The mystery of the presence of methane gas on Mars is complicated.
This gas, which on Earth is largely produced by living things, was previously discovered on a red planet with remote observation. It was also detected on the surface of NASA's Curiosity robot.
But the most specific search carried out ever on the planet, conducted by a European-Russian satellite, did not succeed.
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And that despite the fact that this satellite in orbit, called Orbiter for straight gas (TGO), is capable of detecting methane at incredibly low levels.
Although the concentration of methane (whose chemical formula is CH4) was only a few dozen molecules for each trillion molecules of Mars air, the TGO should be able to identify its presence. If it exists.
The fact that the satellite did not discover during the atmospheric exploration, between April and August 2018, raises some complicated questions and some fascinating opportunities.
Inevitably, some will argue that previous methane discoveries are wrong, but Oleg Korablev of the Institute of Space Studies at the Russian Academy of Sciences in Moscow is not one of them.
"We do not want to criticize the results of others, we can only defend them precision from ours, "the TGO scientist told the BBC.
He added: "We only present the data and let the theorists try to explain what is happening."
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In 2013, samples collected from the Curiosity robot in the Gale crater contained methane peaks in the air of some parts of a billion.
Another satellite in orbit, the European space ship Mars Express, has widely confirmed these data with simultaneous observations.
Thus, if it was accepted that methane actually existed in 2013 and that currently TGO is the standard of Mars quality, it seems that scientists must identify a process that is not yet known which can quickly remove CH4 from the atmosphere for a short period of time.
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This is a working hypothesis, says Manish Patel, another scientist from the Open University of the United Kingdom.
"If we take into account previous measurements and given that we rely on our own results, it means that something happens in the atmosphere between those two points in time, and that's something we did not anticipate, "explains Patel.
"We expect methane to stay in the atmosphere of Mars for hundreds of years, but sunlight destroys it, but in a relatively long time scale in terms of human observation, no matter what was there before. I need to be there today, even if it was at a diluted level, "he adds.
It's like we faced it double puzzle.
He debated where methane could appear on Mars, tempting the source to be CH4-producing microbes. Now we have to debate where methane goes, where she escapes.
One of the answers can be one kind chemical interactionBut Hakan Svedem, a scientist from the TEG project of the European Space Agency, says they should not lose hope for life on the red planet if it is finally found that there is no methane.
"We are a bit obsessed with methane, because we know that more than 95% of this gas comes from biological sources on Earth, but there are abundant forms of life that do not produce methane," Schwehd told the BBC.
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Orbiter for straight gas arrived on Mars in October 2016, but it takes one year to reach its orbit, at an altitude of 400 km.
In April 2018, he began his analysis of the atmosphere with the help of NOMAD and ACS spectrometers.
These devices measure the air components of Mars looking through the atmosphere to the Sun because different molecules absorbs light in a characteristic way.
The accuracy of this solar exclusion method allows the TGO to set the upper methane limit of only 12 parts per billion. In other words, if there is methane, there must be a lower concentration of this.
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The mission team reported on its remarks at a meeting of the European Union of Earth Sciences in Vienna.
The other results they presented included an accurate description of the rise in water in the atmosphere during the recent global dust storm on the planet and a new map of the water below the surface.
This map has a greater resolution than any previous study and, despite the ice of the polar regions, apparently rich in water, detects some "wet" regions previously unknown in the equator.
These data could be important for future robots seeking evidence of current microbial life on the surface of Mars and for astronauts that need local water resources.
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