Not only does the presence of methane on Mars remain unexplained, but the apparent fluctuations of its presence are a real headache for astronomers. Two studies published this summer shed some of the mystery.
Can we find cows on Mars? Clearly, the answer is no. But the discovery of methane on the Red Planet in 2004 made many scientists jump. And for good reason: on Earth, most of the methane present in the atmosphere is emitted by living beings, a large part coming from cattle. Since then, thanks to surveys of Curiosity and several other devices, scientific studies are raining down on the subject, and trying to answer two questions: why there is methane on Mars, and why measurements are also irregular.
But why is there methane on Mars?
For the question of origin, a study published recently in Nature can give good arguments to lovers and lovers of little green men. British scientists have concluded that rock erosion, which was one of the preferred hypotheses for the presence of methane, was certainly not the right answer.
The study led by Jon Telling, a geochemist at Newcastle University, sought to know what kind of rock could erode as much methane as it was detected. Their response ? Any.
To understand their results, one must first know the basic theory. The idea is that some rocks in the Martian subsoil contain methane. Under the effect of erosion, the gas escapes, sometimes by pockets, and is detected to the surface. A hypothesis that would explain why the Curiosity robot's laser spectrometer has detected what appears to be peaks of methane in proportions never observed on the Red Planet.
A tempting theory for geologists, only here, for Jon Telling and his team, no rock, even on Earth could contain as much methane given the measured quantities, namely up to 45 parts per billion by volume (that's ie 45 billionths of methane in a volume of air). " With all the data we have, researchers write, it seems highly unlikely that the mechanism of erosion could produce methane in the quantities observed.. "
Scientists have analyzed several samples of Martian meteorites, but also basalt and sedimentary rocks collected directly on Earth to have several points of comparison. Methane has been detected everywhere and each time there is no doubt about its origin: it is the result of interactions between water and rock (so it was not taken there by an external source, as after a shock for example). They coupled this with the erosion measurements made by the different instruments that approached Mars over time. So Curiosity, but also Pathfinder who took up residence there in 1997. And even being generous with the rate of erosion, they arrive for the basalt results far below what has been observed.
They then tried their luck with the sedimentary rocks, and there the results were more encouraging. To exceed the Curiosity measurements by taking terrestrial rocks very rich in methane. But in these conditions, the presence of methane is also explained by the presence of organic compounds in the rock, which is not on Mars. Another blow, therefore, for the theory of erosion. " These are the only times we have been able to detect satisfactory rates, says Jon Telling in Numerama, but it is with completely unrealistic methane concentration rates compared to what can be measured on Mars. We must find answers elsewhere. "
Bubbles of gas and contradictions
That said, they had one track left: the nodules. A kind of gas bubbles stuck in the basement in the crater of Gale, where the observations were made. These pockets may contain methane, but again the data contradict each other. Indeed, Curiosity has excavated the crater of Gale: he has drilled and he even noted a large seasonal sand flow with an increase in the rate of erosion. However, during these moments likely to release methane, nothing happened. It seems that erosion is not the mechanism involved in this phenomenon.
However, if the sources of methane have not been identified, research continues to know other aspects related to the presence of these mysterious molecules. And especially around their disappearance as sudden as surprising. Indeed, beyond the very presence of methane, it is also the contradictions between the measures that have challenged scientists.
First there were the few discoveries of Curiosity. The rover sniffed methane several times between 2014 and 2018. This confirmed the measurements of the Mars Express probe taken in 2004. But during the summer of 2018, only a few days after the last positive measurements of Curiosity, thunderclap, TGO does not detect anything. TGO is ExoMars Trace Gas Orbiter, a spacecraft launched by the European Space Agency ESA in orbit since late 2016 around the Red Planet. Its role, as its name suggests, is to detect not only methane, but all other gases present in the Martian atmosphere. Renowned ultra-sensitive, its spectrometers are supposed to be 100 times more powerful than Curiosity instruments.
The first reflex was then to say that if TGO had not felt anything, Curiosity must have been wrong. Although it seems odd that the little robot made similar mistakes several times. Another hypothesis then emerged: there is an unknown mechanism that causes an accelerated disappearance of methane.
Danish researchers have proposed a solution. This interdisciplinary group from Aarhus University, led by microbiologist Kai Finster, published a study in the Icarus newspaper last July. For them, the simple irradiation by the rays of the Sun can not explain such a rapid evaporation and the key, this time, is in erosion. " Understanding the source is very interesting, explains Kai Finster, but to understand the variations is too, to identify what constitutes the elementary cycles on Mars. "
They also did laboratory experiments, where they simulated erosion on Mars on rocks similar to those that can be found there.
They discovered that during this phenomenon, not only was methane extracted from the rocks, but also and especially that the molecules were immediately ionized, ie all of a sudden, they were no longer electrically neutral. . Consequence: this ionized methane clings to the surface of the rock in question, and thus becomes undetectable in the atmosphere. It is somehow absorbed in the ore. This mechanism causes the disappearance of methane in an accelerated manner, much more efficiently than what the photochemical processes due to the Sun do. " Our results tell us about the dynamics of methane on Mars, concludes Kai Finster, but not on its origin which can be geochemical or biological. For now, we can not know more, but an isotopic analysis could give us answers. "
In any case, their study suggests that the presence of a life, even bacterial on Mars is very unlikely. The minerals affected by ionized methane would produce chemical reactions not necessarily favorable to the appearance of complex organic molecules. For example reactive derivatives of oxygen, or hydrogen peroxide, toxic for many species. Nothing definite about it for now, but the group is now attacking the study of this methane bound to minerals to know how he can behave.
" If the methane is not in rocks or bubbles below the surface, you may need to look deeper, in very localized seeps Suggests Jon Telling. This is one of the tracks that seems today the most plausible.
The Rosalind Franklin rover, scheduled for launch next year, is equipped with a drill capable of taking rock samples two meters deep. Perhaps the opportunity to know if the methane comes from Martian cows or if the answer is much more down to earth.