There is not much air on Mars – atmospheric pressure is less than a hundredth of what we breathe on Earth – but the little one there 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 rover reported that levels of oxygen unexpectedly varied with the seasons on Mars, at least in the neighborhood that Curiosity has run since 2012.
It follows the rover reading earlier this year of a major methane shortage, another gas that was released onto the earth by living things and which, with confusion, disappeared almost immediately.
"It's confusing but it's exciting," said Sushil K. Atreya, professor of climate and space science and technology at the University of Michigan working with Curiosity's atmospheric measurements. “It keeps us on our toes. March is really not boring. "
The March year lasts for 687 days, so the researchers who studied the oxygen variations could investigate the behavior for almost three March years, through December 2017.
The level of acid "Rising relatively higher in the spring," says Melissa G. Trainer, a research space at NASA's 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. At the poles in winter, it falls out of the air and freezes to ice, and then wires back into the atmosphere when the temperature warms in the spring.
High in the Martian atmosphere, ultraviolet light breaks down carbon dioxide into carbon monoxide and oxygen atoms and then closer to the ground, it interacts with water the oxygen atoms into molecular pairs.
Since oxygen molecules should be quite stable and persist for about a decade, researchers expected the amount of oxygen molecules to remain almost constant.
Curiosity's atmospheric measurements showed exactly that pattern of nitrogen and argon, two other trace gases in the Martian atmosphere. But for oxygen, concentrations shot up by a third in the spring.
"This was a very unexpected result, one unexpectedly phenomenon, ”Dr. Trainer. "There's a lot we don't know about the oxygen cycle on Mars. It's become obvious."
Adding to the mystery, the cycle wasn't the same every year, and the researchers couldn't 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 photosynthesis of plants. But so far, for the Mars scientists, it is far down the list of explanations.
"You have to exclude all other processes first before you go there," Dr. Atreya.
More likely sources are chemicals such as hydrogen peroxide and perchlorates that are known to exist in Martian dirt. "It's pretty clear that you need a flow from the surface," Dr. Atreya. "Nothing in the atmosphere will create this kind of upturn."
But how these chemicals can release and absorb enough oxygen to explain the seasonal increase and fall is hard to find out, especially since there are only 19 oxygen measurements in 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's not clear if there is a correlation or not," Dr. Trainer.
Since 2003 several research groups have reported large outbreaks of methane based on measurements from terrestrial telescopes, orbiting spacecraft and Curiosity rover. Other times, methane has largely been absent.
The presence of methane was one surprise for researchers, 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, it came up empty – less than 1 part per billion.
European Space Agency orbit Mars Express spaceships passed over Gale Crater, the rover's place, about five hours after Curiosity measured the deficiency – and discovered nothing. (The same instrument confirmed a 2013 methane deficiency observed by Curiosity.)
“I would say it seems like this nail measured with curiosity was very short-lived and local, says Marco Giuranna, a researcher at the National Institute for Astrophysics in Italy who is responsible for the Mars Express instrument.
Even between showers, methane on Mars constitutes 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.
Trace Gas Orbiter looks at a region 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 can't be united," Oleg Korablev from the Space Research Institute in Russia wrote in an e-mail. 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 the 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 planned for 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.