martian methane also fluctuates with the seasons, and although both oxygen and methane can be generated by non-biological processes, these two gases are strong indicators of the presence of living organisms.

“We’re beginning to see this tantalizing correlation between methane and oxygen for a good part of the Mars year,” explains study co-author Sushil Atreya, a professor of climate and space sciences at the University of Michigan. “I think there’s something to it. I just don’t have the answers yet. Nobody does.”

Using data collected by Curiosity’s Sample Analysis at Mars (SAM) instrument, the research team found that the oxygen found in the atmosphere at Gale Crater was higher than expected in the spring and summer, and fell to unexpectedly low levels in the fall and winter. Although average oxygen levels only make up 0.16 percent of the Martian atmosphere (CO₂ is the largest component, at 95 percent), it’s important to remember that nearly all of the oxygen present in Earth’s atmosphere is due to biological processes, and the same may be true for Mars’ O₂. Oxygen can still be produced by non-biological processes, but the research team had trouble attributing such processes to the data they were seeing.

The team initially hypothesized that the extra oxygen could be produced by the splitting of carbon dioxide or water molecules over the course of the seasons, but they found that the process would require at least five times more water than what is presently in the Martian atmosphere to hike O₂ levels to what was being observed; additionally, CO₂ molecules don’t split fast enough to account for the seasonal variability. So, in the absence of instruments that could directly detect signs of life, the true cause behind the seasonal changes in oxygen levels remains a mystery.

“We’re struggling to explain this,” according to study leader Melissa Trainer, a planetary scientist at NASA’s Goddard Space Flight Center. “The fact that the oxygen behavior isn’t perfectly repeatable every season makes us think that it’s not an issue that has to do with atmospheric dynamics. It has to be some chemical source and sink that we can’t yet account for.”