The gigantic Martian sandstorm of 2018 wasn’t just a wild ride, it also gave us a gas previously undetected in the planet’s atmosphere. For the first time, the ExoMars orbiter sampled traces of hydrogen chloride, made up of one hydrogen atom and one chlorine atom.
This gas presents Mars scientists with a new mystery to solve: how it got there.
“We have discovered hydrogen chloride for the first time on Mars,” said physicist Kevin Olsen of the University of Oxford in the UK.
“This is the first detection of a halogen gas in the atmosphere of Mars and represents a new chemical cycle to understand.”
Scientists have been keeping an eye out for chlorine-containing gases in Mars’ atmosphere, as they were able to confirm that the planet is volcanically active. However, if hydrogen chloride was produced by volcanic activity, it should only increase very regionally and be accompanied by other volcanic gases.
The hydrogen chloride detected by ExoMars did not, and it was not. It was sniffed in the northern and southern hemispheres of Mars during the dust storm, and the absence of other volcanic gases was evident.
This suggests that the gas was being produced by some other process; Fortunately, we have similar processes here on Earth that can help us understand what it could be.
It is a multi-step process that requires a few key ingredients. First, you need sodium chloride (which is common salt), left over from evaporation processes. There’s a lot of it on Mars, believed to be the remains of ancient salt lakes. When a dust storm churns at the surface, sodium chloride is released into the atmosphere.
Then there are the Martian ice caps which, when heated during the summer, sublimate. When the resulting water vapor mixes with the salt, the resulting reaction liberates chlorine, which then reacts to form hydrogen chloride.
“It takes steam to release chlorine and it takes the by-products of water (hydrogen) to form hydrogen chloride. Water is central to this chemistry,” said Olsen.
“We also see a correlation with dust: we see more hydrogen chloride as dust activity increases, a process related to the seasonal warming of the southern hemisphere.”
This model is supported by a hydrogen chloride detection during the following dusty season of 2019, which the team is still analyzing.
However, confirmation is still pending. Future and ongoing observations will help build a more complete picture of the process cycles.
Meanwhile, laboratory experiments, modeling, and simulations will help scientists rule out or confirm the possible mechanisms behind the release of hydrogen chloride into the Martian atmosphere.
The research has been published in Scientific advances.