A stinky gas, hydrogen chloride, is showing up over a stretch of Mars where fresh fissures have just been mapped. Instruments aboard the ExoMars Trace Gas Orbiter picked it up at parts-per-billion levels near the Alba Mons region. The researchers behind the work say the pattern points toward magma that is still active under the surface.
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The paper appeared in Science Advances on Nov. 4. Its authors are careful to call the evidence a hint rather than a verdict, and the paper’s own wording (“potential signature”) reflects that restraint.
Hydrogen chloride where models say there should be none
The measurements come from the Atmospheric Chemistry Suite on the orbiter. According to the AAAS release distributed through EurekAlert, hydrogen chloride concentrations ran higher than expected during the cold aphelion season, when Mars is farthest from the Sun, and the gas clustered locally over newly discovered fissures rather than spreading evenly across the planet. Lead author Alexander Trokhimovskiy of the Space Research Institute (IKI) of the Russian Academy of Sciences and colleagues argue that the low-altitude detections cannot be explained by the observed atmospheric conditions and existing models.
On Earth the same compound is a staple of volcanic plumes, which is why the authors lean toward an igneous source. Because the signal sat low in the atmosphere and over specific ground, the team argues, a source at the surface is a more natural fit than gas drifting in from elsewhere. The paper itself, on enigmatic hydrogen chloride detections, is Science Advances DOI 10.1126/sciadv.aeb9422, and the release calls the gas one often produced by magmatism.
Fissures as a possible plumbing system
The central hypothesis is geometric. If the newly found cracks reach deep enough to touch active magma, volcanic outgassing would leak hydrogen chloride into the thin air directly above them, which would produce exactly the localized, low-altitude signal the orbiter saw. The authors say this combination, plus the regional geology, hints at an igneous origin for the gas. Gas escaping through a crack would be expected to peak right at the crack and thin out with distance, so a map of where the signal is strongest against where fissures sit is the test the idea has to pass.
The release also points to recently detected marsquakes as separate support for ongoing volcanic activity. NASA’s Mars hub notes that its InSight lander recorded seismic waves from two quakes in 2021, evidence that the planet’s interior is far from silent. Alba Mons itself is a broad volcanic edifice in the northern hemisphere, so a region with a long volcanic history sits underneath the signal. Those marsquakes are a separate line of evidence, which means the gas is not the only clue the authors cite.
The ExoMars programme is a joint effort, and the European Space Agency says the Trace Gas Orbiter has been mapping Martian atmospheric gases from orbit since 2018. That long baseline is what lets scientists notice a local anomaly against a background of seasonal change. Mars sits at aphelion, the far point of its orbit, during the cold season when the unexpected concentrations appeared, which is why the seasonal framing matters to the argument.
Unresolved: how certain is the volcanic reading
The study stops short of proof. The source of the hydrogen chloride is inferred, because current atmospheric models do not fully account for the detections, and the release gives no uncertainty figures or alternative explanations. Exact concentrations, the number of fissures and the observation dates are also absent from the summary, so independent specialists will want the full paper before accepting a living-volcano picture of Mars. The authors’ reasoning rests on three legs: low-altitude detections that atmospheric models cannot reproduce, the newly discovered fissures beneath them, and the regional geology around Alba Mons.
ExoMars has a second act ahead. The European Space Agency says the Rosalind Franklin rover is set to launch in 2028 to search for signs of life, while the orbiter keeps mapping gases from above. Whatever the hydrogen chloride finally turns out to be, the measurement adds to a growing argument about whether the interior of Mars is livelier than long assumed, and the claim stands for now as a published hypothesis awaiting independent follow-up.
Daniel Mege of the Space Research Centre of the Polish Academy of Sciences and the University of Bern is listed as the expert contact for the work, and the release further notes a 2025 Science study that found a highly mixed Martian mantle shaped by early collisions and irregular convection. A mantle that churns unevenly is at least compatible with pockets of melt persisting today.
Whether more orbital passes over the same fissures repeat the signal, and whether it tracks the seasons the way a leaking source should, will decide if the detection holds. The journal, Science Advances, will carry any rebuttal or confirming study, and the orbiter’s continuing observations are the obvious place for one to come from.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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