The European Space Agency (ESA) has confirmed the detection of hydrogen chloride traces in Mars' atmosphere. While not linked to life, this discovery points to a novel chemical cycle that scientists are actively investigating.
For the first time, ESA's ExoMars orbiter has identified hydrogen chloride (HCl, consisting of one hydrogen atom and one chlorine atom) in the Martian atmosphere. "It is incredibly gratifying to see our sensitive instruments detect a gas never before seen on this planet," said Oleg Korablev, principal investigator for the Atmospheric Chemistry Suite (ACS) instrument behind the find. The key question remains: what is the source of this colorless gas?
Its origins are unclear, but experts have promising hypotheses. Volcanic activity could produce HCl, yet no localized concentrations or accompanying sulfur gases were observed, ruling this out for now.
Instead, detections occurred in both northern and southern hemispheres during the massive 2018 dust storm on Mars—the same event that ended NASA's Opportunity rover mission. This timing suggests dust plays a pivotal role, mirroring processes potentially at work on Earth.
The process likely begins with sodium chloride (salt) from ancient evaporated lakes on Mars' surface. Dust storms loft this salt into the atmosphere.
Meanwhile, Martian polar caps sublimate in summer, releasing water vapor that interacts with airborne salt. This reaction frees chlorine, which then combines with hydrogen from water to form hydrogen chloride.
"You need water vapor to release chlorine and hydrogen from water to form hydrogen chloride. Water is essential in this chemistry," explains physicist Kevin Olsen from the University of Oxford, UK. "We also see a correlation with dust: more hydrogen chloride appears as dust activity rises, tied to seasonal warming in the Southern Hemisphere."
Supporting this, a second detection occurred during the 2019 dust storm season, currently under analysis.
These findings offer a compelling explanation for the new cycle. Upcoming observations and lab simulations will validate the mechanisms driving hydrogen chloride release. Full details are published in Science Advances.
