"Where is my methane?" Or the ExoMars TGO probe does not detect CH4 on Mars

diciembre 14, 2018

"Where is my methane?" Or the ExoMars TGO probe does not detect CH4 on Mars https://danielmarin.naukas.com/files/2018/12/exomars-trace-gas-orbiter-getting-ready-to-start-its-mission.jpg

In 2004 the European probe Mars Express gave the bell with the discovery of methane in the Martian atmosphere. Methane is a reactive gas that is highly unstable, which means that it can only be detected if there is a continuous source that is responsible for emitting it into the atmosphere. And the only known mechanisms that could generate methane on Mars are the geological activity -serpentinization- and the biological activity -ie, Martians-, both fascinating cases. But the spectral signature of methane is very elusive and it is easy to confuse it with other compounds if we do not have the proper instrumentation. As of 2004 a debate was opened about the nature of Martian methane and soon the scientific community was divided into three camps. One group was betting that there was no methane to start and that everything was a confusion due to inaccurate observations. Another believed that methane had its origin in the organic substances of meteorites that continuously collide with the planet and, therefore, was not a matter worthy of study. The last group advocated a geological and / or biological origin, for which reason it considered that its analysis was a priority.

ExoMars 2016 TGO has not seen methane on Mars (ESA).

Most planetary scientists at NASA were in favor of the first two options, while at ESA the opinion was given that it was a problem that deserved a specific study from orbit. This is how the ExoMars TGO project was born (Trace Gas Orbiter), a probe to analyze the composition of the Martian atmosphere with exquisite detail. NASA abandoned the project in 2012 with the excuse that the Martian methane was either non-existent or was caused by meteorites, but really the reason was that the US agency was not comfortable with a project led by Europe. ESA was forced to turn to Russia to take forward the TGO probe, which would finally be launched in 2016 (along with the ill-fated Schiaparelli surface probe).

While all this debate was taking place, the Curiosity rover landed in the Gale crater in 2012 with SAM (Sample Analysis at Mars), a very sensitive instrument that allowed him to detect methane in the Martian air. Although the initial results were confusing due to the contamination of the instrument by terrestrial substances, it was finally confirmed that, effectively, there was methane on Mars and that, moreover, a seasonal cycle followed. The concentration of methane is minuscule, with seasonal maximums of the order of 0.7 ppb (parts per billion in volume), but Curiosity has detected a transient peak with a concentration up to ten times higher (7.2 ± 2.1 ppbv) . The "enemy of methane" side changed its discourse in view of the new evidence and concluded that the seasonal variation can be explained by the serpentinization or Martian microbes, yes, but also by meteorites (the organic material of the meteorites contains methane that would be trapped in the ice -clatratos- and it would be released depending on the seasonal temperature).

Martian methane cycle detected by Curiosity (NASA).

Possible sources of methane on Mars (NASA).

And so we come to the present. The ExoMars TGO probe has been analyzing the Martian atmosphere for several months with the Belgian spectrometer NOMAD and the Russian spectrometer ACS. And the result? Nothing, zero, zip, null. There is no methane in the Martian atmosphere as far as ExoMars TGO is concerned. Both spectrometers are complementary and capable of detecting methane at any height on the surface. In theory they should detect concentrations of at least 50 ppt (50 parts per billion). The absence of methane is a hard blow for the supporters of methane originated by geological or biological activity, but it is not, far from it, a cataclysm. Because, after all, Curiosity he detected methane locally. Now it is about reconciling the results of Curiosity, locally, and the TGO, globally.

It is a paradox that the existence of Martian methane has been confirmed by a NASA probe, the agency that vehemently opposed the TGO project, while the TGO probe itself has not been able to detect anything at all. Of course, all experts trust that, over time, TGO will detect methane. Now we just need to wait how much and how. The mystery is still open.

The Martian surface seen by the CaSSIS camera of TGO (ESA / Roscosmos).

References:

https://www.sciencemag.org/news/2018/12/martian-methane-spotted-2004-has-mysteriously-vanished