The 2018 Martian sandstorm was not only a wild walk, but also gave us a previously undetected gas in the planet’s atmosphere. For the first time, the ExoMars orbiter took traces of hydrogen chloride, composed of a hydrogen atom and a chlorine atom.
This gas presents a new mystery to scientists on Mars: how it got there.
“We first discovered hydrogen chloride on Mars,” said physicist Kevin Olsen of Oxford University in the UK.
“This is the first detection of a halogen gas in the atmosphere of Mars and is a new chemical cycle to understand.”
Scientists have been wary of chlorine-containing gases in Mars’ atmosphere because they could confirm that the planet is active volcanic. However, if hydrogen chloride was produced by volcanic activity, it should only grow very regionally and be accompanied by other volcanic gases.
Hydrogen chloride detected by ExoMars was 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 blatant.
This suggests that the gas was produced by another process; Fortunately, we have similar processes here on Earth that can help us understand what they might be.
It is a multi-step process that requires a few key ingredients. First, you need sodium chloride (ie regular salt) left over from evaporation processes. There is much about this on Mars, which is believed to be the remains of ancient salt lakes. When a dust storm shakes the surface, sodium chloride is released into the atmosphere.
Then there are the Martian polar caps that, when heated in summer, sublimate. When the resulting water vapor is mixed with the salt, the resulting reaction releases chlorine, which then reacts further to form hydrogen chloride.
Graph showing the potentially newly detected chemistry cycle 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 essential in this chemistry,” Olsen said.
“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 detection of hydrogen chloride in the next dusty season of 2019, which the team is still analyzing.
However, confirmation is still pending. Future and ongoing observations will contribute to a more comprehensive picture of the process cycles.
Meanwhile, laboratory experiments, modeling and simulations will help scientists rule out or confirm the potential mechanisms behind the release of hydrogen chloride into the Martian atmosphere.
The research was published in Scientific advances.