There is not much air on Mars – the atmospheric pressure there is less than one hundredth of what we breathe on Earth – but what is small there are puzzled planetary scientists.
Oxygen, which represents about 0.13 percent of the Martian atmosphere, is the latest puzzled.
In a paper published this month in the journal Geophysical Research: Planets, scientists working with data collected by NASA's Curiosity Rover report that oxygen levels vary unexpectedly with Mars seasons, at least in the quarter that Curiosity has been circulating since 2012.
"It's confusing, but it's exciting," says Sushil K. Atreia, a professor of climate and science and space engineering at the University of Michigan who works on Curiosity is atmospheric measurements. "That keeps us on our toes. Mars is certainly not boring. "
The Martian year lasts 687 days, so scientists studying the changes in oxygen were able to study the behavior for nearly three Martian years, until December 2017.
The oxygen level " Rises relatively "high in the spring," says Melissa G. Trainer, a space scientist at NASA Goddard Space in Greenbell, Md., and lead author of the new book, "and then it goes lower, we can expect more late in the year. ”
Carbon dioxide is the main constituent of Martian air and scientists For decades, they have understood its efflux and flow, and through the pillars in winter it falls out of the air and freezes to ice, then ejects back into the atmosphere when temperatures warm in the spring. of carbon monoxide and oxygen atoms and then closer to the earth, interactions with water control the oxygen atoms in molecular pairs. Because oxygen molecules have to be fairly stable and last for about a decade, researchers expect that the amount of oxygen molecules will remain almost constant. gases in the Martian atmosphere. But for oxygen concentrations increased by one-third in the spring.
"It was a very unexpected result, an unexpected phenomenon ," says Dr. Coach. "There are many things we do not know about the oxygen cycle of Mars. This becomes obvious. "
Adding to the mystery, the cycle is not the same every year, and scientists could not find an obvious explanation – such as temperature, dust storms or UV – for what changed from year to year.
The most oxygen on Earth is generated from photosynthesis of plants, but for now, for Mars scientists, this is well below in the list of explanations.
"You must first rule out all other processes before you go there, "says Dr. Atreia.
More likely sources are chemicals such as hydrogen isch and perchlorate, which is known to exist in Martian dirt. "It's pretty clear that you need a flow from the surface," said Dr. Atreia. "Nothing in the atmosphere will create such growth."
how these chemicals can release and absorb enough oxygen to account for seasonal rise and fall is difficult to understand, especially since there are only 19 oxygen measurements over five and a half years.
An intriguing possibility is that the oxygen mystery is bound to another gas, methane, which also acts strangely in the Martian atmosphere.
"It's not completely clear if there is a correlation or not," says Dr. Coach.
Since 2003, several teams of scientists have reported major methane bursts based on measurements from Earth-based telescopes, an orbital spacecraft and the Curiosity rover. Other times, methane is largely absent.
The presence of methane was a surprise to scientists because the known gas generation processes are either biological – methane producing methane – or geothermal, which would be a promising environment for the life of today Mars.
Now scientists want to know not only how methane is generated on Mars, but also how it quickly disappears. In June, Curiosity noticed a particularly strong burning of methane – 21 parts per billion by volume. But when he repeated the experiment a few days later he appeared empty – less than 1 part per billion.
The orbit of the European Space Agency's Mars Express spacecraft passed over Galle Crater, the site of the rover, only about five hours after Curiosity measured the burst – and found nothing. (The same instrument confirms the methane collapse in 2013 observed by Curiosity.)
"I would say this spike measured by Curiosity, seems to be very short-lived and local," says Marco Jurana, a scientist at Italy's National Astrophysics Institute, which is responsible for the Mars Express instrument.
Even among the outbursts of Mars methane is a mystery. Curiosity measures the low but persistent presence of methane, about 410 parts per trillion, which rises and falls with the seasons. But a newer European orbiter, the ExoMars Trace Gas Orbiter, with the ability to measure as little methane as 50 parts per trillion, has not yet seen any methane since it began making measurements last April.
The Trace Gas Orbiter looks at a region a few miles above the ground and Curiosity makes surface measurements. But scientists thought that methane near the ground would mix in the higher atmosphere within a few weeks.
"The scientific puzzle is that these two pieces of evidence simply cannot be reconciled." Oleg Korablev of the Russian Space Research Institute wrote in an email . p Korablev is also the principal investigator of one of the two instruments of the Trace Gas Orchestra, which makes methane measurements.
Hakan Swedham, a scientist on the Trace Gas orbit project, says: "We do not know the mechanism that could completely destroy methane in such a short time. So it's really a mystery, unless curiosity sits exactly on the planet's only local source, and even if it did, that source must be small. "
Scientists working on the three missions plan to make close observations at Galle Crater on December 15 and again at the end of December," said Dr. Giuran.
Four missions to Mars are scheduled to be launched next year. Three of them – built by NASA, China and jointly by the European Union and Russia – will try to put new rowers on the surface of the planet. The fourth United Arab Emirates spacecraft will enter orbit. But none of them will carry methane or oxygen measuring instruments.