Hydrogen chloride gas has been detected for the first time in the atmosphere of Mars by the European Space Agency’s (Esa) ExoMars Trace Gas Orbiter and the Russian space company Roscosmos. ‘This is the first detection of a halogen gas in the atmosphere of Mars and represents a new chemical cycle to understand,’ said Kevin Olsen from the University of Oxford, one of the project’s lead scientists.
The discovery of HCl on Mars was the result of a dust storm that hit the planet in 2018. Chlorine-based gases sometimes indicate volcanic activity, but the HCl was found simultaneously in the northern and southern hemispheres of Mars, and no other volcanic gases were detected. Hence, the gas appears to be linked to an entirely new surface-atmosphere interaction driven by dust seasons on Mars.
As here on Earth, sodium chloride – remnants of evaporated oceans and embedded in the dusty surface of Mars – in the Martian environment appears to be lifted into the planet’s atmosphere by the wind. The HCl that was identified disappeared surprisingly quickly at the end of the 2018 dust season, and there is evidence that it could return again next year, Esa said.
‘You need water vapor to release chlorine and you need the byproducts of water – hydrogen – to form hydrogen chloride. Water is of crucial importance in this chemistry, ‘explains 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.” With further reactions, the chlorine or hydrochloric acid-rich dust could return to the surface, perhaps as perchlorates, the agency suggested.
“Regardless of the origin of HCl, it seems unlikely that the processes responsible for its production and destruction will not affect the rest of Mars’ atmospheric chemistry,” the ExoMars team concluded. “Our discovery suggests that photochemistry on Mars needs to be rethought, taking into account reactions with atmospheric dust or surface degassing.”
One of the goals of such a Mars exploration is to identify atmospheric gases indicative of biological or geological activity to determine whether the planet was ever capable of harboring life. Another goal is to clarify how much water there was and is on Mars to help measure its habitability.
ExoMars not only found HCl in the atmosphere of Mars, but also found three cases that accelerated water loss from the atmosphere, including the 2018 global dust storm, as well as a short but powerful regional storm in January 2019 and the release of water from the South Pole. ice cap during the summer months linked to seasonal changes. In particular, Esa pointed to a plume of rising water vapor identified in the Southern Hemisphere during the summer that could potentially inject water into the upper atmosphere on a seasonal and annual basis.
Unsurprisingly, HCl is present on Mars, as another chlorine compound – perchlorate – was detected on Mars in 2008.