There is not much air on Mars – atmospheric pressure is less than a hundredth of what we breathe on Earth – but the little there confuses planetary scientists.
Oxygen, which makes up about 0.13% of the Martian atmosphere, is the latest puzzle.
In an article published this month in the Journal of Geophysical Research: Planets, scientists working with data collected by NASA's Curiosity rover reported that oxygen levels varied unexpectedly with the seasons on Mars, at least in the neighborhood where Curiosity has been driving since 2012..
This follows the spacecraft's reading earlier this year of a large explosion of methane, another gas emitted by the living beings on Earth that disappeared almost perplexingly almost immediately.
"It's confusing but exciting," said Sushil K. Atreya, climate professor and space University of Michigan, who works on Curiosity atmospheric measurements. “It keeps us alert. Mars is certainly not boring.
Since a year on Mars lasts 687 days, scientists studying oxygen variations were able to examine the behavior for nearly three Martian years until December 2017.
The oxygen level "It rises relatively higher in the spring," said Melissa G. Trainer, space scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the new article, "and then down further, lower than we expected at the end. of the year. "
Carbon dioxide is the main ingredient in Martian air, and scientists have understood its ebb and flow for decades. On the poles in winter, it drops from the air and freezes on ice, then returns to the atmosphere as temperatures warm in spring.
High in the Martian atmosphere, ultraviolet light divides carbon dioxide into carbon monoxide and oxygen atoms, and then closer to the ground, interactions with water group oxygen atoms in molecular pairs.
Because oxygen molecules must be very stable, persisting for about a decade, the researchers expected the amount of oxygen molecules to remain almost constant..
The atmospheric measurements of curiosity showed exactly this pattern for nitrogen and argon, two other trace gases in the Martian atmosphere. But for oxygen, concentrations soared by a third during the spring.
“This was a very unexpected result, a unexpected phenomenon, ”said Dr. Trainer. "We don't know much about the oxygen cycle on Mars yet. It has become apparent."
Beyond the mystery, the cycle wasn't the same every year, and scientists couldn't find an obvious explanation – like temperature, dust thunderstorms or ultraviolet radiation – for what has changed from year to year.
On Earth, most of the oxygen is generated by plant photosynthesis. But so far, for Mars scientists, this is far below the list of explanations.
"You need to discard all other processes first before you go there," Atreya said.
The most likely sources are chemicals such as hydrogen peroxide and perchlorates, known to exist in Martian dirt. "It's pretty clear that you need a surface flow," said Atreya. "Nothing in the atmosphere will create that kind of increase."
But it's hard to figure out how these chemicals can release and absorb enough oxygen to explain the seasonal rise and fall, especially since there are only 19 oxygen measurements in five and a half years.
One intriguing possibility is that the mystery of oxygen may be linked to another trace of gas, methane, which is also acting strangely in the Martian atmosphere.
"It is not entirely clear whether there is a correlation or not," said Trainer.
Since 2003, Several teams of scientists have reported large methane explosions based on measurements of ground telescopes, orbiting spacecraft, and the Curiosity rover. Other times, methane was largely absent.
The presence of methane was a surprise for scientists, because the known processes for creating gas are biological – methane-producing microbes – or geothermal, which would be a promising environment for life to exist on Mars today.
Now scientists wants to know not just how methane is generated on Mars, but how it quickly disappears. In June, Curiosity saw a particularly strong methane burp – 21 parts per billion in volume. But when you repeated the experiment a few days later, left empty – less than 1 part per billion.
The European Space Agency's Mars Express spacecraft passed Gale Crater, the location of the spacecraft, just five hours after Curiosity measured the blast – and detected nothing. (The same instrument corroborated a methane explosion in 2013 observed by Curiosity.)
"I'd say it looks like this Spike measured by Curiosity had a short, local life, ”said Marco Giuranna, a scientist at the National Institute of Astrophysics in Italy, responsible for the Mars Express instrument.
Even between gusts, methane on Mars It represents a mystery. Curiosity measured a low but persistent presence of methane, about 410 parts per trillion, which rises and falls with the seasons. But a younger European orbiter, the ExoMars Trace Gas Orbiter, with the ability to measure as little methane as 50 parts per trillion, has not seen any methane since it began taking measurements in April last year.
Trace Gas Orbiter is looking at a region several kilometers above the ground and Curiosity is taking measurements on the surface. But scientists thought that near-ground methane would mix in the higher atmosphere in a few weeks.
"The scientific conundrum is that these two lines of evidence simply cannot be reconciled" Oleg Korablev of the Russian Space Research Institute wrote in an e-mail. Dr. Korablev is also the principal investigator of one of the two Trace Gas Orbiter instruments that makes methane measurements.
Håkan Svedhem, project scientist for Trace Gas Orbiter, said: “We don't know of any mechanism that can completely destroy methane in such a short time. So it's really a mystery, unless Curiosity is on top of the only local source on the planet and even if it was, that source must be small. "
Scientists working on the three missions plan to make almost simultaneous observations of the Gale Crater on December 15 and again in late December, Giuranna said.
Next year, four missions are scheduled to be launched toward Mars. Three of them – built by NASA, China and jointly by the European Union and Russia – will try to place new vehicles on the planet's surface. The fourth, a United Arab Emirates spacecraft, will go into orbit. But none of them will carry instruments to measure methane or oxygen.