There is not much air on Mars – atmospheric pressure is less than one hundredth of what we breathe on earth – but the little that exists has puzzled planetary scientists.
Oxygen, which makes up about 0.13 percent of the Martian atmosphere, is the latest puzzle.
In a paper published this month in the Journal of Geophysical Research: Planets, researchers working on data collected by NASA's Curiosity report reported that oxygen levels unexpectedly varied with the seasons on Mars, at least in the neighborhood that Curiosity has run since 2012.
"It's confusing but it's exciting," said Sushil K. Atreya, a professor of climate and space science at the University of Michigan who works with Curiosity's atmospheric measurements. “It keeps us on our toes. March is really not boring. "
A March year lasts 687 days, so the researchers who studied the oxygen variation could investigate the behavior for nearly three Martian years, through December 2017.
The oxygen level " Rises relatively higher in the spring, "says Melissa G. Trainer, a research space at NASA Goddard Space Flight Center in Greenbelt, Md. And the lead author of the new essay, "And then it comes down lower, below what we would expect later in the year."
Carbon dioxide is the main ingredient in Martian air, and scientists have for decades understood its ebb and flow. in the winter it falls out of the air and freezes to ice, and then waves back into the atmosphere when the temperature is warm in the spring.
Because oxygen molecules should be fairly stable, remaining about a decade, researchers expected the amount of oxygen molecules to remain almost constant
Curiosity's atmospheric measurements showed exactly that pattern for nitrogen and argon, two other the Martian atmosphere, but for oxygen, the concentrations shot up by a third below
"This was a very unexpected result, an unexpected phenomenon ," Dr. Trainer. "There is a lot we do not know about the oxygen cycle on Mars. It has become obvious."
The addition to the mystery was not the same every year, and the researchers could not find an obvious explanation – like temperature, dust storms or ultraviolet radiation – for what changed from year to year.
On Earth, most oxygen is generated by plant photosynthesis, but so far, for Mars scientists, it is far below the list of explanations.
"You have to Exclude all other processes first before you go there, "said Dr. Atreya.
More likely sources are chemicals such as hydrogen peroxide and perchlorates known to be present in Martian's dirt." It's pretty clear that you need a flow from the surface Dr. Atreya said, "Nothing in the atmosphere will create this kind of rise."
But how these chemicals can release and absorb enough oxygen to explain the season's rise and fall is difficult to find out, especially since there are only 19 oxygen measurements over five and a half years.
An exciting possibility is that the oxygen mystery can be linked to another trace gas, methane, which also seems strange in the Martian atmosphere.
"It is not clear if there is a correlation or not," Dr. Trainer.
Since 2003 several research teams have reported large methane blasts based on measurements from terrestrial telescopes, orbiting spacecraft and Curiosity rover. Other times, methane has largely been absent.
The presence of methane was a surprise to scientists because the known processes for creating the gas are either biological – methane producing microbes – or geothermal, which would be a promising environment for life to exist on today's Mars.
Now scientists want to know not just how methane is generated on Mars but how it quickly disappears. In June, curiosity observed a particularly strong burst of methane – 21 parts per billion per volume. But when the repeated experiment a few days later later it came out empty – less than 1 part per billion.
European Space Agency orbits Mars Express spaceships passed over Gale Crater, the rover's place, about five hours after Curiosity detected the defect – and discovered nothing. (The same instrument confirmed a 2013 methane deficiency observed by Curiosity.)
"I would say that this nail measured with Curiosity was very short lived and local," said Marco Giuranna, a researcher at the National Institute for Astrophysics in Italy responsible for the Mars Express instrument.
Even between bursts, methane on Mars is a mystery. Curiosity has measured a low but persistent presence of methane, about 410 parts per trillion, which is rising and falling with the seasons. But a newer European orbiter, ExoMars Trace Gas Orbiter, with the ability to measure as little methane as 50 parts per trillion, has not seen any methane at all since it began measuring last April.
The Trace Gas Orbiter looks at an area several miles above the ground and the curiosity makes measurements at the surface. But scientists thought that methane near the ground would mix through the higher atmosphere within a few weeks.
"The science puzzle is that these two lines of evidence just cannot be joined," Oleg Korablev of the Space Research Institute in Russia wrote in an email . Dr. Korablev is also the principal investigator of one of the two Trace Gas Orbiter instruments that make methane measurements.
Håkan Svedhem, project researcher for Trace Gas Orbiter, said: “We know of no mechanism that can completely destroy methane in such a short time. So it really is a mystery unless curiosity sits just above the only local source on the planet, and even if it did, that source must be a small one. "
Researchers working on the three missions plan to make close simultaneous observations of Gale Crater on December 15 and again at the end of December," Dr. Giuranna.
Next year, four missions are scheduled to be launched against Mars. Three of them – built by NASA, China and together by the European Union and Russia – will try to place new rover on the planet's surface. The fourth, a spacecraft for the United Arab Emirates, enters orbit. But none of them will have instruments for measuring methane or oxygen.