After 2018, the giant desert storm on Mars is more than just a spectacle. Thanks to it, we came across a gas that has never been detected in the planet’s atmosphere. For the first time, the ExoMars spacecraft detected traces of hydrogen chloride, composed of one hydrogen and one chlorine atom.
How did this gas end up there?
“We’ve discovered hydrogen chloride for the first time on Mars. This is the first detection of a halogen gas in the atmosphere of Mars, and represents a new chemical cycle to understand,” says Kevin Olsen from the University of Oxford, UK, one of the lead scientists of the discovery.
Scientists have been keeping a close eye on Mars’ chlorine-containing gases for some time, as they could confirm whether or not the planet is volcanically active. However, if hydrogen chloride was produced as a result of volcanic activity, we would observe periodic peaks of this gas (and it would be accompanied by other volcanic gases). However, this is not the case. Hydrogen chloride was detected in both the northern and southern hemispheres of Mars during the sandstorm, and the absence of other volcanic gases is puzzling.
This means that the gas is produced by other processes. Fortunately, similar processes are taking place on Earth that could potentially explain this mystery. Several key ingredients are needed. First – sodium chloride (ie common salt), obtained by evaporation processes. Mars is full of them, they are believed to be “remnants” of ancient salt lakes. When a desert storm begins to rage on the surface, sodium chloride is released into the atmosphere.
Then comes the polar caps of Mars, which, when warmed in summer, sublimate. When the newly formed water vapor mixes with the salt, the end result is chlorine. Chlorine reacts further and forms hydrogen chloride.
“You need water vapour to free chlorine and you need the by-products of water – hydrogen – to form hydrogen chloride. Water is critical in this chemistry,” says Kevin. “We also observe a correlation to dust: we see more hydrogen chloride when dust activity ramps up, a process linked to the seasonal heating of the southern hemisphere.”
This model is also supported by the fact that hydrogen chloride was discovered in 2019 during the sand season. However, the team is still analyzing these samples. Future observations will allow us to gain a clearer picture of the cycles of these processes.
Meanwhile, laboratory experiments, modeling and simulations will help scientists rule out potential mechanisms associated with the release of hydrogen chloride into the Martian atmosphere.
The study is published in Science Advances.