ESA says recurring hydrogen-chloride detections above fissures on Alba Mons may come from gas rising through volcanic rock. No eruption or new lava flow has been observed, and the proposed magma source remains an interpretation.
Sole source: Hydrogen chloride has repeatedly appeared above volcanic fissures on the flanks of Mars’s Alba Mons volcano in early northern summer, ESA said in an account published on 5 November. The research team proposes that the gas comes from hot magma beneath the surface, a possible sign of dormant volcanism.
The pattern is unusual: hydrogen chloride in the Martian atmosphere normally peaks in northern winter and mostly disappears in summer. ESA says most of that variation appears to be linked to the planet’s dust and water cycles. The ExoMars Trace Gas Orbiter first reported the gas in the atmosphere in 2021.
No new lava flows or eruptions have been observed on Mars, ESA says. The team’s suggested connection to a dormant magma chamber—an underground reservoir of molten rock—remains an interpretation of the observations.
The orbiter’s Atmospheric Chemistry Suite used infrared instruments to detect the gas in a tightly localized area just after winter frosts had turned directly into vapor, a process called sublimation. Images from its CaSSIS camera then showed fresh, narrow fissures, some hundreds of meters long, with changes in surface color.
The researchers suggest that winter ice blocks the fissures and that gas can escape when the ice sublimates in early summer. They also propose that the discoloration could be deposits left by gas escaping from inside Mars, as around volcanic gas vents on Earth. On Earth, hydrogen chloride commonly escapes from volcanic gas vents.
The international team was led by Alexander Trokhimovskiy. Co-lead author Daniel Mège, a professor at the Space Research Centre of the Polish Academy of Sciences, said the investigation involved examining possible atmospheric chemical processes that could explain the gas. The paper, “Enigmatic HCl detections as a potential signature of igneous activity on Mars,” appeared in Science Advances on 4 November 2026.
Analysis
Mège said the fissures would be worth investigating for Martian life. If systems of heated water are active there, he said, they could provide ingredients, energy and shelter for microorganisms. That possibility does not establish that organisms are present.
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