The European Space Agency (ESA) announces that it has isolated traces of hydrogen chloride in the atmosphere of Mars. This gas is not associated with life (quite the contrary). However, its presence implies a new chemical cycle that researchers are trying to understand.
For the first time, the ExoMars orbiter of ESA recorded traces of hydrogen chloride (composed of a hydrogen atom and a chlorine atom) in the atmosphere of the red planet. “It is incredibly gratifying to see our sensitive instruments detect a gas never before seen on this planet “, enthused Oleg Korablev, principal investigator of the Atmospheric Chemistry Suite instrument, which made the discovery. The big question now is:where does this hydrogen chloride come from?
The origin of this colorless gas is still uncertain, but researchers already have some leads. Its presence could for example have meant that the planet is volcanically active . If this hydrogen chloride had been produced by such activity, on the other hand, it should only have appeared in certain areas and been accompanied by other volcanic gases (such as sulfur). But that was not the case here.
On the other hand, we know that this chemical reaction was detected in the northern and southern hemispheres of Mars during the huge dust storm recorded on the planet in 2018 . It is also this huge storm which, we recall, had got the better of the Opportunity rover that NASA had to abandon. The fact that this gas was detected during this huge dust storm is a real lead in that this dust may be involved in similar processes here on Earth.
First you need sodium chloride (salt), available through evaporation processes. Note that there are many on Mars from remnants of ancient salt lakes. When a dust storm stirs the surface, sodium chloride is thrown into the atmosphere.
Then we have the Martian polar caps which, warmed up during the summer, sublimate. You then get water vapour which mixes with salt in the atmosphere. The resulting reaction then releases chlorine which then reacts further to form hydrogen chloride .
"You need water vapor to release chlorine and you need water's by-product, hydrogen, to form chloride of hydrogen. Water is essential in this chemistry “, confirms physicist Kevin Olsen from the University of Oxford in the United Kingdom. “We also see a correlation with dust:we see more hydrogen chloride as dust activity increases, a process related to seasonal warming in the Southern Hemisphere “.
This assumption is also supported by a second detection of hydrogen chloride recorded during the 2019 "dust storm" season , which the team is still analyzing.
A priori, therefore, the researchers have a serious lead to explain this new chemical cycle. Future observations combined with laboratory experiments will help confirm whether or not the potential mechanisms behind the release of hydrogen chloride into the Martian atmosphere.
Details of this study are published in the journal Science Advances.
