Mars shows hydrogen chloride spikes that hint at volcanic activity under its surface

A gas that should not be there

The ExoMars Trace Gas Orbiter, a probe operated by the European Space Agency, first detected hydrogen chloride in the Martian atmosphere in 2021. On Earth that gas is associated with volcanic emissions, which drew immediate interest, but the leading explanation at the time tied it to reactions between wind-lifted salty dust and water vapour. A team led by Alexander Trokhimovskiy now reports spikes of the gas in early northern summer, a season when it should have been almost absent. The work is a joint project between ESA and Russia's Roscosmos, and the paper was published in Science Advances on Wednesday, 4 November 2026. The spikes appeared above fissures on the flanks of Alba Mons, which the researchers say may come from hot magma beneath the surface.

The seasonal puzzle

Hydrogen chloride normally peaks in the northern winter of Mars and nearly disappears in summer, a pattern largely linked to dust and water cycles. The Alba Mons exception occurred in the same season and at the same place, year after year. One possible explanation is that ice forming during the winter sealed the fissures until summer melted it, allowing the gas to escape. To look for visual evidence, the researchers used the CaSSIS camera and observed colour changes in the Alba Mons fissures. The sources describe the ice explanation as plausible rather than proven.

Evidence and open questions

Mars has the largest volcanoes in the Solar System, yet the level of their activity remains unknown. Orbital images suggest some of them have been active in the last few million years, and seismometers, including those on NASA's InSight mission, have detected marsquakes. Neither point confirms present-day volcanism, because no new lava flows or eruptions have been observed. Juan Alday, a researcher at the Instituto de Astrofísica de Andalucía and a co-author, told El Mundo that there is evidence of intense volcanic activity in the past. Daniel Mège, a lead co-author, stressed the limits of the new data.

“We have only hints of gas escaping the surface, not lava.”

Implications for life and crewed missions

Mège said the Alba Mons fissures could be a sensible target for the search for Martian life.

“Looking for Martian life in the seeping fissures of the Alba Mons region would make a lot of sense.”

In his view, the fissures might supply the ingredients and energy that micro-organisms would need to develop. The findings may pose only limited problems for a future human presence on Mars, a goal pursued by some space agencies and billionaires, including Elon Musk. Mège compared the possible dangers to eruptions in Iceland, where tourists can watch from not very far away, and said any risks would be localised. Separately, evidence from NASA's Perseverance rover points to possible ancient microbial activity on Mars, and the new observations add to that body of data.