There is not much air on Mars – the atmospheric pressure there is less than a hundredth of what we breathe on Earth – but there is little to no confused planetary scientists.
Oxygen, which makes up about 0.13 percent of the Martian atmosphere, is the latest puzzler.
In a paper published this month in the Journal of Geophysical Research: Planets, scientists working with data collected by NASA's rover Curiosity reported that oxygen levels unexpectedly varied with the seasons on Mars, at least in the neighborhood curiously driven by 2012. year.
This follows the rover's reading earlier this year of a massive burst of methane, another gas released to the earth by living things, and which strangely disappeared almost immediately.
"It's confusing, but it's exciting," Sushil K. said. Atrea, professor of climate and space science and engineering at the University of Michigan working on atmospheric measurements of curiosity. “He's holding us to our toes. Mars is certainly not boring. "
One year on Mars lasts 687 days, so scientists studying oxygen variations could study the behavior over nearly three Mars years, by December 2017.
The oxygen level "It rises relatively higher in the spring," said Melissa G. Trainer, a NASA Space Research scientist at the Goddard Space Flight in Greenbelt, Madrid, and lead author of the new paper, "and then descends lower, below what we expected later on. the year. "
Carbon dioxide is a major constituent of Martian air, and scientists have understood its abstraction and flow for decades. On the poles in winter, it falls from the air and freezes in ice, then oil returns to the atmosphere as temperatures warm in the spring.
In the Martian atmosphere, ultraviolet light separates carbon dioxide into carbon monoxide and oxygen, and then closer to the ground, interactions with water paste the oxygen atoms into molecular pairs.
Because oxygen molecules need to be fairly stable and last about a decade, researchers expected that the amount of oxygen molecules would remain almost constant..
Atmospheric measurements of curiosity showed exactly that pattern for nitrogen and argon, two other traces of gas in the Martian atmosphere. But for oxygen, concentrations increased by a third during the spring.
"This was a very unexpected result unexpectedly a phenomenon, "said Dr Trainer. "There's a lot we don't know about the oxygen cycle on Mars. It became obvious. "
Adding to the mystery, the cycle was not the same every year, and scientists couldn't find an obvious explanation – like temperature, dust storm or ultraviolet radiation – about what has changed from year to year.
On Earth, most oxygen is produced by photosynthesis of plants. But so far, for Mars scientists, it's far from the list of explanations.
"You need to turn off all other processes before you get there," Dr. Atrea said.
More likely sources are chemicals such as hydrogen peroxide and perchlorate, which are known to exist in Martian dirt. "It is quite clear that you need flux from the surface," said Dr. Atrea. "Nothing in the atmosphere will create this kind of rise."
But how these chemicals can release and absorb enough oxygen to explain seasonal growth and fall is difficult to grasp, especially since there are only 19 oxygen measurements over five and a half years.
An intriguing possibility is that the mystery of oxygen may be tied to yet another trace of gas, methane, which also works strangely in the atmosphere of Mars.
"It is not entirely clear whether there is a correlation or not," Dr Trainer said.
Since 2003 several teams of scientists have reported large methane bursts based on measurements from Earth-based telescopes, orbiting spacecraft and rover infestation. At other times, methane is largely absent.
The presence of methane was a a surprise for scientistsbecause the well-known processes of gas generation are either biological – methane-producing microbes – or geothermal, which would be a promising environment for life on Mars today.
Now the scientists want to know not just how methane is generated on Mars, but also as it disappears rapidly. In June, curiosity saw a particularly strong impact on methane – 21 parts per billion volume. But when he repeated the experiment for several days later, came out empty – less than 1 part per billion.
The spacecraft of the European Space Agency on Mars Express passed over Gale Crater, the rover's place, only about five hours after Curiosity measured the crack – and found nothing. (The same instrument confirmed the 2013 methane firing noted by curiosity.)
"I would say it looks like this spear measured with curiosity was very short-lived and local, "said Marco Ururana, a scientist at the National Institute of Astrophysics in Italy, which is responsible for the Mars Express instrument.
Even between bursts, methane on Mars is a mystery. The curiosity measured a small but persistent presence of methane, about 410 trillion parts, rising and falling with the seasons. But a newer European orbiter, the ExoMars Trace Gas Orbiter, capable of measuring as little methane as 50 parts per trillion, has yet to see any methane since it began taking measurements in April last year.
The Tras Gas Orbiter explores a region a few miles above the ground and curiously makes surface measurements. But scientists thought the methane near the earth would mix through the higher atmosphere in a matter of weeks.
"The science is that these two pieces of evidence cannot be reconciled," Oleg Koralev of the Institute for Space Research in Russia wrote in an e-mail. Dr Korablev is also the lead researcher on one of two trace gas tracer instruments that make methane measurements.
Hakan Swedem, a scientist for the Orbiter Tracer gas project, said: "We don't know a mechanism that can destroy methane completely in such a short time. So it's really a mystery, unless Curious is sitting on top of the planet's only local source, and even if it were, that source must be small. "
Scientists working on the three missions plan to make approximately simultaneous observations of King Gale on December 15 and again at the end of December, said Dr. Ururana.
Next year, four missions to Mars are planned. Three of them – built by NASA, China and jointly by the European Union and Russia – will try to place new rovers on the surface of the planet. Fourth, the UAE spacecraft will enter orbit. But none of them will carry instruments for measuring methane or oxygen.