So far, this sniffing has resulted in some findings that researchers are still trying to understand.
Earlier this year, the rover's tuned laser spectrometer, called SAM, which stands for test analysis on Mars, discovered the largest amount of methane ever measured during its mission.
SAM has also found that over time, oxygen appears in a way that is not explained by any chemical process that scientists currently understand.
SAM has had plenty of time – about six years – to sniff and analyze the atmospheric composition on Mars. The data revealed that at the surface 95% of the atmosphere is carbon dioxide, followed by 2.6% molecular nitrogen, 1
Like Earth, Mars goes through its seasons; over a year the air pressure changes. This happens when carbon dioxide freezes in winter at the poles, causing the air pressure to lower. It rises again in spring and summer and redistributes over Mars as the carbon dioxide evaporates.
Relative to carbon monoxide, nitrogen and argon also follow similar dips and peaks. But oxygen did not.
Surprisingly, oxygen actually increased with a peak increase of 30% in spring and summer before dropping back to normal in the fall.
Given how long the curiosity has been monitoring the atmosphere, one could find that this pattern was repeated, albeit with varying amounts of oxygen.
The variation indicates that oxygen is created by something and then removed.
"The first time we saw it, it was just silent," said Sushil Atreya, study author on a new paper on oxygen levels and professor of climate and space science at the University of Michigan.
To exclude errors, the researchers checked that SAM was working properly, but found no problems.
"We are struggling to explain this," said Melissa Trainer, study author and planetary scientist at NASA's Goddard Space Flight Center. "The fact that oxygen behavior is not completely repeatable every season makes us think that it is not a matter of atmospheric dynamics. It must be some chemical source and sink (of elements in the earth) that we can't explain yet for. "
It goes back to the methane mystery.
The June treatment stated 21 parts per billion units per volume, or ppbv. That means the air volume on Mars is estimated, one billionth of the air volume is methane.
So why is this unusually large amount of methane so interesting? On Earth, microbial life is an important source of But NASA also warned that life expectations should be met because rocks and water interactions can also create methane, and Mars has water and an abundance of rocks.
"With our current measurements, we have no way to tell whether the methane source is biology or geology, or even ancient or modern, "said SAM's chief investigator Paul Mahaffy of NASA's Goddard Spaceflight Center in Greenbelt, Maryland.
tta is not the first time methane has been detected on Mars by Curiosity. During her assignment since landing in August 2012, curiosity has discovered methane many times and studies have been written about how gas levels really seem to rise and fall depending on the season. It can spike as much as 60% during the summer.
Can the two be connected to a similar chemistry because they fluctuate in the same way?
"We are beginning to see this strange connection between methane and oxygen for a good part of the March year," Atreya said. "I think there's something to it. I simply don't have the answer yet. Nobody does."
The origin of methane or oxygen will not be obvious because the rover has no instruments that can track or determine the source. And although both oxygen and methane can be created from biological sources, they can also arise because of chemistry, such as the interaction between water and rocks.
"We haven't been able to come up with a process yet that produces the amount of oxygen we need, but we believe there must be something in the surface soil that changes seasonally because there are not enough oxygen atoms in the atmosphere to create behavior we see, "said Timothy McConnochie, a study co-author and assistant researcher at the University of Maryland.
The researchers wanted to share their findings in the hope that Martian experts can help determine which process creates these increases.
"This is the first time we've seen this interesting behavior for several years. We don't quite understand it," Trainer said. "For me, this is an open conversation to all the smart people out there who are interested in this: See what you can come up with."