Mars is the big thing in the news this week, and the European Space Agency announced the discovery of hydrogen chloride gas in the atmosphere. The detection was made using the ESA-Roscosmos ExoMars Trace Gas Orbiter and could provide insight into surface-atmosphere interactions.

This discovery remains a bit of a mystery, though. Per the press release: Mars scientists were always on the look-out for chlorine- or sulfur-based gases because they are possible indicators of volcanic activity. But the nature of the hydrogen chloride 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.

Here’s one explanation for that different source. The wind lifts dust that contains chloride salts, like NaCl, into the atmosphere. Solar heating warms the surface, causing water vapor to rise from some of that subsurface water ice we keep talking about. Then the salt interacts with the water to release the chlorine, which then reacts with either the hydrogen in the water or free hydrogen gas in the atmosphere to form hydrogen chloride.

Or there could be active volcanoes we just haven’t seen. Either way, we need more data, as always. The results of this research can be found in Science Advances.

More Information

ESA press release

“Transient HCl in the atmosphere of Mars,” Oleg Korablev et al., 2021 February 10, Science Advances