The giant sandstorm on Mars of 2018 wasn’t just a wild ride – it also gave us previously undetected gas in the planet’s atmosphere. For the first time, the ExoMars orbiter sampled traces of hydrogen chloride, consisting of a hydrogen and a chlorine atom.
This gas proposes to Mars scientists to solve a new mystery: how it got there.
“We first discovered hydrogen chloride 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 be understood.”
Scientists have been monitoring gases containing chlorine in the atmosphere of Mars, as they could confirm that the planet is volcanically active. However, if hydrogen chloride were produced by volcanic activity, it would only have to peak very regionally and be accompanied by other volcanic gases.
The hydrogen chloride detected by ExoMars did not and was not. It was sniffed in both the northern and southern hemispheres of Mars during the dust storm, and the absence of other volcanic gases was striking.
This suggests that the gas was produced by a different 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 (that’s just salt) left over from evaporation processes. There are plenty of them on Mars, believed to be the remains of ancient salt lakes. When a dust storm sets the surface in motion, the sodium chloride is kicked into the atmosphere.
Then there are the polar caps of Mars which, when heated in summer, sublimate. When the resulting water vapor mixes with the salt, the resulting reaction releases chlorine, which then further reacts to form hydrogen chloride.
Image showing the possible new chemistry cycle detected on Mars. (ESA)
“You need water vapor to release chlorine and you need the byproducts of water – hydrogen – to form hydrogen chloride. Water is crucial in this chemistry,” said Olsen.
“We also see a connection 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 hydrogen chloride detection during the next dusty 2019 season, which the team is still analyzing.
However, confirmation is still pending. Future and ongoing observations will help provide a more comprehensive picture of the cycles of the process.
Meanwhile, laboratory experiments, modeling and simulations will help scientists rule out or confirm possible mechanisms behind the release of hydrogen chloride into Mars’ atmosphere.
The research is published in Science Advances.