The European Space Agency's ExoMars Trace Gas Orbiter has recorded hydrogen chloride in the Martian atmosphere concentrated over Alba Mons, one of the planet's largest volcanoes, according to SINC. A study published this week in Science Advances, with participation from the Institute of Astrophysics of Andalusia (IAA-CSIC), proposes that the gas may rise from magma beneath the surface. The work was reported by SINC on 5 November 2026.
The orbiter first identified hydrogen chloride in the Martian atmosphere in 2021. On Earth, the compound commonly escapes from magmatic conduits, which prompted researchers to ask whether the same process could occur on Mars.
Why it matters
Whether Mars still has active volcanism is an open question. The planet carries a long volcanic record, but scientists have not established whether any of that activity persists today.
Juan Alday, an IAA-CSIC researcher involved in the study, said the detections at Alba Mons give researchers a new route into the question. According to SINC, he noted that although Mars has an extensive volcanic history, it remains unclear whether that activity continues.
If the interpretation holds, the findings would point to magma that is still warm at depth rather than a geologically finished world — a distinction that bears on how scientists model the planet's interior and its recent evolution.
A seasonal signal with one exception
After the 2021 detection, researchers tracked how hydrogen chloride levels changed with the Martian seasons. Concentrations peak during northern hemisphere winter and fall away to almost nothing by summer, a pattern that appears largely tied to the planet's dust and water cycles.
One observation broke that pattern. In early northern summer, when the atmosphere holds little dust and the gas is normally hard to find, hydrogen chloride appeared only over a single area near Alba Mons. The same location produced the signal at the same time of year repeatedly, which led the team to examine whether a geological feature on the ground could explain it.
Fractures as possible escape routes
The researchers combined atmospheric measurements from the ACS instrument on the Trace Gas Orbiter with images from the orbiter's CaSSIS camera. The images revealed narrow fractures on the slopes of Alba Mons, some hundreds of metres long, accompanied by colour changes in the surrounding terrain.
The team suggests these fractures could serve as pathways for gases moving up from below. One hypothesis is that the gas is magmatic in origin, climbing through old volcanic conduits to reach the surface.
Seasonal ice may govern the timing. Martian winter temperatures can drop to around -120 °C, which would allow ice to build up in the fissures and block the escape of gases. As conditions warm, that ice could sublimate — passing directly from solid to gas — and release the trapped gases into the atmosphere once more.
The colour shifts around the fractures may also be deposits left behind by escaping gases, a process the researchers compare to the surroundings of some fumaroles on Earth.
From these lines of evidence, the team proposes that gas could travel through fractures connected to a magmatic system at depth, which would represent a form of dormant volcanism. Alday described the IAA-CSIC contribution as part of sustained work to monitor known gases, study how they vary, and search for species not yet detected.
What to watch next
The IAA-CSIC is leading the design of new ground-based observation campaigns intended to complement the orbiter's measurements. Alday said the team is preparing observations to look for other gases that might accompany hydrogen chloride under the proposed scenario, particularly sulphur compounds such as sulphur dioxide, which is difficult for the Trace Gas Orbiter to detect. Those observations could add detail on the composition of the atmosphere over the region and on the processes behind the appearance and disappearance of the hydrogen chloride, he said.
