Atmospheric gases on Mars certainly give us a lot of mystery. First it was deals with the disappearing, recurring methane. Now, oxygen levels have been observed to increase and fall over the Gale crater, of amounts that simply do not fit any known chemical processes.
The data comes from Curiosity, the Mars rover who has made his slow and methodical hike across the crater floor and up to the foot of Mount Sharp in the middle of it.
The robot not only looks down at stones under its tread; Curiosity also takes readings of the Martian atmosphere to measure the atmospheric changes of the season. It has been up there for three Mars years now (it is six Earth years), and scientists investigating the measurements have noticed that oxygen in the planet's atmosphere does not appear quite as expected.
There is actually not so much oxygen on Mars. Most of its thin atmosphere (95% by volume) is carbon dioxide or CO2. The remainder consists of 2.6 percent molecular nitrogen (N2), 1
(The Earth's atmosphere, on the other hand, is mostly nitrogen, 78.09% by volume and 20.95% oxygen.)
On Mars, atmospheric pressure changes during the year. In the winter hemisphere, CO2 freezes over the pole, causing the pressure to fall over the hemisphere. This results in a redistribution of gases between the hemisphere and the hemisphere to compare atmospheric pressure worldwide.
In the spring, when the polar lids melt and release CO2, the opposite effect occurs: pressure initially rises on that hemisphere, then evenes out when the gases are redistributed to the winter half color.
So, the second oscillation gases are predictable relative to the carbon dioxide level. Or at least they should be. When it comes to nitrogen and argon it is – these gases have behaved more or less exactly as expected. But acid? Nope.
During the spring and summer, oxygen rose by about 30 percent and dropped back to normal levels during the fall. This happened every year, but as the amount of oxygen increases varies from year to year, it seems that adds something to oxygen and then removes it again.
There is no known process that can produce this result.
The obvious question for such an odd measurement was whether there could be anything wrong with the Quadrupole Mass Spectrometer instrument or software. Several checks saw that everything worked well.
Another possibility was whether oxygen could be produced by water or carbon dioxide in any way decomposed in the atmosphere. This is also quickly eliminated – there is not enough water in the Martian atmosphere, and CO2 is degraded too slowly to accommodate the observed fluctuations.
Martian soil now contains a lot of oxygen. But the conditions for releasing it have not been observed – and that would not explain where it disappears for each year. The process by which solar radiation breaks apart oxygen and spreads it in space is also too slow.
"We are struggling to explain this," said planetary scientist Melissa Trainer of NASA's Goddard Space Flight Center.
"The fact that oxygen behavior is not completely repeatable every season makes us believe that it is not a matter of atmospheric dynamics. It must be some chemical source and sink that we cannot yet account for." 19659003] But there is a clue. Methane. It also rises dramatically during the summer months of March, increasing by up to 60 percent. Sometimes the methane and oxygen levels even seem to rise in pace. It is possible that whatever causes the methane fluctuations also causes the oxygen fluctuations.
What that might be is still a huge issue. Both gases can be produced by organic processes – that is, life – and both can be produced by geological processes.
We do not yet have proof that there is life on Mars, but it cannot be ruled out as a cause. (March 2020 will look for fossils, so maybe we will find out soon.)
But the team believes it is much more likely to be geological.
"We haven't been able to come up with a process yet that produces the amount of oxygen we need," said astronomer Tim McConnochie of the University of Maryland.
"But we think there has to be something in the surface earth that changes seasonally because there is not enough oxygen available atoms in the atmosphere to create the behavior we see."
So … any ideas?
The research has been published in Journal of Geophysical Research: Planets .