The 2018 big Martian sandstorm wasn’t just a wild ride – it gave us a gas never seen before in the planet’s atmosphere. For the first time, the ExoMars orbiter sampled traces of hydrogen chloride, made up of hydrogen and a chlorine atom.
This gas gives Mars scientists a new mystery to unravel: how it got there.
“We have discovered hydrogen chloride for the first time on Mars,” said physicist Kevin Olsen of Oxford University in the UK.
“This is the first discovery of a halogen gas in Mars’ atmosphere, and it represents a new chemical cycle to be understood.”
Scientists have been on the lookout for chlorine-containing gases in Mars’ atmosphere, as they could prove that the planet is volcanically active. However, if hydrogen chloride was produced by volcanic activity, it should be spun only regionally, accompanied by other volcanic gases.
ExoMars did not, and did not, detect the hydrogen chloride. It was emitted in the northern and southern hemispheres of Mars during the dust storm, and other volcanic gases did not emit.
This suggests that the gas was produced by some other process; fortunately, we have similar processes here on Earth that help us understand what could be.
It is a multi-step process that requires a few key ingredients. First, you need sodium chloride (that’s a regular salt), left over from evaporative processes. There is plenty of that on Mars, which is thought to be the remains of old salt lakes. When a dust storm builds up on the surface, the sodium chloride is kicked into the atmosphere.
Then there are the Martian pole ice caps which, when warmed in the summer, will sublimate. When the resulting water mist mixes with the salt, the resulting reaction releases chlorine, which then reacts further to form hydrogen chloride.
Graphic showing the chemical circle that could be found on Mars. (ESA)
“You need a water valve to release chlorine and you need the by-products of water – hydrogen – to form hydrogen chloride. Water is essential in this chemistry,” Olsen said.
“We also see a correlation with dust: we see more hydrogen chloride as dust activity ramps up, a process linked to the seasonal warming of the southern hemisphere.”
This model is supported by the detection of hydrogen chloride in the dusty season following 2019, which the team is still analyzing.
However, confirmation is yet to come. Future and ongoing ideas will help to build a more complete picture of the process’s cycles.
At the same time, laboratory experiments, modeling, and simulations will help scientists to regulate or test mechanisms that may be behind the release of hydrogen chloride in the Martian atmosphere.
The research was published in Advances in science.