Mars may still be volcanically active, as a European orbiter keeps sniffing hydrogen chloride over Alba Mons

Every early northern summer, the same patch of Mars exhales a whiff of hydrogen chloride. The European Space Agency’s ExoMars Trace Gas Orbiter has caught the gas rising over the flanks of Alba Mons in repeated passes, and a team led by Alexander Trokhimovskiy argues that the best explanation is magma still cooling beneath the surface.

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The result appeared in Science Advances on Nov. 4 and drew an ESA write-up the next day. Co-lead author Daniel Mège of the Space Research Centre of the Polish Academy of Sciences says it suggests Mars kept enough internal heat to power volcanism until recently, and possibly to this day.

A gas that should not be there in summer

Hydrogen chloride was first reported in the Martian atmosphere in 2021, and ESA’s own account of the new study explains why the Alba Mons detections are odd. The gas normally peaks in the northern-hemisphere winter and mostly disappears in summer, when it is thought to be taken up by the surface. Over Alba Mons the pattern flips: spikes appear in early summer, in the same place, year after year.

The spikes come from the Atmospheric Chemistry Suite, a set of infrared spectrometers on the orbiter that watch sunlight pass through the atmosphere at sunrise and sunset. Mège compared the work to looking for a needle in a haystack, since the signal is only a handful of molecules and the source may be intermittent. The paper, indexed under 10.1126/sciadv.aeb9422, describes the detections as a potential signature of igneous activity, a deliberate choice of words that stops short of declaring a volcano awake. Solar occultation lets the spectrometers read the chemistry of the air along a long slant path, which is how such a faint trace can be isolated at all.

Fissures on the flanks of the volcano

Orbital images from the CaSSIS camera show fresh, narrow fissures on the flanks of Alba Mons, some hundreds of meters long, with color changes at the surface. The team proposes that ice seals the cracks through winter, and that chloride escapes as the ice sublimates in early summer, much like fumaroles on Earth. The discolored ground may be deposits left by the rising gases, and the seepage is tied to a dormant magma chamber below.

A summary of the paper by The Brighter Side adds numbers: 78 solar occultations across two Martian years, with clear hydrogen chloride in eight of them inside a narrow range of longitude. An early pair of hits prompted a dedicated search in a later Martian year, which found six more. The maximum local concentration was about 1.5 parts per billion. That summary is secondary reporting, and the figures were not cross-checked against the paper.

Another write-up, from Sci.News, quotes a flux estimate from the paper: assuming the gas survives one Martian day in the air, the seepage rate would be below 3 kilograms per second, comparable to the low end of volcanic gas output on Earth. The figure hinges on that one-day lifetime assumption, so it carries real uncertainty, and the same article notes the authors treat the volcanic explanation as best supported by the evidence rather than proven.

What would settle the question

The case is circumstantial. According to National Geographic, the authors themselves say the evidence is not definitive and that they cannot confirm the gas rises from these exact fissures. Hydrogen chloride can also form when hot water reacts with metal chlorides, so hydrothermal activity remains an alternative, and most Martian hydrogen chloride is tied to seasonal ice cycles instead of volcanism. NASA’s InSight lander detected no convincing magma-related seismic signals from Alba Mons, and Tracy Gregg of the University at Buffalo, who was not part of the study, told the magazine the lander’s seismic signals could come from magma or from water, and she suspects both. Nobody is forecasting an imminent eruption.

No new lava flows or eruptions have been observed. Orbital imagery does suggest some Martian volcanoes were active within the last few million years, and seismometers have recorded marsquakes, so a hot interior is not a radical idea. Kevin Olsen of the University of Oxford, another author, called the recurrence at one site the key clue, and described the gas source as unknown.

ESA project scientist Colin Wilson said the findings may prove to be the orbiter’s greatest success yet. The stakes extend past geology: magmatic gases could once have been more abundant and helped thicken the atmosphere and warm the climate, and if lava meets groundwater it can create warm, habitable niches. Mège said looking for Martian life in the seeping fissures of the Alba Mons region would make a lot of sense. More detections from another northern summer are awaiting separate analysis, and they are the next data that could strengthen or weaken the case. A Martian volcano that leaks chlorine gas from hidden magma, rather than erupting lava, would also explain why heat-based surveys from orbit have found no thermal sign of activity.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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