Mars orbiter detects gas pulses that hint the planet's volcanoes are not entirely dead

A European-led team of planetary scientists has reported unusual pulses of hydrogen chloride gas rising from fissures on one of Mars' largest volcanoes, a finding they say is consistent with, though not proof of, magma still churning beneath the planet's crust. The detections, made by the European Space Agency's ExoMars Trace Gas Orbiter, were published on November 4 in the journal Science Advances, in a paper titled "Enigmatic HCl detections as a potential signature of igneous activity on Mars."

The gas showed up above Alba Mons, a broad, low-relief shield volcano roughly the size of the continental United States, in a place and season where it had no obvious reason to be. Hydrogen chloride on Mars typically peaks during the cold aphelion season, when the planet is farthest from the sun, and nearly disappears by summer, a pattern scientists have linked mainly to the seasonal cycling of water ice and dust rather than to anything happening underground. The new readings, gathered by the orbiter's Atmospheric Chemistry Suite, instead turned up in early northern-hemisphere summer, concentrated over Alba Mons at levels researchers say the standard atmospheric models do not explain.

What the instruments found

The lead authors, Alexander Trokhimovskiy of the Space Research Institute of the Russian Academy of Sciences and Daniel Mège of the Centrum Badań Kosmicznych (Polish Academy of Sciences), describe concentrations reaching roughly 1.5 parts per billion by volume at the detection sites, picked up in a handful of the instrument's several dozen observation passes over the region. In the paper's own language, reported by the European Space Agency, the detections of HCl at low altitudes were "unexplained by observed atmospheric state and models," and the regional geology "hint at an igneous origin of this gas."

That geology turned out to matter. Working alongside the gas measurements, the orbiter's Colour and Stereo Surface Imaging System, known as CaSSIS and operated from the University of Bern, photographed fresh, narrow fissures on the volcano's northern flank, some stretching hundreds of metres, several of them associated with visible changes in surface colour that the team interprets as deposits linked to gas escaping from below. The researchers propose a seasonal mechanism to explain the timing: a body of residual heat, or dormant magma, releases gas that migrates upward through these cracks, but winter ice seals the openings until the ice sublimates away in early summer, at which point the trapped gas is released in a short-lived pulse.

Why this adds to, rather than settles, a long debate

Mars has long been assumed to be geologically quiet, its last major eruptions dated by crater counting to somewhere between tens of millions and, on some of Alba Mons' youngest flows, as little as one to two million years ago, according to reporting in National Geographic. That is recent by geological standards but has generally been treated as a closed chapter rather than an ongoing process. The picture has been complicated over the past several years by other lines of evidence: NASA's InSight lander, before its mission ended, recorded marsquakes in the Elysium Planitia region whose characteristics some researchers read as possible signs of fluid or magma movement at depth, and earlier ExoMars observations, going back to the orbiter's first detection of hydrogen chloride in the Martian atmosphere in 2021, established that the gas's seasonal behaviour was not fully understood even without the new, out-of-pattern signal near Alba Mons.

Hydrogen chloride is a plausible volcanic tracer because, on Earth, it is commonly released from magma alongside gases such as sulfur dioxide. Mars' atmosphere is thin and its surface chemistry different enough from Earth's that other explanations remain on the table; the authors themselves note that the gas could, in principle, form from reactions between warm water and metal chlorides in the soil rather than rising directly from magma. ESA's project scientist for the mission, Colin Wilson, was more bullish on the finding's significance, saying it "may prove to be [the orbiter's] greatest success yet," according to the agency's release carried on EurekAlert.

"Mars has retained enough internal heat to support volcanic activity until recently, and possibly still does," said Daniel Mège, one of the study's co-lead authors, in the European Space Agency's release of the findings.

Caution from scientists outside the study

Researchers not involved in the work urged restraint in interpreting the result as confirmation of ongoing eruptions. Kevin Olsen, an atmospheric physicist at the University of Oxford and a co-author on the paper, told National Geographic that the central question remains open: "Is it just pockets of gas underground, or is there actually active magma?" He added that "the nature of the source is unknown." Tracy Gregg, a planetary volcanologist at the University at Buffalo who was not part of the study, offered a similarly guarded assessment, telling the publication that volcanologists have "learned to never say never" about declaring a volcanic system extinct, while stopping short of endorsing the magma explanation outright.

The paper's authors are explicit that their case is circumstantial rather than direct: they have not observed heat, seismicity or any other independent signature of magma at Alba Mons itself, and InSight's seismic data, gathered from a different region of the planet, has not produced a confirmed magmatic signal from this volcano. What they have is a correlation between an anomalous gas signal, its timing relative to the ice cycle, and newly imaged surface fractures that are geologically young enough to be plausible conduits.

What happens next

The team says follow-up observations targeting the same fissures across additional Martian seasons, combined with higher-resolution imaging of the gas plumes' exact source points, would help distinguish a genuinely magmatic origin from the alternative chemical pathways the authors have not ruled out. The orbiter, a joint project of ESA and Russia's Roscosmos that has operated at Mars since 2016, is continuing its atmospheric survey, and the authors note that confirming where, precisely, the gas emerges on the surface has so far eluded them. Scientists outside the project have also pointed out that if warm, chemically active water exists anywhere near such fissures, the sites could be of interest beyond volcanology, as potential niches where microbial life, if it ever existed on Mars, might have found a foothold — a possibility raised in the paper but presented as speculative rather than supported by direct evidence.