Hydrogen Chloride Gas

The source of hydrogen chloride (HCl) in the atmosphere of Mars is recent surface volcanism, subsurface magmatic activity, or aerosol chemistry occurring with the Martian dust particles lofted into the atmosphere. HCl increased during the 2018 global dust storm and declined soon after its end, pointing to the exchange between the dust and the atmosphere of the Red Planet.

Artist’s impression depicting the separation of the Schiaparelli module from ESA’s Trace Gas Orbiter, and heading for Mars. Image credit: D. Ducros / ESA.

HCl was detected by the Atmospheric Chemistry Suite (ACS) and confirmed with Nadir and Occultation for Mars Discovery instruments onboard ESA’s ExoMars Trace Gas Orbiter (TGO) mission.

“We’ve discovered hydrogen chloride for the first time on Mars,” said Dr. Kevin Olsen a researcher in the Department of Physics at the University of Oxford.

“This is the first detection of a halogen gas in the atmosphere of Mars, and represents a new chemical cycle to understand.”

“The discovery of the first new trace gas in the atmosphere of Mars is a major milestone for the Trace Gas Orbiter mission,” said TGO project scientist Dr. Håkan Svedhem, a researcher at ESA.

“This is the first new class of gas discovered since the claimed observation of methane by ESA’s Mars Express in 2004, which motivated the search for other organic molecules and ultimately culminated in the development of the TGO mission, for which detecting new gases is a primary goal.”

Planetary scientists were always on the look-out for chlorine- or sulfur-based gases because they are possible indicators of Martian volcanic activity.

But the nature of the HCl observations — the fact that it was detected in very distant locations at the same time, and the lack of other gases that would be expected from volcanic activity — points to a different source.

That is, the discovery suggests an entirely new surface-atmosphere interaction driven by the dust seasons on Mars that had not previously been explored.

“In a process very similar to that seen on Earth, salts in the form of sodium chloride — remnants of evaporated oceans and embedded in the dusty surface of Mars — are lifted into the atmosphere by winds,” the scientists said.

“Sunlight warms the atmosphere causing dust, together with water vapor released from ice caps, to rise.”

“The salty dust reacts with atmospheric water to release chlorine, which itself then reacts with molecules containing hydrogen to create hydrogen chloride.”

“Further reactions could see the chlorine or hydrochloric acid-rich dust return to the surface, perhaps as perchlorates, a class of salt made of oxygen and chlorine.”

This graphic describes a possible new chemistry cycle on Mars following the discovery of hydrogen chloride in the Martian atmosphere. Salts in the form of sodium chloride (NaCl) are widespread on the surface of Mars. Winds lift this salty dust into the atmosphere. Sunlight warms the dusty atmosphere causing water vapor released from ice caps to rise. The salty dust reacts with atmospheric water to release chlorine (Cl), which itself then reacts with molecules containing hydrogen (H) to create hydrogen chloride (HCl). A similar process takes place on Earth: sea salt is blown into the air, and if it mixes with water vapor, chlorine becomes available for chemical reactions that form HCl. Further reactions could see the chlorine or hydrogen chloride-rich dust return to the surface of Mars perhaps as perchlorates, a class of salt made of oxygen and chlorine. HCl is observed to quickly appear and disappear from the atmosphere so it must be created and destroyed rapidly, with some fraction returned to the surface. The TGO observations suggest this might be an annual process driven by the changing seasons, specifically the warming of the southern hemisphere ice cap during southern summer, which releases water vapor into the atmosphere. The extra warmth also generates strong winds as air moves from warm to cool regions. In turn, the winds lift more dust, triggering regional and global dust storms. Image credit: ESA.

The authors first spotted the HCl gas during the global dust storm in 2018, observing it appear simultaneously in both northern and southern hemispheres, and witnessed its surprisingly quick disappearance again at the end of the seasonal dusty period.

They are already looking into the data collected during the following dust season and see the HCl rising again.

“You need water vapor to free chlorine and you need the by-products of water — hydrogen — to form hydrogen chloride. Water is critical in this chemistry,” Dr. Olsen said.

“We also observe a correlation to dust: we see more hydrogen chloride when dust activity ramps up, a process linked to the seasonal heating of the southern hemisphere.”

“It is incredibly rewarding to see our sensitive instruments detecting a never-before-seen gas in the atmosphere of Mars,” said ACS instrument principal investigator Dr. Oleg Korablev, a researcher at IKI.

“Our analysis links the generation and decline of the hydrogen chloride gas to the surface of Mars.”

The findings appear in two papers in the journal Science Advances and the journal Astronomy Astrophysics.

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Oleg Korablev et al. 2021. Transient HCl in the atmosphere of Mars. Science Advances 7 (7): eabe4386; doi: 10.1126/sciadv.abe4386

K.S. Olsen et al. 2021. Seasonal reappearance of HCl in the atmosphere of Mars during the Mars year 35 dusty season. AA, in press; doi: 10.1051/0004-6361/202140329