Among all the mysteries surrounding the planet Mars, the one about methane is certainly one of the most intriguing. Detected since 2003 in the Martian atmosphere but at extremely low levels, this gas, which is often synonymous with bacterial activity (at least on Earth), has been a subject of intense debate, especially regarding its origin. The debates intensified when the rover Curiosity directly detected methane in the bottom of the Gale crater, where it has been operating since 2012.
Methane Detected by Curiosity in Gale Crater
It appears that methane is seeping from the Martian soil, in very small quantities, enough to be detected by the rover’s instrument named SAM (Sample Analysis at Mars). However, a significant issue arises: this phenomenon seems to be exclusively limited to the Gale crater, with no traces of methane detected elsewhere on the surface of Mars. This is quite a peculiar coincidence. Furthermore, while previous analyses have shown the presence of some methane molecules in the atmosphere, the ExoMars Trace Gas Orbiter, specifically designed to detect gases in the atmosphere above four to eight kilometers in altitude, has not observed anything at all. A previous study suggests an explanation to reconcile the observations of Curiosity and ExoMars by pointing to the timing of the measurements (day or night) as a possible cause. However, this does not explain the origin of Martian methane.
Scientists initially speculated that the materials composing Curiosity could be the source of these emissions, but this hypothesis was quickly discarded due to lack of evidence. Yet, a new study published in the journal JGR Planets reveals that the rover may indeed have something to do with this story.
Methane Trapped Under a Salt Crust and Released by the Rover’s Passage
A team of NASA scientists rolled up their sleeves to tackle the thorny issue of Martian methane. Based on laboratory experiments and modeling, researchers suggest that methane, produced by various geological processes within the Martian subsurface, may be trapped under a layer of surface sulfate. Seasonal variations in temperature, as well as variations between day and night, could alter the sealability of the layer, potentially releasing the trapped methane.
On Mars, methane (CH4) is seen by some, while others do not see it. This has intrigued astronomers for several years. Now, researchers propose an explanation. The detection strongly depends on the time of day when the measurements are taken.
It was in 2004 when methane (CH4) was first spotted on Mars by the European probe Mars Express. This discovery excited astronomers because on Earth, methane is primarily produced by microbes – the ones that help animals digest – which implies life. Although geological processes can also produce methane, finding traces of it on Mars is exciting, especially since this gas has a relatively short lifespan in the Martian atmosphere, around 300 years. Therefore, if our instruments detect methane, it must be from a relatively recent source.
Beyond the mystery of the origin of methane on Mars, astronomers have been facing another puzzle for a few years. Despite the rover Curiosity (NASA) detecting methane several times above the surface of the Gale crater, the European mission ExoMars Trace Gas Orbiter (TGO) has never been able to confirm the presence of this gas higher up in the atmosphere.
The amounts of methane detected by Curiosity are indeed minimal, less than half a part per billion (ppb), equivalent to a pinch of salt in an Olympic-sized swimming pool, with spikes of up to 20 ppb in concentration. When the European team announced they did not see any methane, it was quite surprising.
One theory suggests that the rover’s weight causes tiny fractures in the sulfate crust, allowing for sudden gas release. This phenomenon could explain the variations in methane quantities emitted from the soil, as well as the higher measurements by Curiosity, up to 40 times the background levels, possibly due to the rover’s movements fracturing the crust.
Therefore, the presence of methane on Mars remains a complex puzzle with various factors contributing to its detection and variation in concentrations.Remember Chris Webster, head of the Tunable Laser Spectrometer (TLS) instrument of Curiosity, interviewed by NASA. Because TGO was designed to be the gold standard for measuring gases in the Mars atmosphere.
### What if Curiosity and TGO were both right?
Researchers first suspected Curiosity of producing the CH4 it detected. They analyzed every detail: how it crushes rocks, how its wheels degrade, etc. To unequivocally conclude that the rover’s measurements are correct.
Then, planetary scientists suggested that there was no mystery or error. That Curiosity and TGO could both be right. The measurement discrepancies observed could simply be related to the timing of these measurements.
Researchers point out that the TLS instrument used by Curiosity to measure methane can only operate at night. It is energy-intensive and must wait until all other instruments are shut down before it can wake up. At night, the Martian atmosphere is calm. Methane seeping from the ground will likely accumulate near the surface and be detected by Curiosity.
TGO, on the other hand, needs sunlight to search for methane traces about five kilometers above the ground. But the daytime heat stirs the atmosphere. The methane concentrated at ground level at night gets diluted in the atmosphere a few hours later. At levels suspected to be undetectable.
Astronomers have just confirmed this. They exceptionally mobilized the Curiosity rover for daytime measurements. And the TLS instrument also found nothing! This resolves the issue. At least on the surface. Because if methane is constantly seeping from the Martian soil and its lifespan in the atmosphere is about 300 years, it should have accumulated enough for TGO to detect it.
It is now necessary to understand what destroys methane in the Martian atmosphere. Perhaps very low-intensity electric discharges induced by Martian dust. Or the action of abundant oxygen on the surface that destroys CH4 before it reaches the upper atmosphere. Experiments are ongoing.