The giant Mars sandstorm of 2018 was not just a wild ride; it also gave us a previously unmarked gas in the planet’s atmosphere. The ExoMars orbiter first sampled traces of hydrogen chloride, consisting of hydrogen and chlorine atom.
This gas presents Mars scientists with a new mystery to solve: how it got there.
“We first discovered hydrogen chloride on Mars,” said physicist Kevin Olsen of Oxford University in the United Kingdom.
“This is the first detection of a halogen gas in the atmosphere of Mars, and it represents a new chemical cycle to understand.”
Scientists are monitoring gases containing chlorine in the atmosphere of Mars, as they were able to confirm that the planet is volcanically active. However, if hydrogen chloride is produced by volcanic activity, it should only rise very regionally and be accompanied by other volcanic gases.
The hydrogen chloride detected by ExoMars was not and was not. It was sniffed out in the northern and southern hemispheres of Mars during the dust storm, and the absence of other volcanic gases was noticeable.
This indicates that the gas is 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 process of several steps that requires some important ingredients. First, you need sodium chloride (this is common salt) left over from evaporation processes. There are many of them on Mars, which are presumably the remains of ancient salt lakes. When a dust storm pushes up the surface, the sodium chloride is kicked up into the atmosphere.
Then there are the Martian ice sheets that get hot during the summer sublime. When the resulting water vapor mixes with the salt, the resulting reaction releases chlorine, which then further reacts to form hydrogen chloride.
Graphic version of the potentially new chemical cycle detected on Mars. (ESA)
“You need water vapor to release chlorine and you need the by – products of water – hydrogen – to form hydrogen chloride. Water is critical in this chemistry,” Olsen said.
“We also see a correlation with dust: we see more hydrogen chloride when dust activity picks up, a process related to the seasonal warming of the southern hemisphere.”
This model is supported by the detection of hydrogen chloride during the next dusty season of 2019, which the team is still analyzing.
However, confirmation is still pending. Future and ongoing observations will help to compile a more comprehensive picture of the cycles of the process.
Meanwhile, laboratory experiments, modeling and simulations will help scientists to exclude or confirm potential mechanisms behind the release of hydrogen chloride into the Martian atmosphere.
The research was published in Scientific progress.